path: root/src/H5Oalloc.c

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * Copyright by The HDF Group.                                               *
 * All rights reserved.                                                      *
 *                                                                           *
 * This file is part of HDF5.  The full HDF5 copyright notice, including     *
 * terms governing use, modification, and redistribution, is contained in    *
 * the COPYING file, which can be found at the root of the source code       *
 * distribution tree, or in https://www.hdfgroup.org/licenses.               *
 * If you do not have access to either file, you may request a copy from     *
 * help@hdfgroup.org.                                                        *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

/*-------------------------------------------------------------------------
 *
 * Created:		H5Oalloc.c
 *
 * Purpose:		Object header allocation routines.
 *
 *-------------------------------------------------------------------------
 */

/****************/
/* Module Setup */
/****************/

#include "H5Omodule.h" /* This source code file is part of the H5O module */

/***********/
/* Headers */
/***********/
#include "H5private.h"   /* Generic Functions			*/
#include "H5Eprivate.h"  /* Error handling		  	*/
#include "H5FLprivate.h" /* Free lists                           */
#include "H5MFprivate.h" /* File memory management		*/
#include "H5MMprivate.h" /* Memory management			*/
#include "H5Opkg.h"      /* Object headers			*/

/****************/
/* Local Macros */
/****************/

/******************/
/* Local Typedefs */
/******************/

/********************/
/* Package Typedefs */
/********************/

/********************/
/* Local Prototypes */
/********************/

static herr_t H5O__add_gap(H5F_t *f, H5O_t *oh, unsigned chunkno, hbool_t *chk_dirtied, size_t idx,
                           uint8_t *new_gap_loc, size_t new_gap_size);
static herr_t H5O__eliminate_gap(H5O_t *oh, hbool_t *chk_dirtied, H5O_mesg_t *mesg, uint8_t *new_gap_loc,
                                 size_t new_gap_size);
static herr_t H5O__alloc_null(H5F_t *f, H5O_t *oh, size_t null_idx, const H5O_msg_class_t *new_type,
                              void *new_native, size_t new_size);
static htri_t H5O__alloc_extend_chunk(H5F_t *f, H5O_t *oh, unsigned chunkno, size_t size, size_t *msg_idx);
static herr_t H5O__alloc_find_best_nonnull(const H5F_t *f, const H5O_t *oh, size_t *size,
                                           H5O_msg_alloc_info_t *found_msg);
static herr_t H5O__alloc_new_chunk(H5F_t *f, H5O_t *oh, size_t size, size_t *new_idx);
static herr_t H5O__alloc_find_best_null(const H5O_t *oh, size_t size, size_t *mesg_idx);
static htri_t H5O__move_cont(H5F_t *f, H5O_t *oh, unsigned cont_u);
static htri_t H5O__move_msgs_forward(H5F_t *f, H5O_t *oh);
static htri_t H5O__merge_null(H5F_t *f, H5O_t *oh);
static htri_t H5O__remove_empty_chunks(H5F_t *f, H5O_t *oh);
static herr_t H5O__alloc_shrink_chunk(H5F_t *f, H5O_t *oh, unsigned chunkno);

/*********************/
/* Package Variables */
/*********************/

/* Declare extern the free list for H5O_cont_t's */
H5FL_EXTERN(H5O_cont_t);

/*****************************/
/* Library Private Variables */
/*****************************/

/*******************/
/* Local Variables */
/*******************/

/*-------------------------------------------------------------------------
 * Function:    H5O__add_gap
 *
 * Purpose:     Add a gap to a chunk
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static herr_t
H5O__add_gap(H5F_t H5_ATTR_NDEBUG_UNUSED *f, H5O_t *oh, unsigned chunkno, hbool_t *chk_dirtied, size_t idx,
             uint8_t *new_gap_loc, size_t new_gap_size)
{
    hbool_t merged_with_null;    /* Whether the gap was merged with a null message */
    size_t  u;                   /* Local index variable */
    herr_t  ret_value = SUCCEED; /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh);
    assert(oh->version > H5O_VERSION_1);
    assert(chk_dirtied);
    assert(new_gap_loc);
    assert(new_gap_size);

#ifndef NDEBUG
    if (chunkno > 0) {
        unsigned chk_proxy_status = 0; /* Object header chunk proxy entry cache status */

        /* Check the object header chunk proxy's status in the metadata cache */
        if (H5AC_get_entry_status(f, oh->chunk[chunkno].addr, &chk_proxy_status) < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_CANTGET, FAIL,
                        "unable to check metadata cache status for object header chunk proxy");

        /* Make certain that object header is protected */
        assert(chk_proxy_status & H5AC_ES__IS_PROTECTED);
    }  /* end if */
#endif /* NDEBUG */

    /* Check for existing null message in chunk */
    merged_with_null = FALSE;
    for (u = 0; u < oh->nmesgs && !merged_with_null; u++) {
        /* Find a null message in the chunk with the new gap */
        /* (a null message that's not the one we are eliminating) */
        if (H5O_NULL_ID == oh->mesg[u].type->id && oh->mesg[u].chunkno == chunkno && u != idx) {
            /* Sanity check - chunks with null messages shouldn't have a gap */
            assert(oh->chunk[chunkno].gap == 0);

            /* Eliminate the gap in the chunk */
            if (H5O__eliminate_gap(oh, chk_dirtied, &oh->mesg[u], new_gap_loc, new_gap_size) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, FAIL, "can't eliminate gap in chunk");

            /* Set flag to indicate that the gap was handled */
            merged_with_null = TRUE;
        } /* end if */
    }     /* end for */

    /* If we couldn't find a null message in the chunk, move the gap to the end */
    if (!merged_with_null) {
        /* Adjust message offsets after new gap forward in chunk */
        for (u = 0; u < oh->nmesgs; u++)
            if (oh->mesg[u].chunkno == chunkno && oh->mesg[u].raw > new_gap_loc)
                oh->mesg[u].raw -= new_gap_size;

        /* Slide raw message info forward in chunk image */
        memmove(new_gap_loc, new_gap_loc + new_gap_size,
                (size_t)((oh->chunk[chunkno].image + (oh->chunk[chunkno].size - H5O_SIZEOF_CHKSUM_OH(oh))) -
                         (new_gap_loc + new_gap_size)));

        /* Add existing gap size to new gap size */
        new_gap_size += oh->chunk[chunkno].gap;

        /* Merging with existing gap will allow for a new null message */
        if (new_gap_size >= (size_t)H5O_SIZEOF_MSGHDR_OH(oh)) {
            H5O_mesg_t *null_msg; /* Pointer to new null message */

            /* Check if we need to extend message table to hold the new null message */
            if (oh->nmesgs >= oh->alloc_nmesgs)
                if (H5O__alloc_msgs(oh, (size_t)1) < 0)
                    HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "can't allocate more space for messages");

            /* Increment new gap size */
            oh->chunk[chunkno].gap += new_gap_size;

            /* Create new null message, with the tail of the previous null message */
            null_msg           = &(oh->mesg[oh->nmesgs++]);
            null_msg->type     = H5O_MSG_NULL;
            null_msg->native   = NULL;
            null_msg->raw_size = new_gap_size - (size_t)H5O_SIZEOF_MSGHDR_OH(oh);
            null_msg->raw      = (oh->chunk[chunkno].image + oh->chunk[chunkno].size) -
                            (H5O_SIZEOF_CHKSUM_OH(oh) + null_msg->raw_size);
            null_msg->chunkno = chunkno;

            /* Zero out new null message's raw data */
            if (null_msg->raw_size)
                memset(null_msg->raw, 0, null_msg->raw_size);

            /* Mark message as dirty */
            null_msg->dirty = TRUE;

            /* Reset size of gap in chunk */
            oh->chunk[chunkno].gap = 0;
        } /* end if */
        else
            oh->chunk[chunkno].gap = new_gap_size;

        /* Mark the chunk as modified */
        *chk_dirtied = TRUE;
    } /* end if */

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__add_gap() */

/*-------------------------------------------------------------------------
 * Function:    H5O__eliminate_gap
 *
 * Purpose:     Eliminate a gap in a chunk with a null message.
 *
 * Note:        Sometimes this happens as a result of converting an existing
 *              non-null message to a null message, so we zero out the gap
 *              here, even though it might already be zero (when we're adding
 *              a gap to a chunk with an existing null message).  (Mostly,
 *              this just simplifies the code, esp. with the necessary chunk
 *              locking -QAK)
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static herr_t
H5O__eliminate_gap(H5O_t *oh, hbool_t *chk_dirtied, H5O_mesg_t *mesg, uint8_t *gap_loc, size_t gap_size)
{
    uint8_t *move_start, *move_end; /* Pointers to area of messages to move */
    hbool_t  null_before_gap;       /* Flag whether the null message is before the gap or not */

    FUNC_ENTER_PACKAGE_NOERR

    /* check args */
    assert(oh);
    assert(oh->version > H5O_VERSION_1);
    assert(chk_dirtied);
    assert(mesg);
    assert(gap_loc);
    assert(gap_size);

    /* Check if the null message is before or after the gap produced */
    null_before_gap = (hbool_t)(mesg->raw < gap_loc);

    /* Set up information about region of messages to move */
    if (null_before_gap) {
        move_start = mesg->raw + mesg->raw_size;
        move_end   = gap_loc;
    } /* end if */
    else {
        move_start = gap_loc + gap_size;
        move_end   = mesg->raw - H5O_SIZEOF_MSGHDR_OH(oh);
    } /* end else */

    /* Check for messages between null message and gap */
    if (move_end > move_start) {
        unsigned u; /* Local index variable */

        /* Look for messages that need to move, to adjust raw pointers in chunk */
        /* (this doesn't change the moved messages 'dirty' state) */
        for (u = 0; u < oh->nmesgs; u++) {
            uint8_t *msg_start; /* Start of encoded message in chunk */

            msg_start = oh->mesg[u].raw - H5O_SIZEOF_MSGHDR_OH(oh);
            if (oh->mesg[u].chunkno == mesg->chunkno && (msg_start >= move_start && msg_start < move_end)) {
                /* Move message's raw pointer in appropriate direction */
                if (null_before_gap)
                    oh->mesg[u].raw += gap_size;
                else
                    oh->mesg[u].raw -= gap_size;
            } /* end if */
        }     /* end for */

        /* Slide raw message info in chunk image */
        if (null_before_gap)
            /* Slide messages down */
            memmove(move_start + gap_size, move_start, (size_t)(move_end - move_start));
        else {
            /* Slide messages up */
            memmove(move_start - gap_size, move_start, (size_t)(move_end - move_start));

            /* Adjust start of null message */
            mesg->raw -= gap_size;
        } /* end else */
    }
    else if (move_end == move_start && !null_before_gap) {
        /* Slide null message up */
        memmove(move_start - gap_size, move_start, mesg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh));

        /* Adjust start of null message */
        mesg->raw -= gap_size;
    } /* end if */

    /* Zero out addition to null message */
    memset(mesg->raw + mesg->raw_size, 0, gap_size);

    /* Adjust size of null message */
    mesg->raw_size += gap_size;

    /* Set the gap size to zero for the chunk */
    oh->chunk[mesg->chunkno].gap = 0;

    /* Mark null message as dirty */
    mesg->dirty  = TRUE;
    *chk_dirtied = TRUE;

    FUNC_LEAVE_NOAPI(SUCCEED)
} /* H5O__eliminate_gap() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__alloc_null
 *
 * Purpose:     Allocate room for a new message from a null message
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static herr_t
H5O__alloc_null(H5F_t *f, H5O_t *oh, size_t null_idx, const H5O_msg_class_t *new_type, void *new_native,
                size_t new_size)
{
    H5O_chunk_proxy_t *chk_proxy   = NULL;  /* Chunk that message is in */
    hbool_t            chk_dirtied = FALSE; /* Flags for unprotecting chunk */
    H5O_mesg_t        *alloc_msg;           /* Pointer to null message to allocate out of */
    herr_t             ret_value = SUCCEED; /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh);
    assert(new_type);
    assert(new_size);

    /* Point to null message to allocate out of */
    alloc_msg = &oh->mesg[null_idx];

    /* Protect chunk */
    if (NULL == (chk_proxy = H5O__chunk_protect(f, oh, alloc_msg->chunkno)))
        HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to load object header chunk");

    /* Check if there's a need to split the null message */
    if (alloc_msg->raw_size > new_size) {
        /* Check for producing a gap in the chunk */
        if ((alloc_msg->raw_size - new_size) < (size_t)H5O_SIZEOF_MSGHDR_OH(oh)) {
            size_t gap_size = alloc_msg->raw_size - new_size; /* Size of gap produced */

            /* Adjust the size of the null message being eliminated */
            alloc_msg->raw_size = new_size;

            /* Add the gap to the chunk */
            if (H5O__add_gap(f, oh, alloc_msg->chunkno, &chk_dirtied, null_idx,
                             alloc_msg->raw + alloc_msg->raw_size, gap_size) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, FAIL, "can't insert gap in chunk");
        } /* end if */
        else {
            size_t new_mesg_size =
                new_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh); /* Total size of newly allocated message */
            H5O_mesg_t *null_msg;                            /* Pointer to new null message */

            /* Check if we need to extend message table to hold the new null message */
            if (oh->nmesgs >= oh->alloc_nmesgs) {
                if (H5O__alloc_msgs(oh, (size_t)1) < 0)
                    HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "can't allocate more space for messages");

                /* "Retarget" 'alloc_msg' pointer into newly re-allocated array of messages */
                alloc_msg = &oh->mesg[null_idx];
            } /* end if */

            /* Create new null message, with the tail of the previous null message */
            null_msg           = &(oh->mesg[oh->nmesgs++]);
            null_msg->type     = H5O_MSG_NULL;
            null_msg->native   = NULL;
            null_msg->raw      = alloc_msg->raw + new_mesg_size;
            null_msg->raw_size = alloc_msg->raw_size - new_mesg_size;
            null_msg->chunkno  = alloc_msg->chunkno;

            /* Mark the message as dirty */
            null_msg->dirty = TRUE;
            chk_dirtied     = TRUE;

            /* Check for gap in new null message's chunk */
            if (oh->chunk[null_msg->chunkno].gap > 0) {
                unsigned null_chunkno = null_msg->chunkno; /* Chunk w/gap */

                /* Eliminate the gap in the chunk */
                if (H5O__eliminate_gap(oh, &chk_dirtied, null_msg,
                                       ((oh->chunk[null_chunkno].image + oh->chunk[null_chunkno].size) -
                                        (H5O_SIZEOF_CHKSUM_OH(oh) + oh->chunk[null_chunkno].gap)),
                                       oh->chunk[null_chunkno].gap) < 0)
                    HGOTO_ERROR(H5E_OHDR, H5E_CANTREMOVE, FAIL, "can't eliminate gap in chunk");
            } /* end if */

            /* Set the size of the new "real" message */
            alloc_msg->raw_size = new_size;
        } /* end else */
    }     /* end if */

    /* Initialize the new message */
    alloc_msg->type   = new_type;
    alloc_msg->native = new_native;

    /* Mark the new message as dirty */
    alloc_msg->dirty = TRUE;
    chk_dirtied      = TRUE;

done:
    /* Release chunk */
    if (chk_proxy && H5O__chunk_unprotect(f, chk_proxy, chk_dirtied) < 0)
        HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");

    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__alloc_null() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__alloc_msgs
 *
 * Purpose:     Allocate more messages for a header
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5O__alloc_msgs(H5O_t *oh, size_t min_alloc)
{
    size_t      old_alloc;           /* Old number of messages allocated */
    size_t      na;                  /* New number of messages allocated */
    H5O_mesg_t *new_mesg;            /* Pointer to new message array */
    herr_t      ret_value = SUCCEED; /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh);

    /* Initialize number of messages information */
    old_alloc = oh->alloc_nmesgs;
    na        = oh->alloc_nmesgs + MAX(oh->alloc_nmesgs, min_alloc); /* At least double */

    /* Attempt to allocate more memory */
    if (NULL == (new_mesg = H5FL_SEQ_REALLOC(H5O_mesg_t, oh->mesg, na)))
        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed");

    /* Update ohdr information */
    oh->alloc_nmesgs = na;
    oh->mesg         = new_mesg;

    /* Set new object header info to zeros */
    memset(&oh->mesg[old_alloc], 0, (oh->alloc_nmesgs - old_alloc) * sizeof(H5O_mesg_t));

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__alloc_msgs() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__alloc_extend_chunk
 *
 * Purpose:     Attempt to extend a chunk that is allocated on disk.
 *
 *              If the extension is successful, and if the last message
 *		of the chunk is the null message, then that message will
 *		be extended with the chunk.  Otherwise a new null message
 *		is created.
 *
 *              f is the file in which the chunk will be written.  It is
 *              included to ensure that there is enough space to extend
 *              this chunk.
 *
 * Return:      TRUE:		The chunk has been extended, and *msg_idx
 *				contains the message index for null message
 *				which is large enough to hold size bytes.
 *
 *		FALSE:		The chunk cannot be extended, and *msg_idx
 *				is undefined.
 *
 *		FAIL:		Some internal error has been detected.
 *
 *-------------------------------------------------------------------------
 */
static htri_t
H5O__alloc_extend_chunk(H5F_t *f, H5O_t *oh, unsigned chunkno, size_t size, size_t *msg_idx)
{
    H5O_chunk_proxy_t *chk_proxy   = NULL;  /* Chunk that message is in */
    hbool_t            chk_dirtied = FALSE; /* Flag for unprotecting chunk */
    size_t             delta;               /* Change in chunk's size */
    size_t             aligned_size = H5O_ALIGN_OH(oh, size);
    uint8_t           *old_image;                 /* Old address of chunk's image in memory */
    size_t             old_size;                  /* Old size of chunk */
    htri_t             was_extended;              /* If chunk can be extended */
    size_t             extend_msg        = 0;     /* Index of null message to extend */
    hbool_t            extended_msg      = FALSE; /* Whether an existing message was extended */
    uint8_t            new_size_flags    = 0;     /* New chunk #0 size flags */
    hbool_t            adjust_size_flags = FALSE; /* Whether to adjust the chunk #0 size flags */
    size_t             extra_prfx_size   = 0;     /* Extra bytes added to object header prefix */
    size_t             u;                         /* Local index variable */
    htri_t             ret_value = TRUE;          /* return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(f != NULL);
    assert(oh != NULL);
    assert(chunkno < oh->nchunks);
    assert(size > 0);
    assert(msg_idx != NULL);
    assert(H5_addr_defined(oh->chunk[chunkno].addr));

    /* Test to see if the specified chunk ends with a null messages.
     * If successful, set the index of the null message in extend_msg.
     */
    for (u = 0; u < oh->nmesgs; u++) {
        /* Check for null message at end of proper chunk */
        /* (account for possible checksum at end of chunk) */
        if (oh->mesg[u].chunkno == chunkno && H5O_NULL_ID == oh->mesg[u].type->id &&
            ((oh->mesg[u].raw + oh->mesg[u].raw_size) ==
             ((oh->chunk[chunkno].image + oh->chunk[chunkno].size) -
              (oh->chunk[chunkno].gap + H5O_SIZEOF_CHKSUM_OH(oh))))) {

            extend_msg   = u;
            extended_msg = TRUE;
            break;
        } /* end if */
    }     /* end for */

    /* If we can extend an existing null message, adjust the delta appropriately */
    if (extended_msg) {
        assert(oh->chunk[chunkno].gap == 0);
        delta = aligned_size - oh->mesg[extend_msg].raw_size;
    } /* end if */
    else
        delta = (aligned_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh)) - oh->chunk[chunkno].gap;
    delta = H5O_ALIGN_OH(oh, delta);

    /* Check for changing the chunk #0 data size enough to need adjusting the flags */
    if (oh->version > H5O_VERSION_1 && chunkno == 0) {
        uint64_t chunk0_size;                                                     /* Size of chunk 0's data */
        size_t   orig_prfx_size = (size_t)1 << (oh->flags & H5O_HDR_CHUNK0_SIZE); /* Original prefix size */

        assert(oh->chunk[0].size >= (size_t)H5O_SIZEOF_HDR(oh));
        chunk0_size = oh->chunk[0].size - (size_t)H5O_SIZEOF_HDR(oh);

        /* Check for moving to a 8-byte size encoding */
        if (orig_prfx_size < 8 && (chunk0_size + delta) > 4294967295) {
            extra_prfx_size   = 8 - orig_prfx_size;
            new_size_flags    = H5O_HDR_CHUNK0_8;
            adjust_size_flags = TRUE;
        } /* end if */
        /* Check for moving to a 4-byte size encoding */
        else if (orig_prfx_size < 4 && (chunk0_size + delta) > 65535) {
            extra_prfx_size   = 4 - orig_prfx_size;
            new_size_flags    = H5O_HDR_CHUNK0_4;
            adjust_size_flags = TRUE;
        } /* end if */
        /* Check for moving to a 2-byte size encoding */
        else if (orig_prfx_size < 2 && (chunk0_size + delta) > 255) {
            extra_prfx_size   = 2 - orig_prfx_size;
            new_size_flags    = H5O_HDR_CHUNK0_2;
            adjust_size_flags = TRUE;
        } /* end if */
    }     /* end if */

    /* Protect chunk */
    if (NULL == (chk_proxy = H5O__chunk_protect(f, oh, chunkno)))
        HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to load object header chunk");

    /* Determine whether the chunk can be extended */
    was_extended = H5MF_try_extend(f, H5FD_MEM_OHDR, oh->chunk[chunkno].addr,
                                   (hsize_t)(oh->chunk[chunkno].size), (hsize_t)(delta + extra_prfx_size));
    if (was_extended < 0) /* error */
        HGOTO_ERROR(H5E_OHDR, H5E_CANTEXTEND, FAIL, "can't tell if we can extend chunk");
    else if (was_extended == FALSE) /* can't extend -- we are done */
        HGOTO_DONE(FALSE);

    /* Adjust object header prefix flags */
    if (adjust_size_flags) {
        oh->flags = (uint8_t)(oh->flags & ~H5O_HDR_CHUNK0_SIZE);
        oh->flags |= new_size_flags;

        /* Mark object header as dirty in cache */
        if (H5AC_mark_entry_dirty(oh) < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_CANTMARKDIRTY, FAIL, "unable to mark object header as dirty");
    } /* end if */

    /* If we can extend an existing null message, take care of that */
    if (extended_msg) {
        /* Adjust message size of existing null message */
        oh->mesg[extend_msg].raw_size += delta;
    } /* end if */
    /* Create new null message for end of chunk */
    else {
        /* Create a new null message */
        if (oh->nmesgs >= oh->alloc_nmesgs)
            if (H5O__alloc_msgs(oh, (size_t)1) < 0)
                HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "can't allocate more space for messages");

        /* Set extension message */
        extend_msg = oh->nmesgs++;

        /* Initialize new null message */
        oh->mesg[extend_msg].type   = H5O_MSG_NULL;
        oh->mesg[extend_msg].native = NULL;
        oh->mesg[extend_msg].raw    = ((oh->chunk[chunkno].image + oh->chunk[chunkno].size) -
                                    (H5O_SIZEOF_CHKSUM_OH(oh) + oh->chunk[chunkno].gap)) +
                                   H5O_SIZEOF_MSGHDR_OH(oh);
        oh->mesg[extend_msg].raw_size = (delta + oh->chunk[chunkno].gap) - (size_t)H5O_SIZEOF_MSGHDR_OH(oh);
        oh->mesg[extend_msg].chunkno  = chunkno;
    } /* end else */

    /* Mark the extended message as dirty */
    oh->mesg[extend_msg].dirty = TRUE;
    chk_dirtied                = TRUE;

    /* Allocate more memory space for chunk's image */
    old_image = oh->chunk[chunkno].image;
    old_size  = oh->chunk[chunkno].size;
    oh->chunk[chunkno].size += delta + extra_prfx_size;
    oh->chunk[chunkno].image = H5FL_BLK_REALLOC(chunk_image, old_image, oh->chunk[chunkno].size);
    if (NULL == oh->chunk[chunkno].image)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTALLOC, FAIL, "can't reallocate extended object header chunk");
    oh->chunk[chunkno].gap = 0;

    /* Wipe new space for chunk */
    memset(oh->chunk[chunkno].image + old_size, 0, oh->chunk[chunkno].size - old_size);

    /* Move chunk 0 data up if the size flags changed */
    if (adjust_size_flags)
        memmove(oh->chunk[0].image + H5O_SIZEOF_HDR(oh) - H5O_SIZEOF_CHKSUM_OH(oh),
                oh->chunk[0].image + H5O_SIZEOF_HDR(oh) - H5O_SIZEOF_CHKSUM_OH(oh) - extra_prfx_size,
                old_size - (size_t)H5O_SIZEOF_HDR(oh) + extra_prfx_size);

    /* Spin through existing messages, adjusting them */
    for (u = 0; u < oh->nmesgs; u++) {
        /* Adjust raw addresses for messages in this chunk to reflect new 'image' address */
        if (oh->mesg[u].chunkno == chunkno)
            oh->mesg[u].raw = oh->chunk[chunkno].image + extra_prfx_size + (oh->mesg[u].raw - old_image);

        /* Find continuation message which points to this chunk and adjust chunk's size */
        /* (Chunk 0 doesn't have a continuation message that points to it,
         * its size is directly encoded in the object header) */
        if (chunkno > 0 && (H5O_CONT_ID == oh->mesg[u].type->id) &&
            (((H5O_cont_t *)(oh->mesg[u].native))->chunkno == chunkno)) {
            H5O_chunk_proxy_t *chk_proxy2   = NULL;                /* Chunk that continuation message is in */
            hbool_t            chk_dirtied2 = FALSE;               /* Flag for unprotecting chunk */
            unsigned           cont_chunkno = oh->mesg[u].chunkno; /* Chunk # for continuation message */

            /* Protect chunk containing continuation message */
            if (NULL == (chk_proxy2 = H5O__chunk_protect(f, oh, cont_chunkno)))
                HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to load object header chunk");

            /* Adjust size in continuation message */
            assert(((H5O_cont_t *)(oh->mesg[u].native))->size == old_size);
            ((H5O_cont_t *)(oh->mesg[u].native))->size = oh->chunk[chunkno].size;

            /* Flag continuation message as dirty */
            oh->mesg[u].dirty = TRUE;
            chk_dirtied2      = TRUE;

            /* Release chunk containing continuation message */
            if (H5O__chunk_unprotect(f, chk_proxy2, chk_dirtied2) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");
        } /* end if */
    }     /* end for */

    /* Resize the chunk in the cache */
    if (H5O__chunk_resize(oh, chk_proxy) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTRESIZE, FAIL, "unable to resize object header chunk");

    /* Set new message index */
    *msg_idx = extend_msg;

done:
    /* Release chunk */
    if (chk_proxy && H5O__chunk_unprotect(f, chk_proxy, chk_dirtied) < 0)
        HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");

    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__alloc_extend_chunk() */

/*-------------------------------------------------------------------------
 * Function:    H5O__alloc_find_best_nonnull
 *
 * Purpose:     Find the best fit non-null message for a given size of message
 *              to allocate.
 *
 * Note:	The algorithm for finding a message to replace with a
 *		continuation message is still fairly limited.  It's possible
 *		that two (or more) messages smaller than a continuation message
 *		might occupy a chunk and need to be moved in order to make
 *		room for the continuation message.
 *
 *		Also, we aren't checking for NULL messages in front of another
 *		message right now...
 *
 * Return:      Success:        Index number of the null message for the
 *                              new chunk.  The null message will be at
 *                              least SIZE bytes not counting the message
 *                              ID or size fields.
 *
 *              Failure:        Negative
 *
 *-------------------------------------------------------------------------
 */
static herr_t
H5O__alloc_find_best_nonnull(const H5F_t *f, const H5O_t *oh, size_t *size, H5O_msg_alloc_info_t *found_msg)
{
    H5O_mesg_t *curr_msg;   /* Pointer to current message to operate on */
    size_t      cont_size;  /* Continuation message size */
    size_t      multi_size; /* Size of all the messages in the last chunk */
    unsigned    u;          /* Local index variable */

    FUNC_ENTER_PACKAGE_NOERR

    /* Check args */
    assert(f);
    assert(oh);
    assert(size);
    assert(*size > 0);
    assert(found_msg);

    /*
     * Find the smallest message that could be moved to make room for the
     * continuation message.
     *
     * Don't ever move continuation message from one chunk to another.
     *
     * Avoid moving attributes when possible to preserve their
     * ordering (although ordering is *not* guaranteed!).
     *
     */
    cont_size  = H5O_ALIGN_OH(oh, (size_t)(H5F_SIZEOF_ADDR(f) + H5F_SIZEOF_SIZE(f)));
    multi_size = 0;
    for (u = 0, curr_msg = &oh->mesg[0]; u < oh->nmesgs; u++, curr_msg++) {
        /* Don't consider continuation messages (for now) */
        if (H5O_CONT_ID != curr_msg->type->id) {
            unsigned msg_chunkno = curr_msg->chunkno; /* Chunk that the message is in */
            uint8_t *end_chunk_data =
                (oh->chunk[msg_chunkno].image + oh->chunk[msg_chunkno].size) -
                (H5O_SIZEOF_CHKSUM_OH(oh) + oh->chunk[msg_chunkno].gap); /* End of message data in chunk */
            uint8_t *end_msg    = curr_msg->raw + curr_msg->raw_size;    /* End of current message */
            size_t   gap_size   = 0; /* Size of gap after current message */
            size_t   null_size  = 0; /* Size of NULL message after current message */
            unsigned null_msgno = 0; /* Index of NULL message after current message */
            size_t   total_size;     /* Total size of available space "around" current message */

            /* Check if the message is the last one in the chunk */
            if (end_msg == end_chunk_data)
                gap_size = oh->chunk[msg_chunkno].gap;
            else {
                H5O_mesg_t *tmp_msg; /* Temp. pointer to message to operate on */
                unsigned    v;       /* Local index variable */

                /* Check for null message after this message, in same chunk */
                for (v = 0, tmp_msg = &oh->mesg[0]; v < oh->nmesgs; v++, tmp_msg++) {
                    if (tmp_msg->type->id == H5O_NULL_ID &&
                        (tmp_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh)) == end_msg) {
                        null_msgno = v;
                        null_size  = (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + tmp_msg->raw_size;
                        break;
                    } /* end if */

                    /* XXX: Should also check for NULL message in front of current message... */

                } /* end for */
            }     /* end else */

            /* Add up current message's total available space */
            total_size = curr_msg->raw_size + gap_size + null_size;

            /* Check if message is large enough to hold continuation info */
            if (total_size >= cont_size) {
                hbool_t better = FALSE; /* Whether the current message is better than a previous one */

                /* Check for first message that can be moved */
                if (found_msg->msgno < 0)
                    better = TRUE;
                else {
                    /* Prioritize moving non-attributes above attributes */
                    /* (Even attributes with an otherwise better fit */
                    if (found_msg->id == H5O_ATTR_ID && curr_msg->type->id != H5O_ATTR_ID)
                        better = TRUE;
                    /* Either two attributes, or two non-attributes */
                    else {
                        /* Take a smaller one */
                        if (total_size < found_msg->total_size)
                            better = TRUE;
                        /* If they are the same size, choose the earliest one
                         * in the chunk array */
                        /* (Could also bias toward message earlier / later
                         *      chunk in, but shouldn't be a big deal - QAK, 2016/10/21)
                         */
                        else if (total_size == found_msg->total_size) {
                            if (msg_chunkno < found_msg->chunkno)
                                better = TRUE;
                        } /* end else-if */
                    }     /* end else */
                }         /* end else */

                /* If we found a better message, keep its info */
                if (better) {
                    found_msg->msgno      = (int)u;
                    found_msg->id         = curr_msg->type->id;
                    found_msg->chunkno    = msg_chunkno;
                    found_msg->gap_size   = gap_size;
                    found_msg->null_size  = null_size;
                    found_msg->total_size = total_size;
                    found_msg->null_msgno = null_msgno;
                } /* end if */
            }     /* end if */
            else if (found_msg->msgno < 0 && msg_chunkno == oh->nchunks - 1)
                /* Keep track of the total size of smaller messages in the last
                 * chunk, in case we need to move more than 1 message.
                 */
                multi_size += curr_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh);
        } /* end if */
    }     /* end for */

    /*
     * If we must move some other message to make room for the null
     * message, then make sure the new chunk has enough room for that
     * other message.
     *
     * Move other messages first, and attributes only as a last resort.
     *
     * If all else fails, move every message in the last chunk.
     *
     */
    if (found_msg->msgno < 0)
        *size += multi_size;
    else
        *size += (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + oh->mesg[found_msg->msgno].raw_size;

    FUNC_LEAVE_NOAPI(SUCCEED)
} /* H5O__alloc_find_best_nonnull() */

/*-------------------------------------------------------------------------
 * Function:    H5O__alloc_chunk
 *
 * Purpose:     Allocates and initializes a new chunk for the object header,
 *		including file space.
 *
 * Return:      Success:        SUCCEED, with chunk number for the
 *                              new chunk and a pointer to the location in its
 *				image where the first message should be placed.
 *
 *              Failure:        Negative
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5O__alloc_chunk(H5F_t *f, H5O_t *oh, size_t size, size_t found_null, const H5O_msg_alloc_info_t *found_msg,
                 size_t *new_idx)
{
    H5O_mesg_t        *curr_msg;            /* Pointer to current message to operate on */
    H5O_chunk_proxy_t *chk_proxy;           /* Chunk that message is in */
    size_t             cont_size;           /*continuation message size     */
    size_t             idx;                 /* Message number */
    uint8_t           *p    = NULL;         /* Pointer into new chunk image */
    H5O_cont_t        *cont = NULL;         /*native continuation message   */
    unsigned           chunkno;             /* Chunk allocated */
    haddr_t            new_chunk_addr;      /* Address of new chunk in file */
    unsigned           u;                   /* Local index variable */
    herr_t             ret_value = SUCCEED; /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(f);
    assert(oh);
    assert(found_msg);
    assert(new_idx);

    /*
     * The total chunk size must include the requested space plus enough
     * for the message header.  This must be at least some minimum and
     * aligned properly.
     */
    size = MAX(H5O_MIN_SIZE, size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh));
    assert(size == H5O_ALIGN_OH(oh, size));

    /*
     * The total chunk size must include enough space for the checksum
     * on the chunk and the continuation chunk magic #. (which are only present
     * in later versions of the object header)
     */
    size += H5O_SIZEOF_CHKHDR_OH(oh);

    /* Allocate space in file to hold the new chunk */
    new_chunk_addr = H5MF_alloc(f, H5FD_MEM_OHDR, (hsize_t)size);
    if (!H5_addr_defined(new_chunk_addr))
        HGOTO_ERROR(H5E_OHDR, H5E_CANTALLOC, FAIL, "unable to allocate space for new chunk");

    /* Create the new chunk giving it a file address. */
    if (oh->nchunks >= oh->alloc_nchunks) {
        size_t       na = MAX(H5O_NCHUNKS, oh->alloc_nchunks * 2); /* Double # of chunks allocated */
        H5O_chunk_t *x;

        if (NULL == (x = H5FL_SEQ_REALLOC(H5O_chunk_t, oh->chunk, na)))
            HGOTO_ERROR(H5E_OHDR, H5E_CANTALLOC, FAIL, "can't allocate larger chunk array, na = %zu", na);
        oh->alloc_nchunks = na;
        oh->chunk         = x;
    } /* end if */

    H5_CHECKED_ASSIGN(chunkno, unsigned, oh->nchunks, size_t);
    oh->nchunks++;
    oh->chunk[chunkno].addr = new_chunk_addr;
    oh->chunk[chunkno].size = size;
    oh->chunk[chunkno].gap  = 0;
    if (NULL == (oh->chunk[chunkno].image = p = H5FL_BLK_CALLOC(chunk_image, size)))
        HGOTO_ERROR(H5E_OHDR, H5E_CANTALLOC, FAIL, "can't allocate image for chunk, size = %zu", size);
    oh->chunk[chunkno].chunk_proxy = NULL;

    /* If this is a later version of the object header format, put the magic
     *  # at the beginning of the chunk image.
     */
    if (oh->version > H5O_VERSION_1) {
        H5MM_memcpy(p, H5O_CHK_MAGIC, (size_t)H5_SIZEOF_MAGIC);
        p += H5_SIZEOF_MAGIC;
    } /* end if */

    /*
     * Make sure we have enough space for all possible new messages
     * that could be generated below.
     */
    if (oh->nmesgs + 3 > oh->alloc_nmesgs)
        if (H5O__alloc_msgs(oh, (size_t)3) < 0)
            HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "can't allocate more space for messages");

    /* Check if we need to move multiple messages, in order to make room for the new message */
    cont_size = H5O_ALIGN_OH(oh, (size_t)(H5F_SIZEOF_ADDR(f) + H5F_SIZEOF_SIZE(f)));
    if (found_null >= oh->nmesgs) {
        if (found_msg->msgno < 0) {
            /* Move all non-null messages in the last chunk to the new chunk.  This
             * should be extremely rare so we don't care too much about minimizing
             * the space used.
             */
            H5O_mesg_t *null_msg; /* Pointer to new null message */

            /* Protect last chunk */
            if (NULL == (chk_proxy = H5O__chunk_protect(f, oh, chunkno - 1)))
                HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to load object header chunk");

            /* Copy each message to the new location */
            for (u = 0, curr_msg = &oh->mesg[0]; u < oh->nmesgs; u++, curr_msg++)
                if (curr_msg->chunkno == chunkno - 1) {
                    if (curr_msg->type->id == H5O_NULL_ID) {
                        /* Delete the null message */
                        if (u < oh->nmesgs - 1)
                            memmove(curr_msg, curr_msg + 1, ((oh->nmesgs - 1) - u) * sizeof(H5O_mesg_t));
                        oh->nmesgs--;
                    } /* end if */
                    else {
                        assert(curr_msg->type->id != H5O_CONT_ID);

                        /* Copy the raw data */
                        H5MM_memcpy(p, curr_msg->raw - (size_t)H5O_SIZEOF_MSGHDR_OH(oh),
                                    curr_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh));

                        /* Update the message info */
                        curr_msg->chunkno = chunkno;
                        curr_msg->raw     = p + H5O_SIZEOF_MSGHDR_OH(oh);

                        /* Account for copied message in new chunk */
                        p += (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + curr_msg->raw_size;
                        size -= (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + curr_msg->raw_size;
                    } /* end else */
                }     /* end if */

            /* Create a null message spanning the entire last chunk */
            found_null       = oh->nmesgs++;
            null_msg         = &(oh->mesg[found_null]);
            null_msg->type   = H5O_MSG_NULL;
            null_msg->dirty  = TRUE;
            null_msg->native = NULL;
            null_msg->raw    = oh->chunk[chunkno - 1].image +
                            ((chunkno == 1) ? H5O_SIZEOF_HDR(oh) : H5O_SIZEOF_CHKHDR_OH(oh)) -
                            H5O_SIZEOF_CHKSUM_OH(oh) + H5O_SIZEOF_MSGHDR_OH(oh);
            null_msg->raw_size =
                oh->chunk[chunkno - 1].size -
                ((chunkno == 1) ? (size_t)H5O_SIZEOF_HDR(oh) : (size_t)H5O_SIZEOF_CHKHDR_OH(oh)) -
                (size_t)H5O_SIZEOF_MSGHDR_OH(oh);
            null_msg->chunkno = chunkno - 1;

            assert(null_msg->raw_size >= cont_size);

            /* Remove any gap in the chunk */
            oh->chunk[chunkno - 1].gap = 0;

            /* Release chunk, marking it dirty */
            if (H5O__chunk_unprotect(f, chk_proxy, TRUE) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");
        } /* end if */
        else {
            /* Move message (that will be replaced with continuation message)
             *  to new chunk, if necessary.
             */
            H5O_mesg_t *null_msg; /* Pointer to new null message */

            /* Protect chunk */
            if (NULL == (chk_proxy = H5O__chunk_protect(f, oh, oh->mesg[found_msg->msgno].chunkno)))
                HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to load object header chunk");

            /* Create null message for space that message to copy currently occupies */
            found_null         = oh->nmesgs++;
            null_msg           = &(oh->mesg[found_null]);
            null_msg->type     = H5O_MSG_NULL;
            null_msg->native   = NULL;
            null_msg->raw      = oh->mesg[found_msg->msgno].raw;
            null_msg->raw_size = oh->mesg[found_msg->msgno].raw_size;
            null_msg->chunkno  = oh->mesg[found_msg->msgno].chunkno;

            /* Copy the message to move (& its prefix) to its new location */
            H5MM_memcpy(p, oh->mesg[found_msg->msgno].raw - H5O_SIZEOF_MSGHDR_OH(oh),
                        oh->mesg[found_msg->msgno].raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh));

            /* Switch moved message to point to new location */
            oh->mesg[found_msg->msgno].raw     = p + H5O_SIZEOF_MSGHDR_OH(oh);
            oh->mesg[found_msg->msgno].chunkno = chunkno;

            /* Account for copied message in new chunk */
            p += (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + oh->mesg[found_msg->msgno].raw_size;
            size -= (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + oh->mesg[found_msg->msgno].raw_size;

            /* Add any available space after the message to move to the new null message */
            if (found_msg->gap_size > 0) {
                /* Absorb a gap after the moved message */
                assert(oh->chunk[null_msg->chunkno].gap == found_msg->gap_size);
                null_msg->raw_size += found_msg->gap_size;
                oh->chunk[null_msg->chunkno].gap = 0;
            } /* end if */
            else if (found_msg->null_size > 0) {
                H5O_mesg_t *old_null_msg =
                    &oh->mesg[found_msg->null_msgno]; /* Pointer to NULL message to eliminate */

                /* Absorb a null message after the moved message */
                assert((null_msg->raw + null_msg->raw_size) ==
                       (old_null_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh)));
                null_msg->raw_size += found_msg->null_size;

                /* Release any information/memory for message */
                H5O__msg_free_mesg(old_null_msg);

                /* Remove null message from list of messages */
                if (found_msg->null_msgno < (oh->nmesgs - 1))
                    memmove(old_null_msg, old_null_msg + 1,
                            ((oh->nmesgs - 1) - found_msg->null_msgno) * sizeof(H5O_mesg_t));

                /* Decrement # of messages */
                /* (Don't bother reducing size of message array for now -QAK) */
                oh->nmesgs--;

                /* Adjust message index for new NULL message */
                found_null--;
            } /* end if */

            /* Mark the new null message as dirty */
            null_msg->dirty = TRUE;

            /* Release chunk, marking it dirty */
            if (H5O__chunk_unprotect(f, chk_proxy, TRUE) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");
        } /* end if */
    }     /* end if */

    /* Create null message for [rest of] space in new chunk */
    /* (account for chunk's magic # & checksum) */
    idx                    = oh->nmesgs++;
    oh->mesg[idx].type     = H5O_MSG_NULL;
    oh->mesg[idx].dirty    = TRUE;
    oh->mesg[idx].native   = NULL;
    oh->mesg[idx].raw      = p + H5O_SIZEOF_MSGHDR_OH(oh);
    oh->mesg[idx].raw_size = size - (size_t)(H5O_SIZEOF_CHKHDR_OH(oh) + H5O_SIZEOF_MSGHDR_OH(oh));
    oh->mesg[idx].chunkno  = chunkno;

    /* Insert the new chunk into the cache */
    if (H5O__chunk_add(f, oh, chunkno, oh->mesg[found_null].chunkno) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, FAIL, "can't add new chunk to cache");

    /* Initialize the continuation information */
    if (NULL == (cont = H5FL_MALLOC(H5O_cont_t)))
        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed");
    cont->addr    = oh->chunk[chunkno].addr;
    cont->size    = oh->chunk[chunkno].size;
    cont->chunkno = chunkno;

    /* Split the null message and point at continuation message */
    if (H5O__alloc_null(f, oh, found_null, H5O_MSG_CONT, cont, cont_size) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, FAIL, "can't split null message");

    /* Set new message index value */
    *new_idx = idx;

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__alloc_chunk() */

/*-------------------------------------------------------------------------
 * Function:    H5O__alloc_new_chunk
 *
 * Purpose:     Allocates a new chunk for the object header, including
 *		file space.
 *
 *              One of the other chunks will get an object continuation
 *		message.  If there isn't room in any other chunk for the
 *		object continuation message, then some message from
 *		another chunk is moved into this chunk to make room.
 *
 *              SIZE need not be aligned.
 *
 * Note:	The algorithm for finding a message to replace with a
 *		continuation message is still fairly limited.  It's possible
 *		that two (or more) messages smaller than a continuation message
 *		might occupy a chunk and need to be moved in order to make
 *		room for the continuation message.
 *
 *		Also, we aren't checking for NULL messages in front of another
 *		message right now...
 *
 * Return:      Success:        Index number of the null message for the
 *                              new chunk.  The null message will be at
 *                              least SIZE bytes not counting the message
 *                              ID or size fields.
 *
 *              Failure:        Negative
 *
 *-------------------------------------------------------------------------
 */
static herr_t
H5O__alloc_new_chunk(H5F_t *f, H5O_t *oh, size_t size, size_t *new_idx)
{
    size_t               cont_size;           /*continuation message size     */
    size_t               idx;                 /* Message number */
    H5O_msg_alloc_info_t found_msg;           /* Best fit non-null message */
    herr_t               ret_value = SUCCEED; /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh);
    assert(size > 0);
    size = H5O_ALIGN_OH(oh, size);

    /* Find the smallest null message that could hold a continuation message */
    idx       = oh->nmesgs;
    cont_size = H5O_ALIGN_OH(oh, (size_t)(H5F_SIZEOF_ADDR(f) + H5F_SIZEOF_SIZE(f)));
    if (H5O__alloc_find_best_null(oh, cont_size, &idx) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_NOTFOUND, FAIL, "error while locating best null header message");

    /* If we couldn't find a null message, locate the best message to move to new chunk */
    if (idx >= oh->nmesgs) {
        found_msg.msgno = -1;
        if (H5O__alloc_find_best_nonnull(f, oh, &size, &found_msg) < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_NOTFOUND, FAIL, "error while locating best non-null header message");
    } /* end if */

    /* Allocate and initialize new chunk in the file */
    if (H5O__alloc_chunk(f, oh, size, idx, &found_msg, new_idx) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTALLOC, FAIL, "can't allocate new object header chunk");

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__alloc_new_chunk() */

/*-------------------------------------------------------------------------
 * Function:    H5O__alloc_find_best_null
 *
 * Purpose:     Find the best fit null message for a given size of message
 *              to allocate.
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static herr_t
H5O__alloc_find_best_null(const H5O_t *oh, size_t size, size_t *mesg_idx)
{
    size_t  idx;        /* Index of message which fits allocation */
    ssize_t found_null; /* Best fit null message         */

    FUNC_ENTER_PACKAGE_NOERR

    /* check args */
    assert(oh);
    assert(size > 0);
    assert(mesg_idx);

    /* Find the smallest null message that could hold the new object header message */
    found_null = -1;
    for (idx = 0; idx < oh->nmesgs; idx++) {
        if (H5O_NULL_ID == oh->mesg[idx].type->id) {
            /* If we found an exact fit, use it */
            if (oh->mesg[idx].raw_size == size) {
                /* Keep first exact fit */
                if (found_null < 0)
                    found_null = (ssize_t)idx;
                else
                    /* If we've got more than one exact fit, choose the one in the earliest chunk */
                    if (oh->mesg[idx].chunkno < oh->mesg[found_null].chunkno) {
                        found_null = (ssize_t)idx;

                        /* If we found an exact fit in object header chunk #0, we can get out */
                        /* (Could extend this to look for earliest message in
                         *      chunk #0 - QAK, 2016/10/21)
                         */
                        if (0 == oh->mesg[idx].chunkno)
                            break;
                    } /* end if */
            }         /* end if */
            /* Look for null message that's larger than needed */
            else if (oh->mesg[idx].raw_size > size) {
                /* Keep first one found */
                if (found_null < 0)
                    found_null = (ssize_t)idx;
                /* Check for better fit */
                else if (oh->mesg[idx].raw_size < oh->mesg[found_null].raw_size) {
                    found_null = (ssize_t)idx;
                }
                else {
                    /* If they are the same size, choose the one in the earliest chunk */
                    if (oh->mesg[idx].raw_size == oh->mesg[found_null].raw_size) {
                        if (oh->mesg[idx].chunkno < oh->mesg[found_null].chunkno)
                            found_null = (ssize_t)idx;
                    } /* end if */
                }     /* end else */
            }         /* end else-if */
            /* else: Ignore too-small null messages */
        } /* end if */
    }     /* end for */
    if (found_null >= 0)
        *mesg_idx = (size_t)found_null;

    FUNC_LEAVE_NOAPI(SUCCEED)
} /* H5O__alloc_find_best_null() */

/*-------------------------------------------------------------------------
 * Function:    H5O__alloc
 *
 * Purpose:     Allocate enough space in the object header for this message.
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5O__alloc(H5F_t *f, H5O_t *oh, const H5O_msg_class_t *type, const void *mesg, size_t *mesg_idx)
{
    size_t raw_size;            /* Raw size of message */
    size_t aligned_size;        /* Size of message including alignment */
    size_t idx;                 /* Index of message which fits allocation */
    herr_t ret_value = SUCCEED; /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh);
    assert(type);
    assert(mesg);
    assert(mesg_idx);

    /* Compute the size needed to store the message in the object header */
    raw_size = (type->raw_size)(f, FALSE, mesg);
    if (0 == raw_size)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTINIT, FAIL, "can't compute object header message size");
    if (raw_size >= H5O_MESG_MAX_SIZE)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTINIT, FAIL, "object header message is too large");
    aligned_size = H5O_ALIGN_OH(oh, raw_size);

    /* Find the smallest null message that could hold the new object header message */
    idx = oh->nmesgs;
    if (H5O__alloc_find_best_null(oh, aligned_size, &idx) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_NOTFOUND, FAIL, "error while locating best null header message");

    /* if we didn't find one, then allocate more header space */
    if (idx >= oh->nmesgs) {
        unsigned chunkno;

        /* check to see if we can extend one of the chunks.  If we can,
         * do so.  Otherwise, we will have to allocate a new chunk.
         *
         * Note that in this new version of this function, all chunks
         * must have file space allocated to them.
         */
        for (chunkno = 0; chunkno < oh->nchunks; chunkno++) {
            htri_t tri_result; /* Status from attempting to extend chunk */

            if ((tri_result = H5O__alloc_extend_chunk(f, oh, chunkno, raw_size, &idx)) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTEXTEND, FAIL, "can't extend existing chunk");
            if (tri_result == TRUE)
                break;
        } /* end for */

        /* If we were not able to extend a chunk, create a new one */
        if (idx >= oh->nmesgs)
            if (H5O__alloc_new_chunk(f, oh, raw_size, &idx) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_NOSPACE, FAIL, "unable to create a new object header data chunk");
    } /* end if */
    assert(idx < oh->nmesgs);

    /* Split the null message and point at continuation message */
    if (H5O__alloc_null(f, oh, idx, type, NULL, aligned_size) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, FAIL, "can't split null message");

    /* Mark object header as dirty in cache */
    if (H5AC_mark_entry_dirty(oh) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTMARKDIRTY, FAIL, "unable to mark object header as dirty");

    /* Set message index value */
    *mesg_idx = idx;

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__alloc() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__release_mesg
 *
 * Purpose:     Convert a message into a null message
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5O__release_mesg(H5F_t *f, H5O_t *oh, H5O_mesg_t *mesg, hbool_t adj_link)
{
    H5O_chunk_proxy_t *chk_proxy   = NULL;    /* Chunk that message is in */
    hbool_t            chk_dirtied = FALSE;   /* Flag for unprotecting chunk */
    herr_t             ret_value   = SUCCEED; /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(f);
    assert(oh);
    assert(mesg);

    /* Check if we should operate on the message */
    if (adj_link)
        /* Free any space referred to in the file from this message */
        if (H5O__delete_mesg(f, oh, mesg) < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_CANTDELETE, FAIL,
                        "unable to delete file space for object header message");

    /* Protect chunk */
    if (NULL == (chk_proxy = H5O__chunk_protect(f, oh, mesg->chunkno)))
        HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to protect object header chunk");

    /* Free any native information */
    H5O__msg_free_mesg(mesg);

    /* Change message type to nil and zero it */
    mesg->type = H5O_MSG_NULL;
    assert(mesg->raw + mesg->raw_size <= (oh->chunk[mesg->chunkno].image + oh->chunk[mesg->chunkno].size) -
                                             (H5O_SIZEOF_CHKSUM_OH(oh) + oh->chunk[mesg->chunkno].gap));
    memset(mesg->raw, 0, mesg->raw_size);

    /* Clear message flags */
    mesg->flags = 0;

    /* Mark the message as modified */
    mesg->dirty = TRUE;
    chk_dirtied = TRUE;

    /* Check if chunk has a gap currently */
    if (oh->chunk[mesg->chunkno].gap) {
        /* Eliminate the gap in the chunk */
        if (H5O__eliminate_gap(oh, &chk_dirtied, mesg,
                               ((oh->chunk[mesg->chunkno].image + oh->chunk[mesg->chunkno].size) -
                                (H5O_SIZEOF_CHKSUM_OH(oh) + oh->chunk[mesg->chunkno].gap)),
                               oh->chunk[mesg->chunkno].gap) < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_CANTREMOVE, FAIL, "can't eliminate gap in chunk");
    } /* end if */

done:
    /* Release chunk, if not already done */
    if (chk_proxy && H5O__chunk_unprotect(f, chk_proxy, chk_dirtied) < 0)
        HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");

    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__release_mesg() */

/*-------------------------------------------------------------------------
 * Function:    H5O__move_cont
 *
 * Purpose:     Check and move message(s) forward into a continuation message
 *
 * Return:      Success:        non-negative (TRUE/FALSE)
 *              Failure:        negative
 *
 *-------------------------------------------------------------------------
 */
static htri_t
H5O__move_cont(H5F_t *f, H5O_t *oh, unsigned cont_u)
{
    H5O_chunk_proxy_t *chk_proxy = NULL;    /* Chunk that continuation message is in */
    H5O_mesg_t        *cont_msg;            /* Pointer to the continuation message */
    unsigned           deleted_chunkno;     /* Chunk # to delete */
    hbool_t            chk_dirtied = FALSE; /* Flags for unprotecting chunk */
    htri_t             ret_value   = TRUE;  /* Return value */

    FUNC_ENTER_PACKAGE

    /* Check arguments. */
    assert(f);
    assert(oh);

    /* Get initial information */
    cont_msg = &oh->mesg[cont_u];
    H5O_LOAD_NATIVE(f, 0, oh, cont_msg, FAIL)
    deleted_chunkno = ((H5O_cont_t *)(cont_msg->native))->chunkno;

    /* Check if continuation message is pointing to the last chunk */
    if (deleted_chunkno == (oh->nchunks - 1)) {
        size_t      nonnull_size; /* Total size of nonnull messages in the chunk pointed to by cont message */
        H5O_mesg_t *curr_msg;     /* Pointer to the current message to operate on */
        size_t      gap_size;     /* Size of gap produced */
        size_t      v;            /* Local index variable */

        /* Spin through messages */
        nonnull_size = 0;
        for (v = 0, curr_msg = &oh->mesg[0]; v < oh->nmesgs; v++, curr_msg++) {
            if (curr_msg->chunkno == deleted_chunkno) {
                /* Find size of all non-null messages in the chunk pointed to by the continuation message */
                if (curr_msg->type->id != H5O_NULL_ID) {
                    assert(curr_msg->type->id != H5O_CONT_ID);
                    nonnull_size += curr_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh);
                } /* end if */
            }     /* end if */
        }         /* end for */

        /* Size of gap in chunk w/continuation message */
        gap_size = oh->chunk[cont_msg->chunkno].gap;

        /* Check if messages can fit into the continuation message + gap size */
        /* (Could count any null messages in the chunk w/the continuation
         *      message also, but that is pretty complex. -QAK)
         */
        if (nonnull_size &&
            nonnull_size <= (gap_size + cont_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh))) {
            uint8_t *move_start, *move_end; /* Pointers to area of messages to move */
            unsigned cont_chunkno;          /* Chunk number for continuation message */

            /* Get continuation info */
            move_start   = cont_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh);
            move_end     = cont_msg->raw + cont_msg->raw_size;
            cont_chunkno = cont_msg->chunkno;

            /* Convert continuation message into a null message.  Do not delete
             * the target chunk yet, so we can still copy messages from it. */
            if (H5O__release_mesg(f, oh, cont_msg, FALSE) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTDELETE, FAIL, "unable to convert into null message");

            /* Protect chunk */
            if (NULL == (chk_proxy = H5O__chunk_protect(f, oh, cont_chunkno)))
                HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to protect object header chunk");

            /* Move message(s) forward into continuation message */
            for (v = 0, curr_msg = &oh->mesg[0]; v < oh->nmesgs; v++, curr_msg++)
                /* Look for messages in chunk to delete */
                if (curr_msg->chunkno == deleted_chunkno) {
                    /* Move messages out of chunk to delete */
                    if (curr_msg->type->id != H5O_NULL_ID) {
                        size_t move_size; /* Size of the message to be moved */

                        /* Compute size of message to move */
                        move_size = curr_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh);

                        /* Move message out of deleted chunk */
                        H5MM_memcpy(move_start, curr_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh), move_size);
                        curr_msg->raw     = move_start + H5O_SIZEOF_MSGHDR_OH(oh);
                        curr_msg->chunkno = cont_chunkno;
                        chk_dirtied       = TRUE;

                        /* Adjust location to move messages to */
                        move_start += move_size;
                    } /* end else */
                }     /* end if */

            /* Delete the target chunk */
            if (H5O__chunk_delete(f, oh, deleted_chunkno) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTDELETE, FAIL, "unable to remove chunk from cache");

            assert(move_start <= (move_end + gap_size));

            /* Check if there is space remaining in the continuation message */
            /* (The remaining space can be gap or a null message) */
            gap_size += (size_t)(move_end - move_start);
            if (gap_size >= (size_t)H5O_SIZEOF_MSGHDR_OH(oh)) {
                /* Adjust size of null (was continuation) message */
                cont_msg->raw_size = gap_size - (size_t)H5O_SIZEOF_MSGHDR_OH(oh);
                cont_msg->raw      = move_start + H5O_SIZEOF_MSGHDR_OH(oh);
                cont_msg->dirty    = TRUE;
                chk_dirtied        = TRUE;
            } /* end if */
            else {
                /* Check if there is space that should be a gap */
                if (gap_size > 0) {
                    /* Convert remnant into gap in chunk */
                    if (H5O__add_gap(f, oh, cont_chunkno, &chk_dirtied, cont_u, move_start, gap_size) < 0)
                        HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, FAIL, "can't insert gap in chunk");
                } /* end if */

                /* Release any information/memory for continuation message */
                H5O__msg_free_mesg(cont_msg);
                if (cont_u < (oh->nmesgs - 1))
                    memmove(&oh->mesg[cont_u], &oh->mesg[cont_u + 1],
                            ((oh->nmesgs - 1) - cont_u) * sizeof(H5O_mesg_t));
                oh->nmesgs--;
            } /* end else */

            /* Move message(s) forward into continuation message */
            /*      Note: unsigned v wrapping around at the end */
            for (v = oh->nmesgs - 1, curr_msg = &oh->mesg[v]; v < oh->nmesgs; v--, curr_msg--)
                /* Look for messages in chunk to delete */
                if (curr_msg->chunkno == deleted_chunkno) {
                    /* Remove all null messages in deleted chunk from list of messages */
                    if (curr_msg->type->id == H5O_NULL_ID) {
                        /* Release any information/memory for message */
                        H5O__msg_free_mesg(curr_msg);
                        chk_dirtied = TRUE;

                        /* Remove from message list */
                        if (v < (oh->nmesgs - 1))
                            memmove(&oh->mesg[v], &oh->mesg[v + 1],
                                    ((oh->nmesgs - 1) - v) * sizeof(H5O_mesg_t));
                        oh->nmesgs--;
                    } /* end if */
                }     /* end if */

            /* Remove chunk from list of chunks */
            oh->chunk[deleted_chunkno].image = H5FL_BLK_FREE(chunk_image, oh->chunk[deleted_chunkno].image);
            oh->nchunks--;
        } /* end if */
        else
            ret_value = FALSE;
    } /* end if */
    else
        ret_value = FALSE;

done:
    /* Release chunk, if not already done */
    if (chk_proxy && H5O__chunk_unprotect(f, chk_proxy, chk_dirtied) < 0)
        HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");

    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__move_cont() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__move_msgs_forward
 *
 * Purpose:     Move messages toward first chunk
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static htri_t
H5O__move_msgs_forward(H5F_t *f, H5O_t *oh)
{
    H5O_chunk_proxy_t *null_chk_proxy      = NULL;  /* Chunk that null message is in */
    H5O_chunk_proxy_t *curr_chk_proxy      = NULL;  /* Chunk that message is in */
    H5O_chunk_proxy_t *cont_targ_chk_proxy = NULL;  /* Chunk that continuation message points to */
    hbool_t            null_chk_dirtied    = FALSE; /* Flags for unprotecting null chunk */
    hbool_t            curr_chk_dirtied    = FALSE; /* Flags for unprotecting curr chunk */
    hbool_t            packed_msg;                  /* Flag to indicate that messages were packed */
    hbool_t            did_packing = FALSE;         /* Whether any messages were packed */
    htri_t             ret_value   = FAIL;          /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh);

    /* Loop until no messages packed */
    /* (Double loop is not very efficient, but it would be some extra work to
     *      add a list of messages to each chunk -QAK)
     */
    do {
        H5O_mesg_t *curr_msg; /* Pointer to current message to operate on */
        unsigned    u;        /* Local index variable */

        /* Reset packed messages flag */
        packed_msg = FALSE;

        /* Scan through messages for messages that can be moved earlier in chunks */
        for (u = 0, curr_msg = &oh->mesg[0]; u < oh->nmesgs; u++, curr_msg++) {
            if (H5O_NULL_ID == curr_msg->type->id) {
                H5O_chunk_t *chunk; /* Pointer to chunk that null message is in */

                /* Check if null message is not last in chunk */
                chunk = &(oh->chunk[curr_msg->chunkno]);
                if ((curr_msg->raw + curr_msg->raw_size) !=
                    ((chunk->image + chunk->size) - (H5O_SIZEOF_CHKSUM_OH(oh) + chunk->gap))) {
                    H5O_mesg_t *nonnull_msg; /* Pointer to current message to operate on */
                    unsigned    v;           /* Local index variable */

                    /* Loop over messages again, looking for the message in the chunk after the null message
                     */
                    for (v = 0, nonnull_msg = &oh->mesg[0]; v < oh->nmesgs; v++, nonnull_msg++) {
                        /* Locate message that is immediately after the null message */
                        if ((curr_msg->chunkno == nonnull_msg->chunkno) &&
                            ((curr_msg->raw + curr_msg->raw_size) ==
                             (nonnull_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh)))) {
                            /* Don't swap messages if the second message is also a null message */
                            /* (We'll merge them together later, in another routine) */
                            if (H5O_NULL_ID != nonnull_msg->type->id) {
                                /* Protect chunk */
                                if (NULL == (null_chk_proxy = H5O__chunk_protect(f, oh, curr_msg->chunkno)))
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL,
                                                "unable to load object header chunk");

                                /* Copy raw data for non-null message to new location */
                                memmove(curr_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh),
                                        nonnull_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh),
                                        nonnull_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh));

                                /* Adjust non-null message's offset in chunk */
                                nonnull_msg->raw = curr_msg->raw;

                                /* Adjust null message's offset in chunk */
                                curr_msg->raw =
                                    nonnull_msg->raw + nonnull_msg->raw_size + H5O_SIZEOF_MSGHDR_OH(oh);

                                /* Mark null message dirty */
                                /* (since we need to re-encode its message header) */
                                curr_msg->dirty = TRUE;

                                /* Release chunk, marking it dirty */
                                if (H5O__chunk_unprotect(f, null_chk_proxy, TRUE) < 0)
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                                                "unable to unprotect object header chunk");
                                null_chk_proxy = NULL;

                                /* Set the flag to indicate that the null message
                                 * was packed - if its not at the end its chunk,
                                 * we'll move it again on the next pass.
                                 */
                                packed_msg = TRUE;
                            } /* end if */

                            /* Break out of loop */
                            break;
                        } /* end if */
                    }     /* end for */
                    /* Should have been message after null message */
                    assert(v < oh->nmesgs);
                } /* end if */
            }     /* end if */
            else {
                H5O_mesg_t *null_msg; /* Pointer to current message to operate on */
                size_t      v;        /* Local index variable */

                /* Check if messages in chunk pointed to can replace continuation message */
                if (H5O_CONT_ID == curr_msg->type->id) {
                    htri_t status; /* Status from moving messages */

                    if ((status = H5O__move_cont(f, oh, u)) < 0)
                        HGOTO_ERROR(H5E_OHDR, H5E_CANTDELETE, FAIL,
                                    "Error in moving messages into cont message");
                    else if (status > 0) { /* Message(s) got moved into "continuation" message */
                        packed_msg = TRUE;
                        break;
                    } /* end else-if */
                }     /* end if */

                /* Loop over messages again, looking for large enough null message in earlier chunk */
                for (v = 0, null_msg = &oh->mesg[0]; v < oh->nmesgs; v++, null_msg++) {
                    if (H5O_NULL_ID == null_msg->type->id && curr_msg->chunkno > null_msg->chunkno &&
                        curr_msg->raw_size <= null_msg->raw_size) {
                        unsigned old_chunkno; /* Old message information */
                        uint8_t *old_raw;

                        /* Keep old information about non-null message */
                        old_chunkno = curr_msg->chunkno;
                        old_raw     = curr_msg->raw;

                        /* Protect chunks */
                        if (NULL == (null_chk_proxy = H5O__chunk_protect(f, oh, null_msg->chunkno)))
                            HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL,
                                        "unable to load object header chunk");
                        if (NULL == (curr_chk_proxy = H5O__chunk_protect(f, oh, curr_msg->chunkno)))
                            HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL,
                                        "unable to load object header chunk");

                        /* If the message being moved is a continuation
                         * message and we are doing SWMR writes, we must
                         * update the flush dependencies */
                        if (oh->swmr_write && (H5O_CONT_ID == curr_msg->type->id)) {
                            void *null_chk_mdc_obj; /* The metadata cache object for the null_msg chunk */

                            /* Point to the metadata cache object for the
                             * null message chunk, oh if in chunk 0 or the
                             * proxy otherwise */
                            null_chk_mdc_obj = (null_msg->chunkno == 0 ? (void *)oh : (void *)null_chk_proxy);

                            /* The other chunks involved should never be
                             * chunk 0 */
                            assert(curr_msg->chunkno > 0);
                            assert(((H5O_cont_t *)(curr_msg->native))->chunkno > 0);

                            /* Protect continuation message target chunk */
                            if (NULL == (cont_targ_chk_proxy = H5O__chunk_protect(
                                             f, oh, ((H5O_cont_t *)(curr_msg->native))->chunkno)))
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL,
                                            "unable to load object header chunk");

                            /* Check for flush dependency on previous continuation chunk */
                            /* (As opposed to chunk 0) */
                            if (cont_targ_chk_proxy->fd_parent) {
                                /* Remove flush dependency on old continuation
                                 * message chunk */
                                assert(cont_targ_chk_proxy);
                                assert(curr_chk_proxy);
                                assert((void *)curr_chk_proxy == cont_targ_chk_proxy->fd_parent);

                                if (H5AC_destroy_flush_dependency(curr_chk_proxy, cont_targ_chk_proxy) < 0)
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTUNDEPEND, FAIL,
                                                "unable to destroy flush dependency");

                                cont_targ_chk_proxy->fd_parent = NULL;
                            } /* end if */

                            /* Avoid (another) flush dependency on chunk 0 */
                            if (0 != null_msg->chunkno) {
                                /* Sanity checks */
                                assert(null_chk_mdc_obj);
                                assert(((H5C_cache_entry_t *)null_chk_mdc_obj)->type);
                                assert(((H5C_cache_entry_t *)null_chk_mdc_obj)->type->id == H5AC_OHDR_CHK_ID);

                                /* Create flush dependency on new continuation
                                 * message chunk */
                                if (H5AC_create_flush_dependency(null_chk_mdc_obj, cont_targ_chk_proxy) < 0)
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTDEPEND, FAIL,
                                                "unable to create flush dependency");

                                cont_targ_chk_proxy->fd_parent = null_chk_mdc_obj;
                            } /* end if */

                            /* Unprotect continuation message target chunk */
                            if (H5O__chunk_unprotect(f, cont_targ_chk_proxy, FALSE) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                                            "unable to unprotect object header chunk");
                            cont_targ_chk_proxy = NULL;
                        } /* end if */

                        /* Copy raw data for non-null message to new chunk */
                        H5MM_memcpy(null_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh),
                                    curr_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh),
                                    curr_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh));

                        /* Point non-null message at null message's space */
                        curr_msg->chunkno = null_msg->chunkno;
                        curr_msg->raw     = null_msg->raw;
                        curr_chk_dirtied  = TRUE;

                        /* Change information for null message */
                        if (curr_msg->raw_size == null_msg->raw_size) {
                            /* Point null message at old non-null space */
                            /* (Instead of freeing it and allocating new message) */
                            null_msg->chunkno = old_chunkno;
                            null_msg->raw     = old_raw;

                            /* Mark null message dirty */
                            null_msg->dirty  = TRUE;
                            null_chk_dirtied = TRUE;

                            /* Release current chunk, marking it dirty */
                            if (H5O__chunk_unprotect(f, curr_chk_proxy, curr_chk_dirtied) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                                            "unable to unprotect object header chunk");
                            curr_chk_proxy   = NULL;
                            curr_chk_dirtied = FALSE;

                            /* Check for gap in null message's chunk */
                            if (oh->chunk[old_chunkno].gap > 0) {
                                /* Eliminate the gap in the chunk */
                                if (H5O__eliminate_gap(
                                        oh, &null_chk_dirtied, null_msg,
                                        ((oh->chunk[old_chunkno].image + oh->chunk[old_chunkno].size) -
                                         (H5O_SIZEOF_CHKSUM_OH(oh) + oh->chunk[old_chunkno].gap)),
                                        oh->chunk[old_chunkno].gap) < 0)
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTREMOVE, FAIL,
                                                "can't eliminate gap in chunk");
                            } /* end if */

                            /* Release null chunk, marking it dirty */
                            if (H5O__chunk_unprotect(f, null_chk_proxy, null_chk_dirtied) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                                            "unable to unprotect object header chunk");
                            null_chk_proxy   = NULL;
                            null_chk_dirtied = FALSE;
                        } /* end if */
                        else {
                            size_t new_null_msg; /* Message index for new null message */

                            /* Check if null message is large enough to still exist */
                            if ((null_msg->raw_size - curr_msg->raw_size) <
                                (size_t)H5O_SIZEOF_MSGHDR_OH(oh)) {
                                size_t gap_size =
                                    null_msg->raw_size - curr_msg->raw_size; /* Size of gap produced */

                                /* Adjust the size of the null message being eliminated */
                                null_msg->raw_size = curr_msg->raw_size;

                                /* Mark null message dirty */
                                null_msg->dirty  = TRUE;
                                null_chk_dirtied = TRUE;

                                /* Add the gap to the chunk */
                                if (H5O__add_gap(f, oh, null_msg->chunkno, &null_chk_dirtied, v,
                                                 null_msg->raw + null_msg->raw_size, gap_size) < 0)
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, FAIL, "can't insert gap in chunk");

                                /* Re-use message # for new null message taking place of non-null message */
                                new_null_msg = v;
                            } /* end if */
                            else {
                                /* Adjust null message's size & offset */
                                null_msg->raw += curr_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh);
                                null_msg->raw_size -= curr_msg->raw_size + (size_t)H5O_SIZEOF_MSGHDR_OH(oh);

                                /* Mark null message dirty */
                                null_msg->dirty  = TRUE;
                                null_chk_dirtied = TRUE;

                                /* Create new null message for previous location of non-null message */
                                if (oh->nmesgs >= oh->alloc_nmesgs) {
                                    if (H5O__alloc_msgs(oh, (size_t)1) < 0)
                                        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL,
                                                    "can't allocate more space for messages");

                                    /* "Retarget" 'curr_msg' pointer into newly re-allocated array of messages
                                     */
                                    curr_msg = &oh->mesg[u];
                                } /* end if */

                                /* Get message # for new null message */
                                new_null_msg = oh->nmesgs++;
                            } /* end else */

                            /* Release null message's chunk, marking it dirty */
                            if (H5O__chunk_unprotect(f, null_chk_proxy, null_chk_dirtied) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                                            "unable to unprotect object header chunk");
                            null_chk_proxy   = NULL;
                            null_chk_dirtied = FALSE;

                            /* Initialize new null message to take over non-null message's location */
                            oh->mesg[new_null_msg].type     = H5O_MSG_NULL;
                            oh->mesg[new_null_msg].native   = NULL;
                            oh->mesg[new_null_msg].raw      = old_raw;
                            oh->mesg[new_null_msg].raw_size = curr_msg->raw_size;
                            oh->mesg[new_null_msg].chunkno  = old_chunkno;

                            /* Mark new null message dirty */
                            oh->mesg[new_null_msg].dirty = TRUE;
                            curr_chk_dirtied             = TRUE;

                            /* Check for gap in new null message's chunk */
                            if (oh->chunk[old_chunkno].gap > 0) {
                                /* Eliminate the gap in the chunk */
                                if (H5O__eliminate_gap(
                                        oh, &curr_chk_dirtied, &oh->mesg[new_null_msg],
                                        ((oh->chunk[old_chunkno].image + oh->chunk[old_chunkno].size) -
                                         (H5O_SIZEOF_CHKSUM_OH(oh) + oh->chunk[old_chunkno].gap)),
                                        oh->chunk[old_chunkno].gap) < 0)
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTREMOVE, FAIL,
                                                "can't eliminate gap in chunk");
                            } /* end if */

                            /* Release new null message's chunk, marking it dirty */
                            if (H5O__chunk_unprotect(f, curr_chk_proxy, curr_chk_dirtied) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                                            "unable to unprotect object header chunk");
                            curr_chk_proxy   = NULL;
                            curr_chk_dirtied = FALSE;
                        } /* end else */

                        /* Indicate that we packed messages */
                        packed_msg = TRUE;

                        /* Break out of loop */
                        /* (If it's possible to move message to even earlier chunk
                         *      we'll get it on the next pass - QAK)
                         */
                        break;
                    } /* end if */
                }     /* end for */

                /* If we packed messages, get out of loop and start over */
                /* (Don't know if this has any benefit one way or the other -QAK) */
                if (packed_msg)
                    break;
            } /* end else */
        }     /* end for */

        /* If we did any packing, remember that */
        if (packed_msg)
            did_packing = TRUE;
    } while (packed_msg);

    /* Set return value */
    ret_value = (htri_t)did_packing;

done:
    if (ret_value < 0) {
        if (null_chk_proxy && H5O__chunk_unprotect(f, null_chk_proxy, null_chk_dirtied) < 0)
            HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect null object header chunk");
        if (curr_chk_proxy && H5O__chunk_unprotect(f, curr_chk_proxy, curr_chk_dirtied) < 0)
            HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect current object header chunk");
        if (cont_targ_chk_proxy && H5O__chunk_unprotect(f, cont_targ_chk_proxy, FALSE) < 0)
            HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                        "unable to unprotect continuation message target object header chunk");
    } /* end if */
    else
        assert(!null_chk_proxy && !curr_chk_proxy && !cont_targ_chk_proxy);

    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__move_msgs_forward() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__merge_null
 *
 * Purpose:     Merge neighboring null messages in an object header
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static htri_t
H5O__merge_null(H5F_t *f, H5O_t *oh)
{
    hbool_t merged_msg;          /* Flag to indicate that messages were merged */
    hbool_t did_merging = FALSE; /* Whether any messages were merged */
    htri_t  ret_value   = FAIL;  /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh != NULL);

    /* Loop until no messages merged */
    /* (Double loop is not very efficient, but it would be some extra work to add
     *      a list of messages to each chunk -QAK)
     */
    do {
        H5O_mesg_t *curr_msg; /* Pointer to current message to operate on */
        unsigned    u;        /* Local index variable */

        /* Reset merged messages flag */
        merged_msg = FALSE;

        /* Scan messages for adjacent null messages & merge them */
        for (u = 0, curr_msg = &oh->mesg[0]; u < oh->nmesgs; u++, curr_msg++) {
            if (H5O_NULL_ID == curr_msg->type->id) {
                H5O_mesg_t *curr_msg2; /* Pointer to current message to operate on */
                unsigned    v;         /* Local index variable */

                /* Should be no gaps in chunk with null message */
                assert(oh->chunk[curr_msg->chunkno].gap == 0);

                /* Loop over messages again, looking for null message in same chunk */
                for (v = 0, curr_msg2 = &oh->mesg[0]; v < oh->nmesgs; v++, curr_msg2++) {
                    if (u != v && H5O_NULL_ID == curr_msg2->type->id &&
                        curr_msg->chunkno == curr_msg2->chunkno) {
                        ssize_t adj_raw      = 0; /* Amount to adjust raw message pointer */
                        size_t  adj_raw_size = 0; /* Amount to adjust raw message size */

                        /* Check for second message after first message */
                        if ((curr_msg->raw + curr_msg->raw_size) ==
                            (curr_msg2->raw - H5O_SIZEOF_MSGHDR_OH(oh))) {
                            /* Extend first null message length to cover second null message */
                            adj_raw      = 0;
                            adj_raw_size = (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + curr_msg2->raw_size;

                            /* Message has been merged */
                            merged_msg = TRUE;
                        } /* end if */
                        /* Check for second message before first message */
                        else if ((curr_msg->raw - H5O_SIZEOF_MSGHDR_OH(oh)) ==
                                 (curr_msg2->raw + curr_msg2->raw_size)) {
                            /* Adjust first message address and extend length to cover second message */
                            adj_raw = -((ssize_t)((size_t)H5O_SIZEOF_MSGHDR_OH(oh) + curr_msg2->raw_size));
                            adj_raw_size = (size_t)H5O_SIZEOF_MSGHDR_OH(oh) + curr_msg2->raw_size;

                            /* Message has been merged */
                            merged_msg = TRUE;
                        } /* end if */

                        /* Second message has been merged, delete it */
                        if (merged_msg) {
                            H5O_chunk_proxy_t *curr_chk_proxy; /* Chunk that message is in */
                            htri_t             result;

                            /* Release any information/memory for second message */
                            H5O__msg_free_mesg(curr_msg2);

                            /* Protect chunk */
                            if (NULL == (curr_chk_proxy = H5O__chunk_protect(f, oh, curr_msg->chunkno)))
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL,
                                            "unable to load object header chunk");

                            /* Adjust first message address and extend length to cover second message */
                            curr_msg->raw += adj_raw;
                            curr_msg->raw_size += adj_raw_size;

                            /* Mark first message as dirty */
                            curr_msg->dirty = TRUE;

                            /* Release new null message's chunk, marking it dirty */
                            if (H5O__chunk_unprotect(f, curr_chk_proxy, TRUE) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL,
                                            "unable to unprotect object header chunk");

                            /* Remove second message from list of messages */
                            if (v < (oh->nmesgs - 1))
                                memmove(&oh->mesg[v], &oh->mesg[v + 1],
                                        ((oh->nmesgs - 1) - v) * sizeof(H5O_mesg_t));

                            /* Decrement # of messages */
                            /* (Don't bother reducing size of message array for now -QAK) */
                            oh->nmesgs--;

                            /* The merge null message might span the entire chunk: scan for empty chunk to
                             * remove */
                            if ((result = H5O__remove_empty_chunks(f, oh)) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTPACK, FAIL, "can't remove empty chunk");
                            else if (result > 0)
                                /* Get out of loop */
                                break;

                            /* If the merged message is too large, shrink the chunk */
                            if (curr_msg->raw_size >= H5O_MESG_MAX_SIZE)
                                if (H5O__alloc_shrink_chunk(f, oh, curr_msg->chunkno) < 0)
                                    HGOTO_ERROR(H5E_OHDR, H5E_CANTPACK, FAIL, "unable to shrink chunk");

                            /* Get out of loop */
                            break;
                        } /* end if */
                    }     /* end if */
                }         /* end for */

                /* Get out of loop if we merged messages */
                if (merged_msg)
                    break;
            } /* end if */
        }     /* end for */

        /* If we did any merging, remember that */
        if (merged_msg)
            did_merging = TRUE;
    } while (merged_msg);

    /* Set return value */
    ret_value = (htri_t)did_merging;

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__merge_null() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__remove_empty_chunks
 *
 * Purpose:     Attempt to eliminate empty chunks from object header.
 *
 *              This examines a chunk to see if it's empty
 *              and removes it (and the continuation message that points to it)
 *              from the object header.
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static htri_t
H5O__remove_empty_chunks(H5F_t *f, H5O_t *oh)
{
    hbool_t deleted_chunk;        /* Whether to a chunk was deleted */
    hbool_t did_deleting = FALSE; /* Whether any chunks were deleted */
    htri_t  ret_value    = FAIL;  /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh != NULL);

    /* Loop until no chunks are freed */
    do {
        H5O_mesg_t *null_msg; /* Pointer to null message found */
        H5O_mesg_t *cont_msg; /* Pointer to continuation message found */
        unsigned    u, v;     /* Local index variables */

        /* Reset 'chunk deleted' flag */
        deleted_chunk = FALSE;

        /* Scan messages for null messages that fill an entire chunk */
        for (u = 0, null_msg = &oh->mesg[0]; u < oh->nmesgs; u++, null_msg++) {
            /* If a null message takes up an entire object header chunk (and
             * its not the "base" chunk), delete that chunk from object header
             */
            if (H5O_NULL_ID == null_msg->type->id && null_msg->chunkno > 0 &&
                ((size_t)H5O_SIZEOF_MSGHDR_OH(oh) + null_msg->raw_size) ==
                    (oh->chunk[null_msg->chunkno].size - H5O_SIZEOF_CHKHDR_OH(oh))) {
                H5O_mesg_t *curr_msg;        /* Pointer to current message to operate on */
                unsigned    null_msg_no;     /* Message # for null message */
                unsigned    deleted_chunkno; /* Chunk # to delete */

                /* Locate continuation message that points to chunk */
                for (v = 0, cont_msg = &oh->mesg[0]; v < oh->nmesgs; v++, cont_msg++) {
                    if (H5O_CONT_ID == cont_msg->type->id) {
                        /* Decode current continuation message if necessary */
                        H5O_LOAD_NATIVE(f, 0, oh, cont_msg, FAIL)

                        /* Check if the chunkno needs to be set */
                        /* (should only occur when the continuation message is first decoded) */
                        if (0 == ((H5O_cont_t *)(cont_msg->native))->chunkno) {
                            unsigned w; /* Local index variable */

                            /* Find chunk that this continuation message points to */
                            for (w = 0; w < oh->nchunks; w++)
                                if (oh->chunk[w].addr == ((H5O_cont_t *)(cont_msg->native))->addr) {
                                    ((H5O_cont_t *)(cont_msg->native))->chunkno = w;
                                    break;
                                } /* end if */
                            assert(((H5O_cont_t *)(cont_msg->native))->chunkno > 0);
                        } /* end if */

                        /* Check for correct chunk to delete */
                        if (oh->chunk[null_msg->chunkno].addr == ((H5O_cont_t *)(cont_msg->native))->addr)
                            break;
                    } /* end if */
                }     /* end for */
                /* Must be a continuation message that points to chunk containing null message */
                assert(v < oh->nmesgs);
                assert(cont_msg);
                assert(((H5O_cont_t *)(cont_msg->native))->chunkno == null_msg->chunkno);

                /* Initialize information about null message */
                null_msg_no     = u;
                deleted_chunkno = null_msg->chunkno;

                /* Convert continuation message into a null message */
                if (H5O__release_mesg(f, oh, cont_msg, TRUE) < 0)
                    HGOTO_ERROR(H5E_OHDR, H5E_CANTDELETE, FAIL, "unable to convert into null message");

                /*
                 * Remove chunk from object header's data structure
                 */

                /* Free memory for chunk image */
                oh->chunk[null_msg->chunkno].image =
                    H5FL_BLK_FREE(chunk_image, oh->chunk[null_msg->chunkno].image);

                /* Remove chunk from list of chunks */
                if (null_msg->chunkno < (oh->nchunks - 1)) {
                    memmove(&oh->chunk[null_msg->chunkno], &oh->chunk[null_msg->chunkno + 1],
                            ((oh->nchunks - 1) - null_msg->chunkno) * sizeof(H5O_chunk_t));

                    /* Adjust chunk number for any chunk proxies that are in the cache */
                    for (u = null_msg->chunkno; u < (oh->nchunks - 1); u++) {
                        unsigned chk_proxy_status = 0; /* Metadata cache status of chunk proxy for chunk */

                        /* Check the chunk proxy's status in the metadata cache */
                        if (H5AC_get_entry_status(f, oh->chunk[u].addr, &chk_proxy_status) < 0)
                            HGOTO_ERROR(H5E_OHDR, H5E_CANTGET, FAIL,
                                        "unable to check metadata cache status for chunk proxy");

                        /* If the entry is in the cache, update its chunk index */
                        if (chk_proxy_status & H5AC_ES__IN_CACHE) {
                            if (H5O__chunk_update_idx(f, oh, u) < 0)
                                HGOTO_ERROR(H5E_OHDR, H5E_CANTSET, FAIL,
                                            "unable to update index for chunk proxy");
                        } /* end if */
                    }     /* end for */
                }         /* end if */

                /* Decrement # of chunks */
                /* (Don't bother reducing size of chunk array for now -QAK) */
                oh->nchunks--;

                /*
                 * Delete null message (in empty chunk that was be freed) from list of messages
                 */

                /* Release any information/memory for message */
                H5O__msg_free_mesg(null_msg);

                /* Remove null message from list of messages */
                if (null_msg_no < (oh->nmesgs - 1))
                    memmove(&oh->mesg[null_msg_no], &oh->mesg[null_msg_no + 1],
                            ((oh->nmesgs - 1) - null_msg_no) * sizeof(H5O_mesg_t));

                /* Decrement # of messages */
                /* (Don't bother reducing size of message array for now -QAK) */
                oh->nmesgs--;

                /* Adjust chunk # for messages in chunks after deleted chunk */
                for (u = 0, curr_msg = &oh->mesg[0]; u < oh->nmesgs; u++, curr_msg++) {
                    /* Sanity check - there should be no messages in deleted chunk */
                    assert(curr_msg->chunkno != deleted_chunkno);

                    /* Adjust chunk index for messages in later chunks */
                    if (curr_msg->chunkno > deleted_chunkno)
                        curr_msg->chunkno--;

                    /* Check for continuation message */
                    if (H5O_CONT_ID == curr_msg->type->id) {
                        /* Decode current continuation message if necessary */
                        H5O_LOAD_NATIVE(f, 0, oh, curr_msg, FAIL)

                        /* Check if the chunkno needs to be set */
                        /* (should only occur when the continuation message is first decoded) */
                        if (0 == ((H5O_cont_t *)(curr_msg->native))->chunkno) {
                            unsigned w; /* Local index variable */

                            /* Find chunk that this continuation message points to */
                            for (w = 0; w < oh->nchunks; w++)
                                if (oh->chunk[w].addr == ((H5O_cont_t *)(curr_msg->native))->addr) {
                                    ((H5O_cont_t *)(curr_msg->native))->chunkno = w;
                                    break;
                                } /* end if */
                            assert(((H5O_cont_t *)(curr_msg->native))->chunkno > 0);
                        } /* end if */
                        else {
                            /* Check for pointer to chunk after deleted chunk */
                            if (((H5O_cont_t *)(curr_msg->native))->chunkno > deleted_chunkno)
                                ((H5O_cont_t *)(curr_msg->native))->chunkno--;
                        } /* end else */
                    }     /* end if */
                }         /* end for */

                /* Found chunk to delete */
                deleted_chunk = TRUE;
                break;
            } /* end if */
        }     /* end for */

        /* If we deleted any chunks, remember that */
        if (deleted_chunk)
            did_deleting = TRUE;
    } while (deleted_chunk);

    /* Set return value */
    ret_value = (htri_t)did_deleting;

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__remove_empty_chunks() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__condense_header
 *
 * Purpose:     Attempt to eliminate empty chunks from object header.
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
herr_t
H5O__condense_header(H5F_t *f, H5O_t *oh)
{
    hbool_t rescan_header;       /* Whether to rescan header */
    htri_t  result;              /* Result from packing/merging/etc */
    herr_t  ret_value = SUCCEED; /* return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(oh != NULL);

    /* Loop until no changed to the object header messages & chunks */
    do {
        /* Reset 'rescan chunks' flag */
        rescan_header = FALSE;

        /* Scan for messages that can be moved earlier in chunks */
        result = H5O__move_msgs_forward(f, oh);
        if (result < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_CANTPACK, FAIL, "can't move header messages forward");
        if (result > 0)
            rescan_header = TRUE;

        /* Scan for adjacent null messages & merge them */
        result = H5O__merge_null(f, oh);
        if (result < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_CANTPACK, FAIL, "can't pack null header messages");
        if (result > 0)
            rescan_header = TRUE;

        /* Scan for empty chunks to remove */
        result = H5O__remove_empty_chunks(f, oh);
        if (result < 0)
            HGOTO_ERROR(H5E_OHDR, H5E_CANTPACK, FAIL, "can't remove empty chunk");
        if (result > 0)
            rescan_header = TRUE;
    } while (rescan_header);
#ifdef H5O_DEBUG
    H5O__assert(oh);
#endif /* H5O_DEBUG */

done:
    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__condense_header() */

/*-------------------------------------------------------------------------
 *
 * Function:    H5O__alloc_shrink_chunk
 *
 * Purpose:     Shrinks a chunk, removing all null messages and any gap.
 *
 * Return:	Non-negative on success/Negative on failure
 *
 *-------------------------------------------------------------------------
 */
static herr_t
H5O__alloc_shrink_chunk(H5F_t *f, H5O_t *oh, unsigned chunkno)
{
    H5O_chunk_t       *chunk     = &oh->chunk[chunkno];      /* Chunk to shrink */
    H5O_chunk_proxy_t *chk_proxy = NULL;                     /* Metadata cache proxy for chunk to shrink */
    H5O_mesg_t        *curr_msg;                             /* Current message to examine */
    uint8_t           *old_image = chunk->image;             /* Old address of chunk's image in memory */
    size_t             old_size  = chunk->size;              /* Old size of chunk */
    size_t             new_size  = chunk->size - chunk->gap; /* Size of shrunk chunk */
    size_t             total_msg_size;                       /* Size of the messages in this chunk */
    size_t             min_chunk_size    = H5O_ALIGN_OH(oh, H5O_MIN_SIZE); /* Minimum chunk size */
    size_t             sizeof_chksum     = H5O_SIZEOF_CHKSUM_OH(oh);       /* Size of chunk checksum */
    size_t             sizeof_msghdr     = H5O_SIZEOF_MSGHDR_OH(oh);       /* Size of message header */
    uint8_t            new_size_flags    = 0;                              /* New chunk #0 size flags */
    hbool_t            adjust_size_flags = FALSE; /* Whether to adjust the chunk #0 size flags */
    size_t             less_prfx_size    = 0;     /* Bytes removed from object header prefix */
    size_t             u;                         /* Index */
    herr_t             ret_value = SUCCEED;       /* Return value */

    FUNC_ENTER_PACKAGE

    /* check args */
    assert(f);
    assert(oh);

    /* Protect chunk */
    if (NULL == (chk_proxy = H5O__chunk_protect(f, oh, chunkno)))
        HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to protect object header chunk");

    /* Loop backwards to increase the chance of seeing more null messages at the
     * end of the chunk.  Note that we rely on unsigned u wrapping around at the
     * end.
     */
    for (u = oh->nmesgs - 1, curr_msg = &oh->mesg[u]; u < oh->nmesgs; u--, curr_msg--) {
        if ((H5O_NULL_ID == curr_msg->type->id) && (chunkno == curr_msg->chunkno)) {
            size_t shrink_size = curr_msg->raw_size + sizeof_msghdr; /* Amount to shrink the chunk by */

            /* If the current message is not at the end of the chunk, copy the
             * data after it (except the checksum).
             */
            if (curr_msg->raw + curr_msg->raw_size < old_image + new_size - sizeof_chksum) {
                unsigned    v; /* Index */
                H5O_mesg_t *curr_msg2;
                uint8_t    *src = curr_msg->raw + curr_msg->raw_size; /* Source location */

                /* Slide down the raw data */
                memmove(curr_msg->raw - sizeof_msghdr, src,
                        (size_t)(old_image + new_size - sizeof_chksum - src));

                /* Update the raw data pointers for messages after this one */
                for (v = 0, curr_msg2 = &oh->mesg[0]; v < oh->nmesgs; v++, curr_msg2++)
                    if ((chunkno == curr_msg2->chunkno) && (curr_msg2->raw > curr_msg->raw))
                        curr_msg2->raw -= shrink_size;
            } /* end if */

            /* Adjust the new chunk size */
            new_size -= shrink_size;

            /* Release any information/memory for the message */
            H5O__msg_free_mesg(curr_msg);

            /* Remove the deleted null message from list of messages */
            if (u < (oh->nmesgs - 1))
                memmove(&oh->mesg[u], &oh->mesg[u + 1], ((oh->nmesgs - 1) - u) * sizeof(H5O_mesg_t));

            /* Decrement # of messages */
            /* (Don't bother reducing size of message array for now) */
            oh->nmesgs--;
        } /* end if */
    }     /* end for */

    /* Check if the chunk is too small, extend if necessary */
    total_msg_size = new_size - (size_t)(chunkno == 0 ? H5O_SIZEOF_HDR(oh) : H5O_SIZEOF_CHKHDR_OH(oh));
    if (total_msg_size < min_chunk_size) {
        assert(oh->alloc_nmesgs > oh->nmesgs);
        oh->nmesgs++;

        /* Initialize new null message to make the chunk large enough */
        curr_msg         = &oh->mesg[oh->nmesgs - 1];
        curr_msg->type   = H5O_MSG_NULL;
        curr_msg->dirty  = TRUE;
        curr_msg->native = NULL;
        curr_msg->raw    = old_image + new_size + sizeof_msghdr - sizeof_chksum;
        curr_msg->raw_size =
            MAX(H5O_ALIGN_OH(oh, min_chunk_size - total_msg_size), sizeof_msghdr) - sizeof_msghdr;
        curr_msg->chunkno = chunkno;

        /* update the new chunk size */
        new_size += curr_msg->raw_size + sizeof_msghdr;
    } /* end if */

    /* Check for changing the chunk #0 data size enough to need adjusting the flags */
    if (oh->version > H5O_VERSION_1 && chunkno == 0) {
        uint64_t chunk0_newsize = new_size - (size_t)H5O_SIZEOF_HDR(oh); /* New size of chunk 0's data */
        size_t   orig_prfx_size = (size_t)1 << (oh->flags & H5O_HDR_CHUNK0_SIZE); /* Original prefix size */

        /* Check for moving to a 1-byte size encoding */
        if (orig_prfx_size > 1 && chunk0_newsize <= 255) {
            less_prfx_size    = orig_prfx_size - 1;
            new_size_flags    = H5O_HDR_CHUNK0_1;
            adjust_size_flags = TRUE;
        } /* end if */
        /* Check for moving to a 2-byte size encoding */
        else if (orig_prfx_size > 2 && chunk0_newsize <= 65535) {
            less_prfx_size    = orig_prfx_size - 2;
            new_size_flags    = H5O_HDR_CHUNK0_2;
            adjust_size_flags = TRUE;
        } /* end if */
        /* Check for moving to a 4-byte size encoding */
        else if (orig_prfx_size > 4 && chunk0_newsize <= 4294967295) {
            less_prfx_size    = orig_prfx_size - 4;
            new_size_flags    = H5O_HDR_CHUNK0_4;
            adjust_size_flags = TRUE;
        } /* end if */
    }     /* end if */

    if (adjust_size_flags) {
        /* Adjust object header prefix flags */
        oh->flags = (uint8_t)(oh->flags & ~H5O_HDR_CHUNK0_SIZE);
        oh->flags |= new_size_flags;

        /* Slide chunk 0 data down */
        memmove(chunk->image + H5O_SIZEOF_HDR(oh) - sizeof_chksum,
                chunk->image + H5O_SIZEOF_HDR(oh) - sizeof_chksum + less_prfx_size,
                new_size - (size_t)H5O_SIZEOF_HDR(oh));

        /* Adjust chunk size */
        new_size -= less_prfx_size;
    } /* end if */

    /* Allocate less memory space for chunk's image */
    chunk->size  = new_size;
    chunk->image = H5FL_BLK_REALLOC(chunk_image, old_image, chunk->size);
    chunk->gap   = 0;
    if (NULL == oh->chunk[chunkno].image)
        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed");

    /* Spin through existing messages, adjusting them */
    for (u = 0, curr_msg = &oh->mesg[0]; u < oh->nmesgs; u++, curr_msg++) {
        if (adjust_size_flags || (chunk->image != old_image))
            /* Adjust raw addresses for messages in this chunk to reflect new 'image' address */
            if (curr_msg->chunkno == chunkno)
                curr_msg->raw = chunk->image - less_prfx_size + (curr_msg->raw - old_image);

        /* Find continuation message which points to this chunk and adjust chunk's size */
        /* (Chunk 0 doesn't have a continuation message that points to it and
         * its size is directly encoded in the object header) */
        if (chunkno > 0 && (H5O_CONT_ID == curr_msg->type->id) &&
            (((H5O_cont_t *)(curr_msg->native))->chunkno == chunkno)) {
            H5O_chunk_proxy_t *cont_chk_proxy; /* Chunk that message is in */

            /* Protect chunk */
            if (NULL == (cont_chk_proxy = H5O__chunk_protect(f, oh, curr_msg->chunkno)))
                HGOTO_ERROR(H5E_OHDR, H5E_CANTPROTECT, FAIL, "unable to protect object header chunk");

            /* Adjust size of continuation message */
            assert(((H5O_cont_t *)(curr_msg->native))->size == old_size);
            ((H5O_cont_t *)(curr_msg->native))->size = chunk->size;

            /* Flag continuation message as dirty */
            curr_msg->dirty = TRUE;

            /* Release chunk, marking it dirty */
            if (H5O__chunk_unprotect(f, cont_chk_proxy, TRUE) < 0)
                HGOTO_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");
        } /* end if */
    }     /* end for */

    assert(new_size <= old_size);

    /* Resize the chunk in the cache */
    if (H5O__chunk_resize(oh, chk_proxy) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTRESIZE, FAIL, "unable to resize object header chunk");

    /* Free the unused space in the file */
    if (H5MF_xfree(f, H5FD_MEM_OHDR, chunk->addr + new_size, (hsize_t)(old_size - new_size)) < 0)
        HGOTO_ERROR(H5E_OHDR, H5E_CANTFREE, FAIL, "unable to shrink object header chunk");

done:
    /* Release chunk, marking it dirty */
    if (chk_proxy && H5O__chunk_unprotect(f, chk_proxy, TRUE) < 0)
        HDONE_ERROR(H5E_OHDR, H5E_CANTUNPROTECT, FAIL, "unable to unprotect object header chunk");

    FUNC_LEAVE_NOAPI(ret_value)
} /* H5O__alloc_shrink_chunk() */