[svn-r13226] Description:

	Add attributes to the message classes that use the shared message
method calling interface, completing the transition.  The other mechanism will
be removed shortly.

Tested on:
	Mac OS X/32 10.4.8 (amazon)
	FreeBSD/32 6.2 (duty)

9 files changed, 224 insertions, 379 deletions

diff --git a/src/H5A.c b/src/H5A.c
index ef83826..ac41152 100644
--- a/src/H5A.c
+++ b/src/H5A.c
@@ -359,12 +359,10 @@ H5A_create(const H5G_loc_t *loc, const char *name, const H5T_t *type,
         HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, FAIL, "unable to copy path")
 
     /* Check if any of the pieces should be (or are already) shared in the
-     * SOHM table */
-    /* Data type */
+     * SOHM table
+     */
     if(H5SM_try_share(attr->oloc.file, dxpl_id, H5O_DTYPE_ID, attr->dt) < 0)
 	HGOTO_ERROR(H5E_OHDR, H5E_BADMESG, FAIL, "trying to share datatype failed")
-
-    /* Data space */
     if(H5SM_try_share(attr->oloc.file, dxpl_id, H5O_SDSPACE_ID, attr->ds) < 0)
 	HGOTO_ERROR(H5E_OHDR, H5E_BADMESG, FAIL, "trying to share dataspace failed")
 
@@ -1302,9 +1300,11 @@ H5Arename(hid_t loc_id, const char *old_name, const char *new_name)
     if(H5G_loc(loc_id, & loc) < 0)
 	HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a location")
 
-    /* Call attribute rename routine */
-    if(H5O_attr_rename(loc.oloc, H5AC_dxpl_id, old_name, new_name) < 0)
-	HGOTO_ERROR(H5E_ATTR, H5E_CANTRENAME, FAIL, "can't rename attribute")
+    /* Avoid thrashing things if the names are the same */
+    if(HDstrcmp(old_name, new_name))
+        /* Call attribute rename routine */
+        if(H5O_attr_rename(loc.oloc, H5AC_dxpl_id, old_name, new_name) < 0)
+            HGOTO_ERROR(H5E_ATTR, H5E_CANTRENAME, FAIL, "can't rename attribute")
 
 done:
     FUNC_LEAVE_API(ret_value)
diff --git a/src/H5Adense.c b/src/H5Adense.c
index 7091856..30229f4 100644
--- a/src/H5Adense.c
+++ b/src/H5Adense.c
@@ -334,11 +334,12 @@ H5A_dense_open(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, const char *name)
         if(H5SM_get_fheap_addr(f, dxpl_id, H5O_ATTR_ID, &shared_fheap_addr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, NULL, "can't get shared message heap address")
 
-        /* Open the fractal heap for shared header messages */
+        /* Check if there are any shared messages currently */
         if(H5F_addr_defined(shared_fheap_addr)) {
+            /* Open the fractal heap for shared header messages */
             if(NULL == (shared_fheap = H5HF_open(f, dxpl_id, shared_fheap_addr)))
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, NULL, "unable to open fractal heap")
-        }
+        } /* end if */
     } /* end if */
 
     /* Create the "udata" information for v2 B-tree record modify */
@@ -382,8 +383,7 @@ done:
  *-------------------------------------------------------------------------
  */
 herr_t
-H5A_dense_insert(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, unsigned mesg_flags,
-    const H5A_t *attr)
+H5A_dense_insert(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, H5A_t *attr)
 {
     H5A_bt2_ud_ins_t udata;             /* User data for v2 B-tree insertion */
     H5HF_t *fheap = NULL;               /* Fractal heap handle for attributes */
@@ -391,6 +391,7 @@ H5A_dense_insert(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, unsigned mesg_flags,
     H5O_shared_t sh_mesg;               /* Shared object header message */
     uint8_t attr_buf[H5A_ATTR_BUF_SIZE]; /* Buffer for serializing message */
     void *attr_ptr = NULL;              /* Pointer to serialized message */
+    unsigned mesg_flags = 0;            /* Flags for storing message */
     htri_t attr_sharable;               /* Flag indicating attributes are sharable */
     herr_t ret_value = SUCCEED;         /* Return value */
 
@@ -410,16 +411,33 @@ H5A_dense_insert(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, unsigned mesg_flags,
     /* Get handle for shared message heap, if attributes are sharable */
     if(attr_sharable) {
         haddr_t shared_fheap_addr;      /* Address of fractal heap to use */
+        htri_t shared_mesg;             /* Should this message be stored in the Shared Message table? */
+
+        /* Check if message is already shared */
+        if((shared_mesg = H5O_attr_is_shared(attr)) < 0)
+            HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "error determining if message is shared")
+        else if(shared_mesg > 0)
+            /* Mark the message as shared */
+            mesg_flags |= H5O_MSG_FLAG_SHARED;
+        else {
+            /* Should this attribute be written as a SOHM? */
+            if((shared_mesg = H5SM_try_share(f, dxpl_id, H5O_ATTR_ID, attr)) > 0)
+                /* Mark the message as shared */
+                mesg_flags |= H5O_MSG_FLAG_SHARED;
+            else if(shared_mesg < 0)
+                HGOTO_ERROR(H5E_ATTR, H5E_WRITEERROR, FAIL, "error determining if message should be shared")
+        } /* end else */
 
         /* Retrieve the address of the shared message's fractal heap */
         if(H5SM_get_fheap_addr(f, dxpl_id, H5O_ATTR_ID, &shared_fheap_addr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't get shared message heap address")
 
-        /* Open the fractal heap for shared header messages */
+        /* Check if there are any shared messages currently */
         if(H5F_addr_defined(shared_fheap_addr)) {
+            /* Open the fractal heap for shared header messages */
             if(NULL == (shared_fheap = H5HF_open(f, dxpl_id, shared_fheap_addr)))
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, FAIL, "unable to open fractal heap")
-        }
+        } /* end if */
     } /* end if */
 
     /* Open the fractal heap */
@@ -525,20 +543,10 @@ H5A_dense_write_bt2_cb(void *_record, void *_op_data, hbool_t *changed)
 
     /* Check for modifying shared attribute */
     if(record->flags & H5O_MSG_FLAG_SHARED) {
-        H5O_shared_t sh_mesg;           /* Shared object header message */
-
-        /* Extract shared message info from current attribute */
-        if(NULL == H5O_attr_get_share(op_data->attr, &sh_mesg))
-            HGOTO_ERROR(H5E_ATTR, H5E_BADMESG, FAIL, "can't get shared info")
-
         /* Update the shared attribute in the SOHM info */
-        if(H5O_attr_update_shared(op_data->f, op_data->dxpl_id, op_data->attr, &sh_mesg) < 0)
+        if(H5O_attr_update_shared(op_data->f, op_data->dxpl_id, op_data->attr, NULL) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTUPDATE, FAIL, "unable to update attribute in shared storage")
 
-        /* Extract new shared message info from updated attribute */
-        if(NULL == H5O_attr_get_share(op_data->attr, &sh_mesg))
-            HGOTO_ERROR(H5E_ATTR, H5E_BADMESG, FAIL, "can't get shared info")
-
         /* Update record's heap ID */
         record->id = op_data->attr->sh_loc.u.heap_id;
 
@@ -575,7 +583,7 @@ H5A_dense_write_bt2_cb(void *_record, void *_op_data, hbool_t *changed)
 }
 #endif /* NDEBUG */
         /* Update existing attribute in heap */
-        /* (would be more efficient as fractal heap 'op' callback, but leave that for later -QAK) */
+        /* (might be more efficient as fractal heap 'op' callback, but leave that for later -QAK) */
         if(H5HF_write(op_data->fheap, op_data->dxpl_id, &record->id, changed, attr_ptr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTUPDATE, FAIL, "unable to update attribute in heap")
     } /* end else */
@@ -633,11 +641,12 @@ H5A_dense_write(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, H5A_t *attr)
         if(H5SM_get_fheap_addr(f, dxpl_id, H5O_ATTR_ID, &shared_fheap_addr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't get shared message heap address")
 
-        /* Open the fractal heap for shared header messages */
+        /* Check if there are any shared messages currently */
         if(H5F_addr_defined(shared_fheap_addr)) {
+            /* Open the fractal heap for shared header messages */
             if(NULL == (shared_fheap = H5HF_open(f, dxpl_id, shared_fheap_addr)))
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, FAIL, "unable to open fractal heap")
-        }
+        } /* end if */
     } /* end if */
 
     /* Open the fractal heap */
@@ -741,7 +750,6 @@ H5A_dense_rename(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, const char *old_name,
     H5HF_t *shared_fheap = NULL;        /* Fractal heap handle for shared header messages */
     H5A_t *attr_copy = NULL;            /* Copy of attribute to rename */
     htri_t attr_sharable;               /* Flag indicating attributes are sharable */
-    unsigned mesg_flags = 0;            /* Flags for storing message */
     htri_t shared_mesg;                 /* Should this message be stored in the Shared Message table? */
     herr_t ret_value = SUCCEED;         /* Return value */
 
@@ -767,11 +775,12 @@ H5A_dense_rename(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, const char *old_name,
         if(H5SM_get_fheap_addr(f, dxpl_id, H5O_ATTR_ID, &shared_fheap_addr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't get shared message heap address")
 
-        /* Open the fractal heap for shared header messages */
+        /* Check if there are any shared messages currently */
         if(H5F_addr_defined(shared_fheap_addr)) {
+            /* Open the fractal heap for shared header messages */
             if(NULL == (shared_fheap = H5HF_open(f, dxpl_id, shared_fheap_addr)))
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, FAIL, "unable to open fractal heap")
-        }
+        } /* end if */
     } /* end if */
 
     /* Open the fractal heap */
@@ -795,17 +804,27 @@ H5A_dense_rename(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, const char *old_name,
         HGOTO_ERROR(H5E_ATTR, H5E_CANTINSERT, FAIL, "unable to find record in v2 B-tree")
     HDassert(attr_copy);
 
+    /* Check if message is already shared */
+    if((shared_mesg = H5O_attr_is_shared(attr_copy)) < 0)
+        HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "error determining if message is shared")
+    else if(shared_mesg > 0) {
+        /* Reset shared status of copy */
+        /* (so it will get shared again if necessary) */
+        attr_copy->sh_loc.flags = 0;
+    } /* end if */
+
     /* Change name of attribute */
     H5MM_xfree(attr_copy->name);
     attr_copy->name = H5MM_xstrdup(new_name);
 
-    /* Should this attribute be written as a SOHM? */
-    /* (allows for attributes that change "shared" status) */
-    if((shared_mesg = H5SM_try_share(f, dxpl_id, H5O_ATTR_ID, attr_copy)) > 0) {
-        hsize_t attr_rc;                /* Attribute's ref count in shared message storage */
+    /* Insert renamed attribute back into dense storage */
+    /* (Possibly making it shared) */
+    if(H5A_dense_insert(f, dxpl_id, oh, attr_copy) < 0)
+        HGOTO_ERROR(H5E_ATTR, H5E_CANTINSERT, FAIL, "unable to add to dense storage")
 
-        /* Mark the message as shared */
-        mesg_flags |= H5O_MSG_FLAG_SHARED;
+    /* Was this attribute shared? */
+    if((shared_mesg = H5O_attr_is_shared(attr_copy)) > 0) {
+        hsize_t attr_rc;                /* Attribute's ref count in shared message storage */
 
         /* Retrieve ref count for shared attribute */
         if(H5SM_get_refcount(f, dxpl_id, H5O_ATTR_ID, &attr_copy->sh_loc, &attr_rc) < 0)
@@ -833,10 +852,6 @@ H5A_dense_rename(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, const char *old_name,
     else if(shared_mesg < 0)
 	HGOTO_ERROR(H5E_ATTR, H5E_WRITEERROR, FAIL, "error determining if message should be shared")
 
-    /* Insert renamed attribute back into dense storage */
-    if(H5A_dense_insert(f, dxpl_id, oh, mesg_flags, attr_copy) < 0)
-        HGOTO_ERROR(H5E_ATTR, H5E_CANTINSERT, FAIL, "unable to add to dense storage")
-
     /* Delete old attribute from dense storage */
     if(H5A_dense_remove(f, dxpl_id, oh, old_name) < 0)
         HGOTO_ERROR(H5E_ATTR, H5E_CANTDELETE, FAIL, "unable to delete attribute in dense storage")
@@ -851,7 +866,7 @@ done:
         H5O_msg_free(H5O_ATTR_ID, attr_copy);
 
     FUNC_LEAVE_NOAPI(ret_value)
-} /* end H5A_dense_write() */
+} /* end H5A_dense_rename() */
 
 
 /*-------------------------------------------------------------------------
@@ -997,11 +1012,12 @@ H5A_dense_iterate(H5F_t *f, hid_t dxpl_id, hid_t loc_id, haddr_t attr_fheap_addr
             if(H5SM_get_fheap_addr(f, dxpl_id, H5O_ATTR_ID, &shared_fheap_addr) < 0)
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't get shared message heap address")
 
-            /* Open the fractal heap for shared header messages */
+            /* Check if there are any shared messages currently */
             if(H5F_addr_defined(shared_fheap_addr)) {
+                /* Open the fractal heap for shared header messages */
                 if(NULL == (shared_fheap = H5HF_open(f, dxpl_id, shared_fheap_addr)))
                     HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, FAIL, "unable to open fractal heap")
-            }
+            } /* end if */
         } /* end if */
 
         /* Construct the user data for v2 B-tree iterator callback */
@@ -1141,11 +1157,12 @@ H5A_dense_remove(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, const char *name)
         if(H5SM_get_fheap_addr(f, dxpl_id, H5O_ATTR_ID, &shared_fheap_addr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't get shared message heap address")
 
-        /* Open the fractal heap for shared header messages */
+        /* Check if there are any shared messages currently */
         if(H5F_addr_defined(shared_fheap_addr)) {
+            /* Open the fractal heap for shared header messages */
             if(NULL == (shared_fheap = H5HF_open(f, dxpl_id, shared_fheap_addr)))
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, FAIL, "unable to open fractal heap")
-        }
+        } /* end if */
     } /* end if */
 
     /* Set up the user data for the v2 B-tree 'record remove' callback */
@@ -1225,11 +1242,12 @@ H5A_dense_exists(H5F_t *f, hid_t dxpl_id, const H5O_t *oh, const char *name)
         if(H5SM_get_fheap_addr(f, dxpl_id, H5O_ATTR_ID, &shared_fheap_addr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't get shared message heap address")
 
-        /* Open the fractal heap for shared header messages */
+        /* Check if there are any shared messages currently */
         if(H5F_addr_defined(shared_fheap_addr)) {
+            /* Open the fractal heap for shared header messages */
             if(NULL == (shared_fheap = H5HF_open(f, dxpl_id, shared_fheap_addr)))
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTOPENOBJ, FAIL, "unable to open fractal heap")
-        }
+        } /* end if */
     } /* end if */
 
     /* Create the "udata" information for v2 B-tree record 'find' */
diff --git a/src/H5Aint.c b/src/H5Aint.c
index 456717b..3bfb494 100644
--- a/src/H5Aint.c
+++ b/src/H5Aint.c
@@ -135,20 +135,9 @@ H5A_compact_build_table_cb(H5O_t UNUSED *oh, H5O_mesg_t *mesg/*in,out*/,
         udata->atable->nattrs = n;
     } /* end if */
 
-    /* Check for shared message */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-	/*
-	 * If the message is shared then then the native pointer points to an
-	 * H5O_MSG_SHARED message.  We use that information to look up the real
-	 * message in the global heap or some other object header.
-	 */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &udata->atable->attrs[udata->curr_attr]))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTCOPY, H5_ITER_ERROR, "unable to read shared attribute")
-    } /* end if */
-    else {
-        if(NULL == H5A_copy(&udata->atable->attrs[udata->curr_attr], (const H5A_t *)mesg->native))
-            HGOTO_ERROR(H5E_ATTR, H5E_CANTCOPY, H5_ITER_ERROR, "can't copy attribute")
-    } /* end else */
+    /* Copy attribute into table */
+    if(NULL == H5A_copy(&udata->atable->attrs[udata->curr_attr], (const H5A_t *)mesg->native))
+        HGOTO_ERROR(H5E_ATTR, H5E_CANTCOPY, H5_ITER_ERROR, "can't copy attribute")
 
     /* Increment current attribute */
     udata->curr_attr++;
diff --git a/src/H5Apkg.h b/src/H5Apkg.h
index 6fa92e8..3194494 100644
--- a/src/H5Apkg.h
+++ b/src/H5Apkg.h
@@ -161,8 +161,11 @@ H5_DLL herr_t H5A_free(H5A_t *attr);
 H5_DLL herr_t H5A_close(H5A_t *attr);
 
 /* Attribute "dense" storage routines */
+H5_DLL herr_t H5A_dense_create(H5F_t *f, hid_t dxpl_id, H5O_t *oh);
 H5_DLL H5A_t *H5A_dense_open(H5F_t *f, hid_t dxpl_id, const H5O_t *oh,
     const char *name);
+H5_DLL herr_t H5A_dense_insert(H5F_t *f, hid_t dxpl_id, const H5O_t *oh,
+    H5A_t *attr);
 H5_DLL herr_t H5A_dense_write(H5F_t *f, hid_t dxpl_id, const H5O_t *oh,
     H5A_t *attr);
 H5_DLL herr_t H5A_dense_rename(H5F_t *f, hid_t dxpl_id, const H5O_t *oh,
@@ -197,7 +200,7 @@ H5_DLL htri_t H5O_attr_is_shared(const void *_mesg);
 
 /* Attribute operations */
 H5_DLL herr_t H5O_attr_update_shared(H5F_t *f, hid_t dxpl_id, H5A_t *attr, 
-    const H5O_shared_t *sh_mesg);
+    H5O_shared_t *sh_mesg);
 
 /* Testing functions */
 #ifdef H5A_TESTING
diff --git a/src/H5Aprivate.h b/src/H5Aprivate.h
index ca14ebc..88b7f32 100644
--- a/src/H5Aprivate.h
+++ b/src/H5Aprivate.h
@@ -54,9 +54,6 @@ H5_DLL struct H5O_loc_t *H5A_oloc(H5A_t *attr);
 H5_DLL H5G_name_t *H5A_nameof(H5A_t *attr);
 
 /* Attribute "dense" storage routines */
-H5_DLL herr_t H5A_dense_create(H5F_t *f, hid_t dxpl_id, struct H5O_t *oh);
-H5_DLL herr_t H5A_dense_insert(H5F_t *f, hid_t dxpl_id, const struct H5O_t *oh,
-    unsigned mesg_flags, const H5A_t *attr);
 H5_DLL herr_t H5A_dense_delete(H5F_t *f, hid_t dxpl_id, struct H5O_t *oh);
 
 #endif /* _H5Aprivate_H */
diff --git a/src/H5Oattr.c b/src/H5Oattr.c
index b2d2712..2997705 100644
--- a/src/H5Oattr.c
+++ b/src/H5Oattr.c
@@ -65,20 +65,20 @@ const H5O_msg_class_t H5O_MSG_ATTR[1] = {{
     H5O_ATTR_ID,		/* message id number            */
     "attribute",		/* message name for debugging   */
     sizeof(H5A_t),		/* native message size          */
-    H5O_attr_decode,		/* decode message               */
-    H5O_attr_encode,		/* encode message               */
+    H5O_attr_shared_decode,		/* decode message               */
+    H5O_attr_shared_encode,		/* encode message               */
     H5O_attr_copy,		/* copy the native value        */
-    H5O_attr_size,		/* size of raw message          */
+    H5O_attr_shared_size,		/* size of raw message          */
     H5O_attr_reset,		/* reset method                 */
     H5O_attr_free,	        /* free method			*/
-    H5O_attr_delete,		/* file delete method		*/
-    NULL /* H5O_attr_link */,		/* link method			*/
+    H5O_attr_shared_delete,		/* file delete method		*/
+    H5O_attr_shared_link,		/* link method			*/
     H5O_attr_get_share,		/* get share method		*/
     H5O_attr_set_share,		/* set share method		*/
     NULL,		    	/*can share method		*/
     H5O_attr_is_shared,		/*is shared method		*/
     H5O_attr_pre_copy_file,	/* pre copy native value to file */
-    H5O_attr_copy_file,		/* copy native value to file    */
+    H5O_attr_shared_copy_file,		/* copy native value to file    */
     NULL,			/* post copy native value to file    */
     NULL /* H5O_attr_get_crt_index */,	/* get creation index		*/
     NULL /* H5O_attr_set_crt_index */,	/* set creation index		*/
diff --git a/src/H5Oattribute.c b/src/H5Oattribute.c
index 62e86e0..5d6d1aa 100644
--- a/src/H5Oattribute.c
+++ b/src/H5Oattribute.c
@@ -161,9 +161,8 @@ H5O_attr_to_dense_cb(H5O_t *oh, H5O_mesg_t *mesg/*in,out*/,
     unsigned UNUSED sequence, unsigned *oh_flags_ptr, void *_udata/*in,out*/)
 {
     H5O_iter_cvt_t *udata = (H5O_iter_cvt_t *)_udata;   /* Operator user data */
-    H5A_t shared_attr;                  /* Copy of shared attribute */
-    H5A_t *attr = NULL;                 /* Pointer to attribute to insert */
-    herr_t ret_value = H5_ITER_CONT;    /* Return value */
+    H5A_t *attr = (H5A_t *)mesg->native;        /* Pointer to attribute to insert */
+    herr_t ret_value = H5_ITER_CONT;            /* Return value */
 
     FUNC_ENTER_NOAPI_NOINIT(H5O_attr_to_dense_cb)
 
@@ -171,20 +170,8 @@ H5O_attr_to_dense_cb(H5O_t *oh, H5O_mesg_t *mesg/*in,out*/,
     HDassert(oh);
     HDassert(mesg);
 
-    /* Check for shared attribute */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-        /* Read the shared attribute in */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &shared_attr))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, H5_ITER_ERROR, "unable to read shared attribute")
-
-        /* Point attribute to insert at shared attribute read in */
-        attr = &shared_attr;
-    } /* end if */
-    else
-        attr = (H5A_t *)mesg->native;
-
     /* Insert attribute into dense storage */
-    if(H5A_dense_insert(udata->f, udata->dxpl_id, oh, mesg->flags, attr) < 0)
+    if(H5A_dense_insert(udata->f, udata->dxpl_id, oh, attr) < 0)
         HGOTO_ERROR(H5E_OHDR, H5E_CANTINSERT, H5_ITER_ERROR, "unable to add to dense storage")
 
     /* Convert message into a null message in the header */
@@ -196,10 +183,6 @@ H5O_attr_to_dense_cb(H5O_t *oh, H5O_mesg_t *mesg/*in,out*/,
     *oh_flags_ptr |= H5AC__DIRTIED_FLAG;
 
 done:
-    /* Release copy of shared attribute */
-    if(attr == &shared_attr)
-        H5O_attr_reset(&shared_attr);
-
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5O_attr_to_dense_cb() */
 
@@ -221,7 +204,6 @@ H5O_attr_create(const H5O_loc_t *loc, hid_t dxpl_id, H5A_t *attr)
 {
     H5O_t *oh = NULL;                   /* Pointer to actual object header */
     unsigned oh_flags = H5AC__NO_FLAGS_SET;     /* Metadata cache flags for object header */
-    unsigned mesg_flags = 0;            /* Flags for storing message */
     htri_t shared_mesg;                 /* Should this message be stored in the Shared Message table? */
     herr_t ret_value = SUCCEED;         /* Return value */
 
@@ -231,48 +213,6 @@ H5O_attr_create(const H5O_loc_t *loc, hid_t dxpl_id, H5A_t *attr)
     HDassert(loc);
     HDassert(attr);
 
-    /* Should this message be written as a SOHM? */
-    if((shared_mesg = H5SM_try_share(loc->file, dxpl_id, H5O_ATTR_ID, attr)) > 0) {
-        hsize_t attr_rc;                /* Attribute's ref count in shared message storage */
-
-        /* Mark the message as shared */
-        mesg_flags |= H5O_MSG_FLAG_SHARED;
-
-        /* Retrieve ref count for shared attribute */
-        if(H5SM_get_refcount(loc->file, dxpl_id, H5O_ATTR_ID, &attr->sh_loc, &attr_rc) < 0)
-            HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't retrieve shared message ref count")
-
-        /* If this is not the first copy of the attribute in the shared message
-         *      storage, decrement the reference count on any shared components
-         *      of the attribute.  This is done because the shared message
-         *      storage's "try delete" call doesn't call the message class's
-         *      "delete" callback until the reference count drops to zero.
-         *      However, attributes have already increased the reference
-         *      count on shared components before passing the attribute
-         *      to the shared message code to manage, causing an asymmetry
-         *      in the reference counting for any shared components.
-         *
-         *      The alternate solution is to have the shared message's "try
-         *      delete" code always call the message class's "delete" callback,
-         *      even when the reference count is positive.  This can be done
-         *      without an appreciable performance hit (by using H5HF_op() in
-         *      the shared message comparison v2 B-tree callback), but it has
-         *      the undesirable side-effect of leaving the reference count on
-         *      the attribute's shared components artificially (and possibly
-         *      misleadingly) high, because there's only one shared attribute
-         *      referencing the shared components, not <refcount for the
-         *      shared attribute> objects referencing the shared components.
-         *
-         *      *ick* -QAK, 2007/01/08
-         */
-        if(attr_rc > 1) {
-            if(H5O_attr_delete(loc->file, dxpl_id, attr, TRUE) < 0)
-                HGOTO_ERROR(H5E_ATTR, H5E_CANTDELETE, FAIL, "unable to delete attribute")
-        } /* end if */
-    } /* end if */
-    else if(shared_mesg < 0)
-	HGOTO_ERROR(H5E_ATTR, H5E_WRITEERROR, FAIL, "error determining if message should be shared")
-
     /* Protect the object header to iterate over */
     if(NULL == (oh = (H5O_t *)H5AC_protect(loc->file, dxpl_id, H5AC_OHDR, loc->addr, NULL, NULL, H5AC_WRITE)))
 	HGOTO_ERROR(H5E_ATTR, H5E_CANTLOAD, FAIL, "unable to load object header")
@@ -313,15 +253,54 @@ H5O_attr_create(const H5O_loc_t *loc, hid_t dxpl_id, H5A_t *attr)
     /* Check for storing attribute with dense storage */
     if(H5F_addr_defined(oh->attr_fheap_addr)) {
         /* Insert attribute into dense storage */
-        if(H5A_dense_insert(loc->file, dxpl_id, oh, mesg_flags, attr) < 0)
+        if(H5A_dense_insert(loc->file, dxpl_id, oh, attr) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTINSERT, FAIL, "unable to add to dense storage")
     } /* end if */
     else {
         /* Append new message to object header */
-        if(H5O_msg_append_real(loc->file, dxpl_id, oh, H5O_MSG_ATTR, mesg_flags, 0, attr, &oh_flags) < 0)
+        if(H5O_msg_append_real(loc->file, dxpl_id, oh, H5O_MSG_ATTR, 0, 0, attr, &oh_flags) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTINSERT, FAIL, "unable to create new attribute in header")
     } /* end else */
 
+    /* Was new attribugte shared? */
+    if((shared_mesg = H5O_attr_is_shared(attr)) > 0) {
+        hsize_t attr_rc;                /* Attribute's ref count in shared message storage */
+
+        /* Retrieve ref count for shared attribute */
+        if(H5SM_get_refcount(loc->file, dxpl_id, H5O_ATTR_ID, &attr->sh_loc, &attr_rc) < 0)
+            HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't retrieve shared message ref count")
+
+        /* If this is not the first copy of the attribute in the shared message
+         *      storage, decrement the reference count on any shared components
+         *      of the attribute.  This is done because the shared message
+         *      storage's "try delete" call doesn't call the message class's
+         *      "delete" callback until the reference count drops to zero.
+         *      However, attributes have already increased the reference
+         *      count on shared components before passing the attribute
+         *      to the shared message code to manage, causing an asymmetry
+         *      in the reference counting for any shared components.
+         *
+         *      The alternate solution is to have the shared message's "try
+         *      delete" code always call the message class's "delete" callback,
+         *      even when the reference count is positive.  This can be done
+         *      without an appreciable performance hit (by using H5HF_op() in
+         *      the shared message comparison v2 B-tree callback), but it has
+         *      the undesirable side-effect of leaving the reference count on
+         *      the attribute's shared components artificially (and possibly
+         *      misleadingly) high, because there's only one shared attribute
+         *      referencing the shared components, not <refcount for the
+         *      shared attribute> objects referencing the shared components.
+         *
+         *      *ick* -QAK, 2007/01/08
+         */
+        if(attr_rc > 1) {
+            if(H5O_attr_delete(loc->file, dxpl_id, attr, TRUE) < 0)
+                HGOTO_ERROR(H5E_ATTR, H5E_CANTDELETE, FAIL, "unable to delete attribute")
+        } /* end if */
+    } /* end if */
+    else if(shared_mesg < 0)
+	HGOTO_ERROR(H5E_ATTR, H5E_WRITEERROR, FAIL, "error determining if message should be shared")
+
     /* Update the modification time, if any */
     if(H5O_touch_oh(loc->file, dxpl_id, oh, FALSE, &oh_flags) < 0)
         HGOTO_ERROR(H5E_ATTR, H5E_CANTUPDATE, FAIL, "unable to update time on object")
@@ -362,39 +341,12 @@ H5O_attr_open_cb(H5O_t UNUSED *oh, H5O_mesg_t *mesg/*in,out*/,
     HDassert(mesg);
     HDassert(!udata->attr);
 
-    /* Check for shared message */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-        H5A_t shared_attr;             /* Copy of shared attribute */
-
-	/*
-	 * If the message is shared then then the native pointer points to an
-	 * H5O_MSG_SHARED message.  We use that information to look up the real
-	 * message in the global heap or some other object header.
-	 */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &shared_attr))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, H5_ITER_ERROR, "unable to read shared attribute")
-
-        /* Check for correct attribute message to modify */
-        if(HDstrcmp(shared_attr.name, udata->name) == 0) {
-            /* Make a copy of the attribute to return */
-            if(NULL == (udata->attr = H5A_copy(NULL, &shared_attr)))
-                HGOTO_ERROR(H5E_ATTR, H5E_CANTCOPY, H5_ITER_ERROR, "unable to copy attribute")
-        } /* end if */
-
-        /* Release copy of shared attribute */
-        H5O_attr_reset(&shared_attr);
-    } /* end if */
-    else {
-        /* Check for correct attribute message to modify */
-        if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->name) == 0) {
-            /* Make a copy of the attribute to return */
-            if(NULL == (udata->attr = H5A_copy(NULL, (H5A_t *)mesg->native)))
-                HGOTO_ERROR(H5E_ATTR, H5E_CANTCOPY, H5_ITER_ERROR, "unable to copy attribute")
-        } /* end if */
-    } /* end else */
+    /* Check for correct attribute message to modify */
+    if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->name) == 0) {
+        /* Make a copy of the attribute to return */
+        if(NULL == (udata->attr = H5A_copy(NULL, (H5A_t *)mesg->native)))
+            HGOTO_ERROR(H5E_ATTR, H5E_CANTCOPY, H5_ITER_ERROR, "unable to copy attribute")
 
-    /* Set common info, if we found the correct attribute */
-    if(udata->attr) {
         /* Stop iterating */
         ret_value = H5_ITER_STOP;
     } /* end if */
@@ -557,9 +509,10 @@ done:
  *-------------------------------------------------------------------------
  */
 herr_t
-H5O_attr_update_shared(H5F_t *f, hid_t dxpl_id, H5A_t *attr, 
-    const H5O_shared_t *sh_mesg)
+H5O_attr_update_shared(H5F_t *f, hid_t dxpl_id, H5A_t *attr,
+    H5O_shared_t *update_sh_mesg)
 {
+    H5O_shared_t sh_mesg;               /* Shared object header message */
     hsize_t attr_rc;                    /* Attribute's ref count in shared message storage */
     htri_t shared_mesg;                 /* Whether the message should be shared */
     herr_t ret_value = SUCCEED;         /* Return value */
@@ -569,7 +522,10 @@ H5O_attr_update_shared(H5F_t *f, hid_t dxpl_id, H5A_t *attr,
     /* check args */
     HDassert(f);
     HDassert(attr);
-    HDassert(sh_mesg);
+
+    /* Extract shared message info from current attribute (for later use) */
+    if(NULL == H5O_attr_get_share(attr, &sh_mesg))
+        HGOTO_ERROR(H5E_ATTR, H5E_BADMESG, FAIL, "can't get shared info")
 
     /* Store new version of message as a SOHM */
     /* (should always work, since we're not changing the size of the attribute) */
@@ -596,9 +552,14 @@ H5O_attr_update_shared(H5F_t *f, hid_t dxpl_id, H5A_t *attr,
     } /* end if */
 
     /* Remove the old attribute from the SOHM storage */
-    if(H5SM_try_delete(f, dxpl_id, H5O_ATTR_ID, sh_mesg) < 0)
+    if(H5SM_try_delete(f, dxpl_id, H5O_ATTR_ID, &sh_mesg) < 0)
         HGOTO_ERROR(H5E_ATTR, H5E_CANTFREE, FAIL, "unable to delete shared attribute in shared storage")
 
+    /* Extract updated shared message info from modified attribute, if requested */
+    if(update_sh_mesg)
+        if(NULL == H5O_attr_get_share(attr, update_sh_mesg))
+            HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "can't get shared info")
+
 done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5O_attr_update_shared() */
@@ -632,61 +593,33 @@ H5O_attr_write_cb(H5O_t UNUSED *oh, H5O_mesg_t *mesg/*in,out*/,
     HDassert(mesg);
     HDassert(!udata->found);
 
-    /* Check for shared message */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-        H5A_t shared_attr;             /* Copy of shared attribute */
-
-	/*
-	 * If the message is shared then then the native pointer points to an
-	 * H5O_MSG_SHARED message.  We use that information to look up the real
-	 * message in the global heap or some other object header.
-	 */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &shared_attr))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, H5_ITER_ERROR, "unable to read shared attribute")
-
-        /* Check for correct attribute message to modify */
-        if(HDstrcmp(shared_attr.name, udata->attr->name) == 0) {
-            /* Update the shared attribute in the SOHM storage */
-            if(H5O_attr_update_shared(udata->f, udata->dxpl_id, udata->attr, (const H5O_shared_t *)mesg->native) < 0)
+    /* Check for correct attribute message to modify */
+    if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->attr->name) == 0) {
+        /* Update the shared attribute in the SOHM storage */
+        if(mesg->flags & H5O_MSG_FLAG_SHARED) {
+            if(H5O_attr_update_shared(udata->f, udata->dxpl_id, udata->attr, (H5O_shared_t *)mesg->native) < 0)
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTUPDATE, H5_ITER_ERROR, "unable to update attribute in shared storage")
-
-            /* Extract updated shared message info from modified attribute */
-            if(NULL == H5O_attr_get_share(udata->attr, (H5O_shared_t *)mesg->native))
-                HGOTO_ERROR(H5E_ATTR, H5E_BADMESG, H5_ITER_ERROR, "can't get shared info")
-
-            /* Indicate that we found the correct attribute */
-            udata->found = TRUE;
         } /* end if */
 
-        /* Release copy of shared attribute */
-        H5O_attr_reset(&shared_attr);
-    } /* end if */
-    else {
-        /* Check for correct attribute message to modify */
-        if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->attr->name) == 0) {
-            /* Allocate storage for the message's data, if necessary */
-            if(((H5A_t *)mesg->native)->data == NULL)
-                if(NULL == (((H5A_t *)mesg->native)->data = H5FL_BLK_MALLOC(attr_buf, udata->attr->data_size)))
-                    HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, H5_ITER_ERROR, "memory allocation failed")
-
-            /* Copy the data */
-            HDmemcpy(((H5A_t *)mesg->native)->data, udata->attr->data, udata->attr->data_size);
-
-            /* Indicate that we found the correct attribute */
-            udata->found = TRUE;
-        } /* end if */
-    } /* end else */
+        /* Allocate storage for the message's data, if necessary */
+        if(((H5A_t *)mesg->native)->data == NULL)
+            if(NULL == (((H5A_t *)mesg->native)->data = H5FL_BLK_MALLOC(attr_buf, udata->attr->data_size)))
+                HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, H5_ITER_ERROR, "memory allocation failed")
+
+        /* Copy the data into the header message */
+        HDmemcpy(((H5A_t *)mesg->native)->data, udata->attr->data, udata->attr->data_size);
 
-    /* Set common info, if we found the correct attribute */
-    if(udata->found) {
         /* Mark message as dirty */
         mesg->dirty = TRUE;
 
-        /* Stop iterating */
-        ret_value = H5_ITER_STOP;
-
         /* Indicate that the object header was modified */
         *oh_flags_ptr |= H5AC__DIRTIED_FLAG;
+
+        /* Indicate that the attribute was found */
+        udata->found = TRUE;
+
+        /* Stop iterating */
+        ret_value = H5_ITER_STOP;
     } /* end if */
 
 done:
@@ -782,49 +715,22 @@ H5O_attr_rename_chk_cb(H5O_t UNUSED *oh, H5O_mesg_t *mesg/*in,out*/,
     H5O_iter_ren_t *udata = (H5O_iter_ren_t *)_udata;   /* Operator user data */
     herr_t ret_value = H5_ITER_CONT;   /* Return value */
 
-    FUNC_ENTER_NOAPI_NOINIT(H5O_attr_rename_chk_cb)
+    FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5O_attr_rename_chk_cb)
 
     /* check args */
     HDassert(oh);
     HDassert(mesg);
     HDassert(!udata->found);
 
-    /* Check for shared message */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-        H5A_t shared_attr;              /* Copy of shared attribute */
-
-	/*
-	 * If the message is shared then then the native pointer points to an
-	 * H5O_MSG_SHARED message.  We use that information to look up the real
-	 * message in the global heap or some other object header.
-	 */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &shared_attr))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, H5_ITER_ERROR, "unable to read shared attribute")
-
-        /* Check for existing attribute with new name */
-        if(HDstrcmp(shared_attr.name, udata->new_name) == 0) {
-            /* Indicate that we found an existing attribute with the new name*/
-            udata->found = TRUE;
-
-            /* Stop iterating */
-            ret_value = H5_ITER_STOP;
-        } /* end if */
+    /* Check for existing attribute with new name */
+    if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->new_name) == 0) {
+        /* Indicate that we found an existing attribute with the new name*/
+        udata->found = TRUE;
 
-        /* Release copy of shared attribute */
-        H5O_attr_reset(&shared_attr);
+        /* Stop iterating */
+        ret_value = H5_ITER_STOP;
     } /* end if */
-    else {
-        /* Check for existing attribute with new name */
-        if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->new_name) == 0) {
-            /* Indicate that we found an existing attribute with the new name*/
-            udata->found = TRUE;
 
-            /* Stop iterating */
-            ret_value = H5_ITER_STOP;
-        } /* end if */
-    } /* end else */
-
-done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5O_attr_rename_chk_cb() */
 
@@ -862,51 +768,24 @@ H5O_attr_rename_mod_cb(H5O_t *oh, H5O_mesg_t *mesg/*in,out*/,
     HDassert(mesg);
     HDassert(!udata->found);
 
-    /* Check for shared message */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-        H5A_t shared_attr;             /* Copy of shared attribute */
-
-	/*
-	 * If the message is shared then then the native pointer points to an
-	 * H5O_MSG_SHARED message.  We use that information to look up the real
-	 * message in the global heap or some other object header.
-	 */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &shared_attr))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, H5_ITER_ERROR, "unable to read shared attribute")
+    /* Find correct attribute message to rename */
+    if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->old_name) == 0) {
+        /* Change the name for the attribute */
+        H5MM_xfree(((H5A_t *)mesg->native)->name);
+        ((H5A_t *)mesg->native)->name = H5MM_xstrdup(udata->new_name);
 
-        /* Check for correct attribute message to modify */
-        if(HDstrcmp(shared_attr.name, udata->old_name) == 0) {
-            /* Change the name for the attribute */
-            H5MM_xfree(shared_attr.name);
-            shared_attr.name = H5MM_xstrdup(udata->new_name);
-
-            /* Mark message as dirty */
-            mesg->dirty = TRUE;
+        /* Mark message as dirty */
+        mesg->dirty = TRUE;
 
+        /* Check for shared message */
+        if(mesg->flags & H5O_MSG_FLAG_SHARED) {
             /* Update the shared attribute in the SOHM storage */
-            if(H5O_attr_update_shared(udata->f, udata->dxpl_id, &shared_attr, (const H5O_shared_t *)mesg->native) < 0)
+            if(H5O_attr_update_shared(udata->f, udata->dxpl_id, mesg->native, NULL) < 0)
                 HGOTO_ERROR(H5E_ATTR, H5E_CANTUPDATE, H5_ITER_ERROR, "unable to update attribute in shared storage")
-
-            /* Extract updated shared message info from modified attribute */
-            if(NULL == H5O_attr_get_share(&shared_attr, (H5O_shared_t *)mesg->native))
-                HGOTO_ERROR(H5E_ATTR, H5E_BADMESG, H5_ITER_ERROR, "can't get shared info")
-
-            /* Indicate that we found the correct attribute */
-            udata->found = TRUE;
         } /* end if */
-
-        /* Release copy of shared attribute */
-        H5O_attr_reset(&shared_attr);
-    } /* end if */
-    else {
-        /* Find correct attribute message to rename */
-        if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->old_name) == 0) {
-            /* Change the name for the attribute */
-            H5MM_xfree(((H5A_t *)mesg->native)->name);
-            ((H5A_t *)mesg->native)->name = H5MM_xstrdup(udata->new_name);
-
-            /* Mark message as dirty */
-            mesg->dirty = TRUE;
+        else {
+            /* Sanity check */
+            HDassert(H5O_attr_is_shared((H5A_t *)mesg->native) == FALSE);
 
             /* Check for attribute message changing size */
             if(HDstrlen(udata->new_name) != HDstrlen(udata->old_name)) {
@@ -947,27 +826,26 @@ H5O_attr_rename_mod_cb(H5O_t *oh, H5O_mesg_t *mesg/*in,out*/,
                     oh->nattrs++;
 
                 /* Append renamed attribute to object header */
-                /* (doesn't increment the link count on shared components because
-                 *      attributes no longer have a 'link' callback) */
-                if(H5O_msg_append_real(udata->f, udata->dxpl_id, oh, H5O_MSG_ATTR, 0, 0, attr, oh_flags_ptr) < 0)
+                /* (Don't let it become shared) */
+                if(H5O_msg_append_real(udata->f, udata->dxpl_id, oh, H5O_MSG_ATTR, (mesg->flags | H5O_MSG_FLAG_DONTSHARE), 0, attr, oh_flags_ptr) < 0)
                     HGOTO_ERROR(H5E_ATTR, H5E_CANTINSERT, H5_ITER_ERROR, "unable to relocate renamed attribute in header")
 
+                /* Sanity check */
+                HDassert(H5O_attr_is_shared(attr) == FALSE);
+
                 /* Release the local copy of the attribute */
                 H5O_msg_free_real(H5O_MSG_ATTR, attr);
             } /* end if */
+        } /* end else */
 
-            /* Indicate that we found an existing attribute with the old name */
-            udata->found = TRUE;
-        } /* end if */
-    } /* end else */
+        /* Indicate that the object header was modified */
+        *oh_flags_ptr |= H5AC__DIRTIED_FLAG;
+
+        /* Indicate that we found an existing attribute with the old name */
+        udata->found = TRUE;
 
-    /* Set common info, if we found the correct attribute */
-    if(udata->found) {
         /* Stop iterating */
         ret_value = H5_ITER_STOP;
-
-        /* Indicate that the object header was modified */
-        *oh_flags_ptr |= H5AC__DIRTIED_FLAG;
     } /* end if */
 
 done:
@@ -1036,6 +914,10 @@ H5O_attr_rename(const H5O_loc_t *loc, hid_t dxpl_id, const char *old_name,
         op.lib_op = H5O_attr_rename_mod_cb;
         if(H5O_msg_iterate_real(loc->file, oh, H5O_MSG_ATTR, TRUE, op, &udata, dxpl_id, &oh_flags) < 0)
             HGOTO_ERROR(H5E_ATTR, H5E_CANTUPDATE, FAIL, "error updating attribute")
+
+        /* Check that we found the attribute to rename */
+        if(!udata.found)
+            HGOTO_ERROR(H5E_ATTR, H5E_NOTFOUND, FAIL, "can't locate attribute with old name")
     } /* end else */
 
     /* Update the modification time, if any */
@@ -1162,35 +1044,8 @@ H5O_attr_remove_cb(H5O_t *oh, H5O_mesg_t *mesg/*in,out*/,
     HDassert(mesg);
     HDassert(!udata->found);
 
-    /* Check for shared message */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-        H5A_t shared_attr;             /* Copy of shared attribute */
-
-	/*
-	 * If the message is shared then then the native pointer points to an
-	 * H5O_MSG_SHARED message.  We use that information to look up the real
-	 * message in the global heap or some other object header.
-	 */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &shared_attr))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, H5_ITER_ERROR, "unable to read shared attribute")
-
-        /* Check for correct attribute message to modify */
-        if(HDstrcmp(shared_attr.name, udata->name) == 0)
-            /* Indicate that this message is the attribute to be deleted */
-            udata->found = TRUE;
-
-        /* Release copy of shared attribute */
-        H5O_attr_reset(&shared_attr);
-    } /* end if */
-    else {
-        /* Check for correct attribute message to modify */
-        if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->name) == 0)
-            /* Indicate that this message is the attribute to be deleted */
-            udata->found = TRUE;
-    } /* end else */
-
-    /* Check for finding correct message to delete */
-    if(udata->found) {
+    /* Check for correct attribute message to modify */
+    if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->name) == 0) {
         /* If the later version of the object header format, decrement attribute */
         /* (must be decremented before call to H5O_release_mesg(), in order for
          *      sanity checks to pass - QAK)
@@ -1202,11 +1057,14 @@ H5O_attr_remove_cb(H5O_t *oh, H5O_mesg_t *mesg/*in,out*/,
         if(H5O_release_mesg(udata->f, udata->dxpl_id, oh, mesg, TRUE, TRUE) < 0)
             HGOTO_ERROR(H5E_OHDR, H5E_CANTDELETE, H5_ITER_ERROR, "unable to convert into null message")
 
-        /* Stop iterating */
-        ret_value = H5_ITER_STOP;
-
         /* Indicate that the object header was modified */
         *oh_flags_ptr |= H5AC__DIRTIED_FLAG;
+
+        /* Indicate that this message is the attribute to be deleted */
+        udata->found = TRUE;
+
+        /* Stop iterating */
+        ret_value = H5_ITER_STOP;
     } /* end if */
 
 done:
@@ -1303,30 +1161,35 @@ H5O_attr_remove(const H5O_loc_t *loc, const char *name, hid_t dxpl_id)
             if(can_convert) {
                 /* Iterate over attributes, to put them into header */
                 for(u = 0; u < oh->nattrs; u++) {
-                    htri_t shared_mesg;     /* Should this message be stored in the Shared Message table? */
-                    unsigned mesg_flags = 0;    /* Message flags for attribute */
+                    htri_t shared_mesg;             /* Should this message be stored in the Shared Message table? */
 
-                    /* Should this message be written as a SOHM? */
-                    if((shared_mesg = H5SM_try_share(loc->file, dxpl_id, H5O_ATTR_ID, &(atable.attrs[u]))) > 0)
-                        /* Mark the message as shared */
-                        mesg_flags |= H5O_MSG_FLAG_SHARED;
+                    /* Check if attribute is shared */
+                    if((shared_mesg = H5O_attr_is_shared(&(atable.attrs[u]))) < 0)
+                        HGOTO_ERROR(H5E_ATTR, H5E_CANTGET, FAIL, "error determining if message is shared")
                     else if(shared_mesg == 0) {
                         /* Increment reference count on attribute components */
                         /* (so that they aren't deleted when the dense attribute storage is deleted) */
                         if(H5O_attr_link(loc->file, dxpl_id, &(atable.attrs[u])) < 0)
                             HGOTO_ERROR(H5E_ATTR, H5E_LINKCOUNT, FAIL, "unable to adjust attribute link count")
                     } /* end if */
-                    else if(shared_mesg < 0)
-                        HGOTO_ERROR(H5E_OHDR, H5E_WRITEERROR, FAIL, "error determining if message should be shared")
+                    else {
+                        /* Reset 'shared' status, so attributes will be shared again */
+                        atable.attrs[u].sh_loc.flags = 0;
+                    } /* end else */
 
                     /* Insert attribute message into object header */
-                    if(H5O_msg_append_real(loc->file, dxpl_id, oh, H5O_MSG_ATTR, mesg_flags, H5O_UPDATE_TIME, &(atable.attrs[u]), &oh_flags) < 0)
+                    /* (Will increment reference count on shared attributes) */
+                    if(H5O_msg_append_real(loc->file, dxpl_id, oh, H5O_MSG_ATTR, 0, 0, &(atable.attrs[u]), &oh_flags) < 0)
                         HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, FAIL, "can't create message")
                 } /* end for */
 
                 /* Remove the dense storage */
                 if(H5A_dense_delete(loc->file, dxpl_id, oh) < 0)
                     HGOTO_ERROR(H5E_ATTR, H5E_CANTDELETE, FAIL, "unable to delete dense attribute storage")
+
+                /* Update the modification time, if any */
+                if(H5O_touch_oh(loc->file, dxpl_id, oh, FALSE, &oh_flags) < 0)
+                    HGOTO_ERROR(H5E_ATTR, H5E_CANTUPDATE, FAIL, "unable to update time on object")
             } /* end if */
 
             /* Free attribute table information */
@@ -1411,46 +1274,21 @@ H5O_attr_exists_cb(H5O_t UNUSED *oh, H5O_mesg_t *mesg/*in,out*/,
     H5O_iter_rm_t *udata = (H5O_iter_rm_t *)_udata;   /* Operator user data */
     herr_t ret_value = H5_ITER_CONT;    /* Return value */
 
-    FUNC_ENTER_NOAPI_NOINIT(H5O_attr_exists_cb)
+    FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5O_attr_exists_cb)
 
     /* check args */
     HDassert(mesg);
     HDassert(!udata->found);
 
-    /* Check for shared message */
-    if(mesg->flags & H5O_MSG_FLAG_SHARED) {
-        H5A_t shared_attr;             /* Copy of shared attribute */
-
-	/*
-	 * If the message is shared then then the native pointer points to an
-	 * H5O_MSG_SHARED message.  We use that information to look up the real
-	 * message in the global heap or some other object header.
-	 */
-        if(NULL == H5O_shared_read(udata->f, udata->dxpl_id, (const H5O_shared_t *)mesg->native, H5O_MSG_ATTR, &shared_attr))
-	    HGOTO_ERROR(H5E_ATTR, H5E_CANTINIT, H5_ITER_ERROR, "unable to read shared attribute")
-
-        /* Check for correct attribute message */
-        if(HDstrcmp(shared_attr.name, udata->name) == 0)
-            /* Indicate that this message is the attribute sought */
-            udata->found = TRUE;
-
-        /* Release copy of shared attribute */
-        H5O_attr_reset(&shared_attr);
-    } /* end if */
-    else {
-        /* Check for correct attribute message */
-        if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->name) == 0)
-            /* Indicate that this message is the attribute sought */
-            udata->found = TRUE;
-    } /* end else */
+    /* Check for correct attribute message */
+    if(HDstrcmp(((H5A_t *)mesg->native)->name, udata->name) == 0) {
+        /* Indicate that this message is the attribute sought */
+        udata->found = TRUE;
 
-    /* Check for finding correct message to delete */
-    if(udata->found) {
         /* Stop iterating */
         ret_value = H5_ITER_STOP;
     } /* end if */
 
-done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5O_attr_exists_cb() */
 
diff --git a/src/H5Omessage.c b/src/H5Omessage.c
index 6d4a15c..1c9235e 100644
--- a/src/H5Omessage.c
+++ b/src/H5Omessage.c
@@ -2086,7 +2086,7 @@ H5O_new_mesg(H5F_t *f, H5O_t *oh, unsigned *mesg_flags, const H5O_msg_class_t *o
 
         /* Check if message is already shared */
         if((shared_mesg = H5O_msg_is_shared((*new_type)->id, (*new_mesg))) < 0)
-            HGOTO_ERROR(H5E_OHDR, H5E_WRITEERROR, UFAIL, "error determining if message is shared")
+            HGOTO_ERROR(H5E_OHDR, H5E_CANTGET, UFAIL, "error determining if message is shared")
         else if(shared_mesg > 0) {
             /* Increment message's reference count */
             if((*new_type)->link && ((*new_type)->link)(f, dxpl_id, (*new_mesg)) < 0)
@@ -2094,7 +2094,7 @@ H5O_new_mesg(H5F_t *f, H5O_t *oh, unsigned *mesg_flags, const H5O_msg_class_t *o
             *mesg_flags |= H5O_MSG_FLAG_SHARED;
         } /* end if */
         else {
-            /* Check for unsharable message */
+            /* Avoid unsharable messages */
             if(!(*mesg_flags & H5O_MSG_FLAG_DONTSHARE)) {
                 /* Attempt to share message */
                 if((shared_mesg = H5SM_try_share(f, dxpl_id, (*new_type)->id, (*new_mesg))) > 0)
@@ -2222,7 +2222,7 @@ H5O_msg_delete(H5F_t *f, hid_t dxpl_id, unsigned type_id, const void *mesg)
     HDassert(type);
 
     /* delete */
-    if((type->del) && (type->del)(f, dxpl_id, mesg, 1) < 0)
+    if((type->del) && (type->del)(f, dxpl_id, mesg, TRUE) < 0)
         HGOTO_ERROR(H5E_OHDR, H5E_CANTDELETE, FAIL, "unable to delete file space for object header message")
 
 done:
diff --git a/src/H5Opkg.h b/src/H5Opkg.h
index 83065d4..0a0310d 100644
--- a/src/H5Opkg.h
+++ b/src/H5Opkg.h
@@ -160,7 +160,7 @@
 /* Temporary macro to define which message classes are using the "new"
  * shared message "interface" for their methods.
  */
-#define H5O_NEW_SHARED(T)       ((T) == H5O_MSG_PLINE || (T) == H5O_MSG_FILL_NEW || (T) == H5O_MSG_FILL || (T) == H5O_MSG_SDSPACE || (T) == H5O_MSG_DTYPE)
+#define H5O_NEW_SHARED(T)       ((T) == H5O_MSG_PLINE || (T) == H5O_MSG_FILL_NEW || (T) == H5O_MSG_FILL || (T) == H5O_MSG_SDSPACE || (T) == H5O_MSG_DTYPE || (T) == H5O_MSG_ATTR)
 
 
 /* The "message class" type */