Fixed HDFFV-10214:

* Updated comments in the library to accurately reflect the semantics
  of the H5FD_FEAT_POSIX_COMPAT_HANDLE flag.
* Removed the check for the POSIX compatible handle feature flag
  when determining if a VFD supports SWMR.
Partial work for HDFFV-10197:
* Added a new H5FD_FEAT_DEFAULT_VFD_COMPATIBLE feature flag that is
  set when a VFD creates output that is compatible with the default
  VFD. Will be used in the testing but might also be generally useful
  to users.

8 files changed, 28 insertions, 17 deletions

diff --git a/src/H5FDcore.c b/src/H5FDcore.c
index d100a8b..b980b7e 100644
--- a/src/H5FDcore.c
+++ b/src/H5FDcore.c
@@ -982,9 +982,11 @@ H5FD__core_query(const H5FD_t * _file, unsigned long *flags /* out */)
         *flags |= H5FD_FEAT_ALLOW_FILE_IMAGE;               /* OK to use file image feature with this VFD                       */
         *flags |= H5FD_FEAT_CAN_USE_FILE_IMAGE_CALLBACKS;   /* OK to use file image callbacks with this VFD                     */
 
-        /* If the backing store is open, a POSIX file handle is available */
-        if(file && file->fd >= 0 && file->backing_store)
-            *flags |= H5FD_FEAT_POSIX_COMPAT_HANDLE; /* VFD handle is POSIX I/O call compatible */
+        /* These feature flags are only applicable if the backing store is enabled */
+        if(file && file->fd >= 0 && file->backing_store) {
+            *flags |= H5FD_FEAT_POSIX_COMPAT_HANDLE;        /* get_handle callback returns a POSIX file descriptor              */
+            *flags |= H5FD_FEAT_DEFAULT_VFD_COMPATIBLE;     /* VFD creates a file which can be opened with the default VFD      */
+        }
     } /* end if */
 
     FUNC_LEAVE_NOAPI(SUCCEED)
diff --git a/src/H5FDdirect.c b/src/H5FDdirect.c
index 1487cda..7a4c99c 100644
--- a/src/H5FDdirect.c
+++ b/src/H5FDdirect.c
@@ -698,6 +698,7 @@ H5FD_direct_query(const H5FD_t H5_ATTR_UNUSED * _f, unsigned long *flags /* out
         *flags |= H5FD_FEAT_ACCUMULATE_METADATA;    /* OK to accumulate metadata for faster writes                      */
         *flags |= H5FD_FEAT_DATA_SIEVE;             /* OK to perform data sieving for faster raw data reads & writes    */
         *flags |= H5FD_FEAT_AGGREGATE_SMALLDATA;    /* OK to aggregate "small" raw data allocations                     */
+        *flags |= H5FD_FEAT_DEFAULT_VFD_COMPATIBLE; /* VFD creates a file which can be opened with the default VFD      */
     }
 
     FUNC_LEAVE_NOAPI(SUCCEED)
diff --git a/src/H5FDlog.c b/src/H5FDlog.c
index 75333c2..7c6bbd4 100644
--- a/src/H5FDlog.c
+++ b/src/H5FDlog.c
@@ -895,8 +895,9 @@ H5FD_log_query(const H5FD_t *_file, unsigned long *flags /* out */)
         *flags |= H5FD_FEAT_ACCUMULATE_METADATA;    /* OK to accumulate metadata for faster writes                      */
         *flags |= H5FD_FEAT_DATA_SIEVE;             /* OK to perform data sieving for faster raw data reads & writes    */
         *flags |= H5FD_FEAT_AGGREGATE_SMALLDATA;    /* OK to aggregate "small" raw data allocations                     */
-        *flags |= H5FD_FEAT_POSIX_COMPAT_HANDLE;    /* VFD handle is POSIX I/O call compatible                          */
+        *flags |= H5FD_FEAT_POSIX_COMPAT_HANDLE;    /* get_handle callback returns a POSIX file descriptor              */
         *flags |= H5FD_FEAT_SUPPORTS_SWMR_IO;       /* VFD supports the single-writer/multiple-readers (SWMR) pattern   */
+        *flags |= H5FD_FEAT_DEFAULT_VFD_COMPATIBLE; /* VFD creates a file which can be opened with the default VFD      */
 
         /* Check for flags that are set by h5repart */
         if(file && file->fam_to_sec2)
diff --git a/src/H5FDmpio.c b/src/H5FDmpio.c
index ace91f8..f594d8e 100644
--- a/src/H5FDmpio.c
+++ b/src/H5FDmpio.c
@@ -1150,10 +1150,11 @@ H5FD_mpio_query(const H5FD_t H5_ATTR_UNUSED *_file, unsigned long *flags /* out
     /* Set the VFL feature flags that this driver supports */
     if(flags) {
         *flags=0;
-        *flags|=H5FD_FEAT_AGGREGATE_METADATA;  /* OK to aggregate metadata allocations */
-        *flags|=H5FD_FEAT_AGGREGATE_SMALLDATA; /* OK to aggregate "small" raw data allocations */
-        *flags|=H5FD_FEAT_HAS_MPI;             /* This driver uses MPI */
-        *flags|=H5FD_FEAT_ALLOCATE_EARLY;      /* Allocate space early instead of late */
+        *flags |= H5FD_FEAT_AGGREGATE_METADATA;     /* OK to aggregate metadata allocations                             */
+        *flags |= H5FD_FEAT_AGGREGATE_SMALLDATA;    /* OK to aggregate "small" raw data allocations                     */
+        *flags |= H5FD_FEAT_HAS_MPI;                /* This driver uses MPI                                             */
+        *flags |= H5FD_FEAT_ALLOCATE_EARLY;         /* Allocate space early instead of late                             */
+        *flags |= H5FD_FEAT_DEFAULT_VFD_COMPATIBLE; /* VFD creates a file which can be opened with the default VFD      */
     } /* end if */
 
     FUNC_LEAVE_NOAPI(SUCCEED)
diff --git a/src/H5FDpublic.h b/src/H5FDpublic.h
index 3032e8a..45b1237 100644
--- a/src/H5FDpublic.h
+++ b/src/H5FDpublic.h
@@ -250,6 +250,14 @@ typedef enum H5F_mem_t	H5FD_mem_t;
      * This is specifically used for the multi/split driver.
      */
 #define H5FD_FEAT_PAGED_AGGR		0x00004000
+    /*
+     * Defining H5FD_FEAT_DEFAULT_VFD_COMPATIBLE for a VFL driver
+     * that creates a file which is compatible with the default VFD.
+     * Generally, this means that the VFD creates a single file that follows
+     * the canonical HDF5 file format.
+     */
+#define H5FD_FEAT_DEFAULT_VFD_COMPATIBLE        0x00008000
+
 
 /* Forward declaration */
 typedef struct H5FD_t H5FD_t;
diff --git a/src/H5FDsec2.c b/src/H5FDsec2.c
index 26913e2..b6385fb 100644
--- a/src/H5FDsec2.c
+++ b/src/H5FDsec2.c
@@ -527,8 +527,9 @@ H5FD_sec2_query(const H5FD_t *_file, unsigned long *flags /* out */)
         *flags |= H5FD_FEAT_ACCUMULATE_METADATA;    /* OK to accumulate metadata for faster writes                      */
         *flags |= H5FD_FEAT_DATA_SIEVE;             /* OK to perform data sieving for faster raw data reads & writes    */
         *flags |= H5FD_FEAT_AGGREGATE_SMALLDATA;    /* OK to aggregate "small" raw data allocations                     */
-        *flags |= H5FD_FEAT_POSIX_COMPAT_HANDLE;    /* VFD handle is POSIX I/O call compatible                          */
+        *flags |= H5FD_FEAT_POSIX_COMPAT_HANDLE;    /* get_handle callback returns a POSIX file descriptor              */
         *flags |= H5FD_FEAT_SUPPORTS_SWMR_IO;       /* VFD supports the single-writer/multiple-readers (SWMR) pattern   */
+        *flags |= H5FD_FEAT_DEFAULT_VFD_COMPATIBLE; /* VFD creates a file which can be opened with the default VFD      */
 
         /* Check for flags that are set by h5repart */
         if(file && file->fam_to_sec2)
diff --git a/src/H5FDstdio.c b/src/H5FDstdio.c
index 5023af3..861c6a6 100644
--- a/src/H5FDstdio.c
+++ b/src/H5FDstdio.c
@@ -555,10 +555,11 @@ H5FD_stdio_query(const H5FD_t *_f, unsigned long /*OUT*/ *flags)
      */
     if(flags) {
         *flags = 0;
-        *flags|=H5FD_FEAT_AGGREGATE_METADATA; /* OK to aggregate metadata allocations */
-        *flags|=H5FD_FEAT_ACCUMULATE_METADATA; /* OK to accumulate metadata for faster writes */
-        *flags|=H5FD_FEAT_DATA_SIEVE;       /* OK to perform data sieving for faster raw data reads & writes */
-        *flags|=H5FD_FEAT_AGGREGATE_SMALLDATA; /* OK to aggregate "small" raw data allocations */
+        *flags |= H5FD_FEAT_AGGREGATE_METADATA;     /* OK to aggregate metadata allocations                             */
+        *flags |= H5FD_FEAT_ACCUMULATE_METADATA;    /* OK to accumulate metadata for faster writes                      */
+        *flags |= H5FD_FEAT_DATA_SIEVE;             /* OK to perform data sieving for faster raw data reads & writes    */
+        *flags |= H5FD_FEAT_AGGREGATE_SMALLDATA;    /* OK to aggregate "small" raw data allocations                     */
+        *flags |= H5FD_FEAT_DEFAULT_VFD_COMPATIBLE; /* VFD creates a file which can be opened with the default VFD      */
     }
 
     return 0;
diff --git a/src/H5Fint.c b/src/H5Fint.c
index fe532b2..e52d539 100644
--- a/src/H5Fint.c
+++ b/src/H5Fint.c
@@ -715,10 +715,6 @@ H5F_new(H5F_file_t *shared, unsigned flags, hid_t fcpl_id, hid_t fapl_id, H5FD_t
         if(!H5F_HAS_FEATURE(f, H5FD_FEAT_SUPPORTS_SWMR_IO) && (H5F_INTENT(f) & (H5F_ACC_SWMR_WRITE | H5F_ACC_SWMR_READ)))
             HGOTO_ERROR(H5E_FILE, H5E_BADVALUE, NULL, "must use a SWMR-compatible VFD when SWMR is specified")
 
-        /* Require a POSIX compatible VFD to use SWMR feature */
-        /* (It's reasonable to try to expand this to other VFDs eventually -QAK) */
-        if(!H5F_HAS_FEATURE(f, H5FD_FEAT_POSIX_COMPAT_HANDLE) && (H5F_INTENT(f) & (H5F_ACC_SWMR_WRITE | H5F_ACC_SWMR_READ)))
-            HGOTO_ERROR(H5E_FILE, H5E_BADVALUE, NULL, "must use POSIX compatible VFD with SWMR write access")
         if(H5FD_get_fs_type_map(lf, f->shared->fs_type_map) < 0)
             HGOTO_ERROR(H5E_FILE, H5E_CANTGET, NULL, "can't get free space type mapping from VFD")
         if(H5MF_init_merge_flags(f) < 0)