Repository: Unidata/netcdf-c (c)
Issue: calling nc_redef() twice on a netCDF/HDF5 file returns NC_EINDEFINE, but should not, unless classic flag is used
According to netcdf.h:

```
/** Operation not allowed in define mode.

The specified netCDF is in define mode rather than data mode.

With netCDF-4/HDF5 files, this error will not occur, unless
::NC_CLASSIC_MODEL was used in nc_create().
 */
#define	NC_EINDEFINE	(-39)
```

Yet calling nc_redef() twice on a netCDF/HDF5 file results in this error. The problem is in libhdf5/hdf5file.c, line 429:
```
    /* If we're already in define mode, return an error. */
    if (nc4_info->flags & NC_INDEF)
        return NC_EINDEFINE;

```

Instead this should check the classic flag, and return error if it is set, and NC_NOERR if it is not set.

Pull request description: Fix nc_redef() - when called twice on netcdf-4 file, it should return no error
For classic files (or netCDF/HDF5 files with classic model), calling nc_redef() twice should and does result in NC_EINDEFINE error.

But, in accordance with the automatic handling of enddef/redef for netCDF/HDF5 files, extra calls to redef should just be ignored for netCDF/HDF5 files.

THis PR fixes this issue and makes the code work in accordance with the documentation.

Fixes #1779 

Patch:
diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 48a8b2fcb..5fe366f57 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -7,6 +7,7 @@ This file contains a high-level description of this package's evolution. Release
 
 ## 4.8.0 - TBD
 
+* [Bug Fix] Don't return error for extra calls to nc_redef() for netCDF/HDF5 files, unless classic model is in use. See [https://github.com/Unidata/netcdf-c/issues/1779].
 * [Bug Fix] Now allow szip to be used on variables with unlimited dimension [https://github.com/Unidata/netcdf-c/issues/1774].
 * [Enhancement] Add support for cloud storage using a variant of the Zarr storage format. Warning: this feature is highly experimental and is subject to rapid evolution [https://www.unidata.ucar.edu/blogs/developer/en/entry/overview-of-zarr-support-in].
 * [Bug Fix] Fix nccopy to properly set default chunking parameters when not otherwise specified. This can significantly improve performance in selected cases. Note that if seeing slow performance with nccopy, then, as a work-around, specifically set the chunking parameters. [https://github.com/Unidata/netcdf-c/issues/1763].
diff --git a/libhdf5/hdf5file.c b/libhdf5/hdf5file.c
index 197b8441f..131a28569 100644
--- a/libhdf5/hdf5file.c
+++ b/libhdf5/hdf5file.c
@@ -425,9 +425,10 @@ NC4_redef(int ncid)
         return retval;
     assert(nc4_info);
 
-    /* If we're already in define mode, return an error. */
+    /* If we're already in define mode, return an error for classic
+     * files, or netCDF/HDF5 files when classic mode is in use. */
     if (nc4_info->flags & NC_INDEF)
-        return NC_EINDEFINE;
+	return (nc4_info->cmode & NC_CLASSIC_MODEL) ? NC_EINDEFINE : NC_NOERR;
 
     /* If the file is read-only, return an error. */
     if (nc4_info->no_write)


Test patch:
diff --git a/nc_test4/tst_files.c b/nc_test4/tst_files.c
index d8927d8ec..e0f7ddf30 100644
--- a/nc_test4/tst_files.c
+++ b/nc_test4/tst_files.c
@@ -497,6 +497,20 @@ test_redef(int format)
    if (strcmp(var_name, REDEF_VAR2_NAME) || xtype_in != NC_BYTE || ndims != REDEF_NDIMS ||
        dimids_in[0] != dimids[0] || dimids_in[1] != dimids[1]) ERR;
 
+   /* Try enddef/redef. */
+   if (nc_enddef(ncid)) ERR;
+   if (nc_redef(ncid)) ERR;
+
+   /* NetCDF/HDF5 files ignore the repeated redef(). */
+   if (format != NC_FORMAT_NETCDF4)
+   {
+       if (nc_redef(ncid) != NC_EINDEFINE) ERR;
+   }
+   else
+   {
+       if (nc_redef(ncid)) ERR;
+   }
+   
    /* Close it up. */
    if (format != NC_FORMAT_NETCDF4)
       if (nc_enddef(ncid)) ERR;