Improve the fix for Unidata#350 included in Unidata#1119

1) We have to use H5Tequal() to compare HDF5 type IDs.
2) When checking if we can re-use an NC_CHAR attribute it is enough to
   compare data types (H5Tequal() takes care of the size comparison).
3) This commit adds missing code (reuse_att was set but not used).

Now an attribute in a NetCDF-4 file can be modified as many times as
necessary, as long its type and length remain the same.

Modifications changing either type or length of an attribute require
deleting and re-creating an attribute which increments the attribute
order creation index. Once this index reaches 65535 all attribute
modifications (for a particular group or variable) will fail.

For reference:

Issue 350 title: NetCDF-4 limits the number of times an attribute can
be modified

Pull request 1119 title: Fix checking for HDF5 max dims, no longer
re-create atts if not needed, confirm behavior for HDF5 cyclical
files, allow user to set mpiexec

libhdf5/nc4hdf.c

@@ -512,7 +512,6 @@ put_att_grpa(NC_GRP_INFO_T *grp, int varid, NC_ATT_INFO_T *att)
  hid_t existing_att_typeid = 0, existing_attid = 0, existing_spaceid = 0;
  hsize_t dims[1]; /* netcdf attributes always 1-D. */
  htri_t attr_exists;
- int reuse_att = 0; /* Will be true if we can re-use an existing att. */
  void *data;
  int phoney_data = 99;
  int retval = NC_NOERR;
@@ -612,31 +611,59 @@ put_att_grpa(NC_GRP_INFO_T *grp, int varid, NC_ATT_INFO_T *att)
  BAIL(NC_EATTMETA);
 
  /* How big is the attribute? */
- if ((existing_spaceid = H5Aget_space(existing_attid)) < 0)
+ if (att->nc_typeid == NC_CHAR) {
+ /* For text attributes the size is specified in the datatype
+ and it is enough to compare types using H5Tequal(). Here
+ we set npoints to disable the second part of the
+ comparison below. */
+ npoints = att->len;
+ }
+ else
+ {
+ if ((existing_spaceid = H5Aget_space(existing_attid)) < 0)
  BAIL(NC_EATTMETA);
- if ((npoints = H5Sget_simple_extent_npoints(existing_spaceid)) < 0)
+  if ((npoints = H5Sget_simple_extent_npoints(existing_spaceid)) < 0)
  BAIL(NC_EATTMETA);
+ }
 
- /* Delete the attribute. */
- if (file_typeid != existing_att_typeid || npoints != att->len)
+ if (!H5Tequal(file_typeid, existing_att_typeid) || npoints != att->len)
  {
+ /* The attribute exists but we cannot re-use it. */
+
+ /* Delete the attribute. */
  if (H5Adelete(locid, att->hdr.name) < 0)
  BAIL(NC_EHDFERR);
+
+ /* Re-create the attribute with the type and length
+ reflecting the new value (or values). */
+ if ((attid = H5Acreate(locid, att->hdr.name, file_typeid, spaceid,
+ H5P_DEFAULT)) < 0)
+ BAIL(NC_EATTMETA);
+
+ /* Write the values, (even if length is zero). */
+ if (H5Awrite(attid, file_typeid, data) < 0)
+ BAIL(NC_EATTMETA);
  }
  else
  {
- reuse_att++;
+ /* The attribute exists and we can re-use it. */
+
+ /* Write the values, re-using the existing attribute. */
+ if (H5Awrite(existing_attid, file_typeid, data) < 0)
+ BAIL(NC_EATTMETA);
  }
- }
+ } else {
+ /* The attribute does not exist yet. */
 
- /* Create the attribute. */
- if ((attid = H5Acreate(locid, att->hdr.name, file_typeid, spaceid,
- H5P_DEFAULT)) < 0)
+  /* Create the attribute. */
+  if ((attid = H5Acreate(locid, att->hdr.name, file_typeid, spaceid,
+  H5P_DEFAULT)) < 0)
  BAIL(NC_EATTMETA);
 
- /* Write the values, (even if length is zero). */
- if (H5Awrite(attid, file_typeid, data) < 0)
+  /* Write the values, (even if length is zero). */
+  if (H5Awrite(attid, file_typeid, data) < 0)
  BAIL(NC_EATTMETA);
+ }
 
 exit:
  if (file_typeid && H5Tclose(file_typeid))