Address memory issues when copying empty enums (#3088)

When copying an empty enum type (including implicitly, as when an enum
is contained in a compound type), the library would allocate 0-size
blocks of memory and attempt to memcpy 0 bytes from NULL pointers, which
are undefined behavior. In debug mode, the library would raise an
assert in H5MM.

The library now avoid undefined memory operations when copying empty
enum types and a test that copies empty enums has been added.

1 files changed, 11 insertions, 0 deletions

diff --git a/release_docs/RELEASE.txt b/release_docs/RELEASE.txt
index 7153c4e..559d474 100644
--- a/release_docs/RELEASE.txt
+++ b/release_docs/RELEASE.txt
@@ -228,6 +228,17 @@ Bug Fixes since HDF5-1.14.0 release
 ===================================
     Library
     -------
+    - Fixed a potential bug when copying empty enum datatypes
+
+      Copying an empty enum datatype (including implicitly, as when an enum
+      is a part of a compound datatype) would fail in an assert in debug
+      mode and could fail in release mode depending on how the platform
+      handles undefined behavior regarding size 0 memory allocations and
+      using memcpy with a NULL src pointer.
+
+      The library is now more more careful about using memory operations when
+      copying empty enum datatypes and will not error or raise an assert.
+
     - Added an AAPL check to H5Acreate
 
       A check was added to H5Acreate to ensure that a failure is correctly