Always unlink first segment for AO/AOCO temp relation.

[reshke]

Usually relfiles from deleted relation persist until next CHECKPOINT, and unlinked only then. For temporary relation. this is not the case, and this is explicitly handled in mdunlinkfork

https://github.com/postgres/postgres/blob/868825a/src/backend/storage/smgr/md.c#L384-L385

With GPDB 6 -> 7 refactorings we ended up using separate unlink functions for relation obsoletion. In gpdb 6 we used to call mdunlink_ao from mdunlinkfork while in gpdb 7 we are not. This leads to orphan files after temporal relation drop like in scenario:

create tablespace  t location '/tmp/tts';
create temp table t (i int) using ao_row tablespace t distributed by (i);
drop table t;

Fix this by importing logic from mdunlinkfork

#1644

[my-ship-it]

Thanks for catching this！

Minor comments below:

int ret; is declared at function scope.
Better to be scoped inside the if block, or removed entirely since it's only checked against < 0 immediately.

[my-ship-it]

Should also unlink the segment files beyond segment 0?

How about modifying mdunlink_ao_base_relfile() and mdunlink_ao_perFile() to check RelFileNodeBackendIsTemp(rnode) and unlink immediately (mirroring the heap mdunlinkfork pattern), rather than appending a separate unlink at the end of mdunlink_ao?

Something like

  if (!unlinkFiles->isRedo)
  {
      if (RelFileNodeBackendIsTemp(unlinkFiles->rnode))
      {
          ret = unlink(baserel);
          if (ret < 0 && errno != ENOENT)
              ereport(WARNING, ...);
      }
      else
      {
          // Existing truncate + deferred unlink logic
          ...
      }
  }