Usually the committer. Most patches sent to the mailing list does not contain a commit message.

Generally the committer writes the commit message. There is occassionally discussion of specific wording ahead of time.

Yep this is what I meant. I could have chosen my words in a better way though, since we were talking about a git commit. Having said that quoting commit message 2nd last paragraph:

The Postgres project thanks Mitsumasa Kondo for discovering this bug, Kyotaro Horiguchi for drafting the fix, and Noah Misch for recognizing the full extent of the danger.

We can't exclude without further info that Tom contributed to the fix, just saying.

Knowing him after a few years spent following postgres dev process I bet on co-authoring at least :)