I realized that for our use case, we'd ideally wait for holders of RowExclusiveLock only, and not e.g. VACUUM holding ShareUpdateExclusiveLock. Waiting for lockers in a specific mode seems possible by generalizing/duplicating WaitForLockersMultiple() and GetLockConflicts(), but I'd love to have a sanity check before attempting that. Also, I imagine those semantics might be too different to make sense as part of the LOCK command.
Alternatively, I had originally been trying to use the pg_locks view, which obviously provides flexibility in identifying existing lock holders. But I couldn't find a way to wait for the locks to be released / transactions to finish, and I was a little concerned about the performance impact of selecting from it frequently when we only care about a subset of the locks, although I didn't try to assess that in our particular application. In any case, I'm looking forward to hearing more feedback from reviewers and potential users. :-)
