How to keep an unique counter per row with PostgreSQL?

I need to keep an unique (per-row) revision number in a document_revisions table, where the revision number is scoped to a document, so it’s not unique to the whole table, only to the related document.

I initially came up with something like:

current_rev = SELECT MAX(rev) FROM document_revisions WHERE document_id = 123;
INSERT INTO document_revisions(rev) VALUES(current_rev + 1);

But there is a race condition!

I’m trying to solve it with pg_advisory_lock, but the documentation is a bit scarce and I don’t fully understand it, and I don’t want to lock something by mistake.

Is the following acceptable, or am I doing it wrong, or is there a better solution?

SELECT pg_advisory_lock(123);
current_rev = SELECT MAX(rev) FROM document_revisions WHERE document_id = 123;
INSERT INTO document_revisions(rev) VALUES(current_rev + 1);
SELECT pg_advisory_unlock(123);

Shouldn’t I lock the document row (key1) for a given operation (key2) instead? So that would be the proper solution:

SELECT pg_advisory_lock(id, 1) FROM documents WHERE id = 123;
current_rev = SELECT MAX(rev) FROM document_revisions WHERE document_id = 123;
INSERT INTO document_revisions(rev) VALUES(current_rev + 1);
SELECT pg_advisory_unlock(id, 1) FROM documents WHERE id = 123;

Maybe I’m not used to PostgreSQL and a SERIAL can be scoped, or maybe a sequence and nextval() would do the job better?