BUG #7594: "tuple concurrently updated" error on concurrent GRANT ON DATABASE statements

The following bug has been logged on the website:

Bug reference: 7594
Logged by: Reinhard Max
Email address: max(at)suse(dot)de
PostgreSQL version: 9.1.6
Operating system: openSUSE
Description:

Is it intentional, that concurrent GRANT statements for the same database
may result in a "tuple concurrently updated" error?

It can be reproduced by running the following loop from two shells at the
same time:

while true; do psql -c "GRANT ALL ON DATABASE foo TO foo" ; done

I found that wrapping the GRANT statement in a transaction and explicitly
locking pg_table in SHARE UPDATE EXCLUSIVE mode makes the error disappear.
According to the documentation of the lock modes, statements like VACUUM
acquire this lock by itself, so I wonder if GRANT ON DATABASE should do the
same.