Thread

injection_points: Switch wait/wakeup to use atomics rather than latches

Michael Paquier <michael@paquier.xyz> — 2026-05-28T02:43:29Z

Hi all,
(Adding Andrey in CC, as I'm sure he is interested in that.)

While looking at the test proposed on the thread about the ProcKill(),
I have been reminded about the fact that relying on latches and a
condition variable for the wait and the wakeups has its limits:
https://www.postgresql.org/message-id/aheVjCHmcbXBtiy0%40paquier.xyz

In this case, we are trying to synchronize backends once they don't
have latch assigned anymore, which defeats the purpose of wait/wakeup
because the condition variable used in injection_points while waiting
expects a Latch to be set for the processes we are waiting on.

Folks have complained about this limitation a couple of times in the
past, and I never got around to do something about it.  While looking
at that I have finished with the patch attached, which was
surprisingly simpler than what I thought was needed.  This replaces
the condition variable with a set of atomic counters.  The counters
are incremented at wakeup, and the wait checks them on a periodic
basis.  The wait loop uses a delay that increases over time, maxed at
100ms so as we can get a good responsiveness on fast machines, without
burning CPU for nothing in tests that require more wait time due to a
tight loop with the counter checks.

One thing worth noticing is the CHECK_FOR_INTERRUPTS() in the wait
loop, which is something we need for the autovacuum test in test_misc
that requires some signaling and interrupt processing.

It may make sense to be conservative and limit ourselves to do this
change on HEAD, but I'd like to suggest a backpatch down to v17 so as
future tests that rely on a such change can be backpatched.  I would
need this change for the other test, still consistency in the facility
primes for me here.

Note: The CI seems happy with the patch.

Thoughts or comments?
--
Michael