Re: Let's drop support for named POSIX semaphores

Hi,

On 2026-10-26 11:42:56 -0400, Tom Lane wrote:
> So I think the direction we'd have to go in to make that happen is to
> replace all the semaphore stuff with futexes or some other low-level
> primitive, as Munro speculated about in one of the threads I cited.

Agreed, that's the right direction. We really aren't using semaphores in a way
that benefits from them being semaphores. In fact, we have to fix up the
semaphore counters in a bunch of places that would not need such complication
with a better primitive.

FWIW, I recalled Thomas posting patches for macos futexes before. I didn't
immediately find them, but did quickly find the branch that implements futexes
on macos:

https://github.com/macdice/postgres/commits/pg_futex/
https://github.com/macdice/postgres/commit/bf2ceb73fe8a48ece981a82ef103284e5dfc0a54

Given that, as Thomas notes in [1], macos finally documents the futex APIs,
that seems like the obvious way forward.

I also found an older branch of Thomas' that abstracts futexes across a number
of platforms [2] and then implements semaphores on top of those futexes.

It's *really* annoying that window's WaitOnAddress() equivalent explicitly
requires the waiter and waker to be threads in the same process :(.

E.g. lwlocks would be faster *and* the code would be simpler if futexes could
be used everywhere.

Greetings,

Andres

[1] https://www.postgresql.org/message-id/CA%2BhUKGKAPR%3DNXxMTCmEomCUpq%2BmrPLDVyfVa4bKZqfDjwyzD7Q%40mail.gmail.com
[2] https://github.com/macdice/postgres/commits/pg_futex/