semaphore: Bad file descriptor errors

[langston-barrett]

I generally set semaphore: True in my cabal.project.local to speed up my builds. Occasionally, I see cabal build fail with messages like this:

semWaitInterrruptible: invalid argument (Bad file descriptor)
cabal_semaphore_3: semUnlink: does not exist (No such file or directory)

I'm afraid I don't have a reliable reproducer to share, but I didn't see any other issues in the tracker mentioning semaphores and bad file descriptors, so I figured I'd file this anyway just in case.

Expected behavior The build simply completes, but faster than with semaphore: False.

System information

• macOS 15.5
• cabal: 3.14.2.0
• ghc: 9.8.4

[ulysses4ever]

thanks for the report! tagging our semaphores guru @mpickering

[langston-barrett]

I also sometimes see this right when starting a build, just after Cabal finishes planning:

Created semaphore called cabal_semaphore_3 with 14 slots.
cabal: semWait: invalid argument (Bad file descriptor)

Then cabal just hangs.

[Bodigrim]

I've seen the same intermittent error, on macOS and on Ubuntu.

[geekosaur]

The question then becomes, is this the fault of cabal somehow, or is it a bug in semaphore-compat?

[Bodigrim]

(It might be a manifestation of a bug in semaphore-compat as well, but) there is certainly a fault on Cabal side, because it should not fail hard but rather degrade gracefully to a non-semaphore mode.

[geekosaur]

Pinging @bgamari who added this support in #9139.

[geekosaur]

(of course, it's not exactly the best time for him…)

[wraithm]

I see this error quite a bit on:

• macOS Tahoe 26.0.1
• cabal 3.14.2.0
• ghc 9.10.3