Disable `--semaphore` functionality downstream

[hasufell]

It's effectively a broken feature:

• https://gitlab.haskell.org/ghc/ghc/-/issues/25087
• [Ubuntu] [3.12.0.0-prerelease] Semaphore does not exist cabal#9993

So we should consider patching it out, until it is fixed and works reliably.

[mpilgrem]

Is it 'broken' only on Linux distributions (other than Alpine Linux)? I appreciate that Linux is far more popular with Haskell developers than it is more generally, but is that a reason to create a fork of the executable? (Also, I was not clear if you were proposing to fork only the Cabal (the tool) executable or also GHC executables? Stack users are not affected by the former but could be affected by the latter.)

With Stack, my own approach has been label it as 'experimental' and ensure that its limitation on Linux is clearly documented. This is not a relevant consideration for the GHCup project, but, for Stack, I understand that a work-around for people wanting to use it on Linux is to use Stack to build Stack from source (which can be done with a single command and takes about 20 minutes to complete, if starting from scratch).

[hasufell]

Yes, I will rip it out of cabal.

[mpilgrem]

One other clarification: will you fork the Cabal (the tool) executable for all operating systems, or only for Linux (I assume you would like to revert to providing a statically-linked Cabal (the tool) executable on Linux)?

[MangoIV]

I think it's broken on BSD, in some configurations, too, afaiu. However, I think it's generally a very useful feature. Since it's currently opt in, I think it should be left in. The speedups you can get from using semaphore is really considerable.

[AndreasPK]

It's effectively a broken feature ...

It worked for me reliably in most configurations across multiple operating systems and platforms I use regularly. So really depends on how you define effectively broken.

so we should consider patching it out, until it is fixed and works reliably.

There are configurations on which it's broken, but given it's an opt-in feature removing the option to use it for all users seems unproductive to me.

If you think it's worth patching Cabal in your distribution I would suggest making it output a warning when used without making the feature unusable. Something along the lines of:

--semaphore is known to cause build failures on certain platform configurations. See for details. Use at your own peril.

[hasufell]

So really depends on how you define effectively broken.

The default configuration for cabal (static musl binary distributed on glibc systems) didn't work. That's how I define effectively broken. Now distributors have to play tricks in order to restore it (which is not without side effects). I can just keep distributing musl binaries and have cabal barf out with obscure errors.

This isn't a minor oversight in how the semaphore feature was designed, implemented and shipped. It's a major oversight.

But I'm waiting for ghc-proposals/ghc-proposals#673