jobserver updates jobserver-auth from old style R,W to new style fifo:PATH for children though children may not be compatible with fifo:PATH style auth

[IceTDrinker]

Hello,

As discussed in this zulip thread: https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/does.20Cargo.20inject.20fifo.3APATH.20style.20jobserver-auth.20in.20MAKEFLAG/near/447522824

We have a build case with rust that started failing on newer nightly toolchains, the crash stemmed from a fifo:PATH style jobserver-auth being forwarded in a build.rs to a make command (with version < 4.4) that is not compatible with the fifo:PATH style auth, when the make command tried to run it crashed indicating it did not support the fifo:PATH auth.

@the8472 pointed out this piece of code

jobserver-rs/src/unix.rs

Lines 168 to 181 in c0c2898

	// Optimization: Try converting it to a fifo by using /dev/fd
	//
	// On linux, opening `/dev/fd/$fd` returns a fd with a new file description,
	// so we can set `O_NONBLOCK` on it without affecting other processes.
	//
	// On macOS, opening `/dev/fd/$fd` seems to be the same as `File::try_clone`.
	//
	// I tested this on macOS 14 and Linux 6.5.13
	#[cfg(target_os = "linux")]
	if let Ok(Some(jobserver)) =
	Self::from_fifo(&format!("fifo:/dev/fd/{}", read.as_raw_fd()))
	{
	return Ok(Some(jobserver));
	}

which breaks compatibility if the new fifo:PATH auth is forwarded to incompatible children processes.

Thanks a lot for the swift support!

Cheers

[NobodyXu]

I think we can fix this, by pass the fd + the path for fifo?

[the8472]

We should pass-through whatever the parent gave us and as an optimization open a separate set of FDs for internal use on linux and not pass.