Enable file locking support in illumos

[pietroalbini]

#132977 introduced an allow-list of targets supporting file locking, but forgot to add illumos to it (which introduced support for it in ~2015). File::lock and friends are now stable, and the ecosystem is slowly replacing custom libc calls with the standard library. Crucially, in 1.91 both Cargo and bootstrap switched to File::lock, both breaking build directory locking.

This PR enables file locking on illumos. Fixes #146312.

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[pietroalbini]

r? @tgross35

Currently testing this on my illumos VM, will report back once the tests pass.

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[pietroalbini]

Ran the tests locally, I can confirm the changes work.

[ChrisDenton]

This looks right to me and your tests pass so r=me if CI is happy.

[RalfJung]

I figured we'd land Illumos first since as you said it is tier-2-with-host-tools so cargo has battle-tested this in the past. Then we can separately discuss what to do on Android. I pinged the target maintainers in #148325.

[pietroalbini]

Ok, on second thought, let's just focus this PR on the illumos changes.

@bors r=ChrisDenton

[bors]

📌 Commit 3d9c69b has been approved by ChrisDenton

It is now in the queue for this repository.

[pietroalbini]

Nominated for stable and beta backport.

File::lock was stabilized in Rust 1.89.0 with an allow-list of targets supporting it. illumos was excluded from that list, even though it does support the underlying syscalls to make it work since around 2015.

While normally adding a new feature (support for illumos in File::lock) would not be eligible for beta backport, let alone stable backport, the ecosystem is migrating from custom libc calls to the standard library API when file locking is required.

The most visible breakage is Cargo, which replaced its own file locking code with File::lock in Rust 1.91.0. When std returns "unsupported" from that API call, Cargo proceeds as if the build directory was locked, even though std never did such a thing. As a result of this, parallel Cargo invocations on illumos interfere with each other, instead of the usual behavior of one invocation waiting for the previous one to finish.

I am aware that the reason for the backport is a Cargo regression, not a standard library regression, but this seems the best fix for the problem. Approving the stable backport would be appreciated, as Cargo 1.91.0 on illumos (a tier 2 target) is misbehaving.