[PATCH] bootstrap: use fcntl locking on Solaris

[psumbera]

Solaris does not provide flock, so std::fs::File deliberately does not support its locking API there. Its earlier fcntl emulation was removed because fcntl locks are process-scoped rather than handle-scoped.

The bootstrap build lock only coordinates separate bootstrap processes and is held for the lifetime of the process, making fcntl locking suitable for this narrower use. Add a Solaris-specific backend while retaining std file locking on other platforms.

Note: LLM was used to generate it.

[rustbot]

r? @clubby789

rustbot has assigned @clubby789.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: bootstrap
• bootstrap expanded to 6 candidates
• Random selection from Mark-Simulacrum, clubby789, jieyouxu

[bjorn3]

How hard would it be to add flock support to solaris? And how long would that take before we could depend on that support from the rust side?

Edit: Forgot Solaris is no longer open source.

[psumbera]

How hard would it be to add flock support to solaris? And how long would that take before we could depend on that support from the rust side?

Edit: Forgot Solaris is no longer open source.

It would be pretty hard. Plus we would then need to wait for new Solaris 11.4 CBE release. Also Rust is expected to run/build on older Solaris 11.4 releases too...

[bjorn3]

While it would be a while before rust can depend on it, adding flock support to Solaris now would fix other programs that use File::lock. And the fact that fcntl is process-scoped means I will likely have to complicate #159287 a fair bit by adding a second layer of locking to ensure multiple rustc sessions in the same process don't think they all have exclusive access to the same incr comp dir. The sooner Solaris (and whichever other Unix OSes supported by rustc are missing it) adds flock support, the sooner people can stop using unsafe fcntl based locking.

But if it is hard to implement, I can understand it not getting implemented any time soon.