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use_file: std::sync::Mutex, dropping all libpthread use. #457

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use_file: std::sync::Mutex, dropping all libpthread use. #457

wants to merge 2 commits into from

Commits on Jun 5, 2024

  1. Move wait_until_rng_ready to linux_android_with_fallback.rs. 

    It is only used in one configuration, so put it with the other
stuff from that configuration.
    @briansmith
    briansmith committed Jun 5, 2024
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  2. use_file: std::sync::Mutex, dropping all libpthread use. 

    pthreads mutexes are not safe to move. While it is very unlikely that
the mutex we create will ever be moved, we don't actively do anything
to actively prevent it from being moved. (libstd, when it used/uses
pthreads mutexes, would box them to prevent them from being moved.)

Also, now on Linux and Android (and many other targets for which we
don't use use_std), libstd uses futexes instead of pthreads mutexes.
Thus using libstd's Mutex will be more efficient and avoid adding an
often-otherwise-unnecessary libpthreads dependency on these targets.

  * Linux, Android: Futex [1].
  * Haiku, Redox, NTO, AIX: pthreads [2].
  * others: not using `use_file`.

This will not affect our plans for *-*-linux-none, since we don't
plan to use `use_file` for it. OnceLock

This breaks 32-bit x86 QNX Neutrino, which doesn't have libstd
because the target itself is abandoned [3]. the other QNX Neutrino
targets didn't get libstd support until Rust 1.69, so this
effectively raises the MSRV for them to 1.69.

Otherwise, the MSRV increases to 1.63 for the above-mentioned targets,
as that's when `Mutex::new()` became a `const fn`.

I tried to use `Once` to avoid the MSRV increase but it doesn't support
fallible initialization even in Nightly. `OnceLock` wasn't added until
1.70.

On x86_64 Linux, this change removes all libpthreads dependencies:

```diff
-       pthread_mutex_lock
-       pthread_mutex_unlock
```

and adds these libstd dependencies:

```diff
+       std::panicking::panic_count::GLOBAL_PANIC_COUNT
+       std::panicking::panic_count::is_zero_slow_path
+       std::sys::sync::mutex::futex::Mutex::lock_contended
+       std::sys::sync::mutex::futex::Mutex::wake
```

as measured using `cargo asm`.

[1] https://github.com/rust-lang/rust/blob/c1dba09f263cbff6170f130aa418e28bdf22bd96/library/std/src/sys/sync/mutex/mod.rs#L4-L10
[2] https://github.com/rust-lang/rust/blob/c1dba09f263cbff6170f130aa418e28bdf22bd96/library/std/src/sys/sync/mutex/mod.rs#L17-L20
[3] rust-random#453 (comment)
    @briansmith
    briansmith committed Jun 5, 2024
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