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MSRV 1.63: Use libstd instead of libc::open and instead of libpthreads's Mutex. #458

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MSRV 1.63: Use libstd instead of libc::open and instead of libpthreads's Mutex. #458

wants to merge 3 commits into from

Commits on Jun 18, 2024

  1. use_file: Use std::File instead of DropGuard. 

    For now, still use `libc::{poll,read}`. But use `File::open` to open
the files, instead of using `DropGuard`.

While doing this, switch to the `RawFd` type alias from `libc::c_int`.
    @briansmith
    briansmith committed Jun 18, 2024
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  2. MSRV 1.63: use_file: Clarify I/O safety using BorrowedFd.

    @briansmith
    briansmith committed Jun 18, 2024
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  3. 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.

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.

I tried to use `Once` to avoid the MSRV increase but it doesn't support
fallible initialization. `OnceLock` wasn't added until 1.70 but even
then, `OnceLock::get_or_try_init` is still unstable.

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 18, 2024
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