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use_file: Use std::fs::File instead of libc and once_cell::sync::OnceCell instead of pthreads mutex #481

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use_file: Use std::fs::File instead of libc and once_cell::sync::OnceCell instead of pthreads mutex #481

wants to merge 2 commits into from

Commits on Jun 19, 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 19, 2024
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  2. use_file: Use once_cell::sync::OnceCell instead of libpthreads. 

    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 `std::sync::Once` to avoid adding an external dependency
but it doesn't support fallible initialization. `OnceLock` wasn't added
until 1.70, and even then `OnceLock::get_or_try_init` is still
unstable.
    @briansmith
    briansmith committed Jun 19, 2024
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