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

[briansmith]

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:

-       pthread_mutex_lock
-       pthread_mutex_unlock

and adds these libstd dependencies:

+       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] #453 (comment)

[briansmith]

As explained in the first commit, This code is only used on Linux/Android. With the other changes in this PR, DropGuard also became Linux/Android-specific. Instead of adding more #[cfg(...)] I just moved it all.

[josephlr]

If we are adding a std dependency for these targets, should we also move to using std::File so that we don't even need DropGuard anymore? Also, should use of std require enabling the "std" cargo feature?

[briansmith]

If we are adding a std dependency for these targets, should we also move to using std::File so that we don't even need DropGuard anymore?

Unless/until we can use std::syc::OnceLock or a polyfill for it, it doesn't make much sense right not to use std::File, because we'd end up using std::File::open() and then immediately calling into_raw_fd().

Also, should use of std require enabling the "std" cargo feature?

No. Note that this PR effectively only uses libstd on targets for which we know libstd is already available.

[briansmith]

Unless/until we can use std::syc::OnceLock or a polyfill for it, it doesn't make much sense right not to use std::File, because we'd end up using std::File::open() and then immediately calling into_raw_fd().

Well, actually, it is possible to use std::File in wait_until_rng_ready now.

[briansmith]

In #458, instead of moving wait_until_rng_ready, I took @josephlr suggestion and replaced DropGuard with std::fs::File.

[briansmith]

Closing this in favor of #458 based on @newpavlov's comment in the issue.