portable-atomic 1.0.1

// Based on https://github.com/rust-embedded/critical-section/blob/v1.1.1/src/std.rs,
// but compatible with Rust 1.59 that we run test.

use std::{
    cell::Cell,
    mem::MaybeUninit,
    sync::{Mutex, MutexGuard},
};

use once_cell::sync::Lazy;

static GLOBAL_MUTEX: Lazy<Mutex<()>> = Lazy::new(|| Mutex::new(()));

// This is initialized if a thread has acquired the CS, uninitialized otherwise.
static mut GLOBAL_GUARD: MaybeUninit<MutexGuard<'static, ()>> = MaybeUninit::uninit();

std::thread_local!(static IS_LOCKED: Cell<bool> = Cell::new(false));

struct StdCriticalSection;
critical_section::set_impl!(StdCriticalSection);

unsafe impl critical_section::Impl for StdCriticalSection {
    unsafe fn acquire() -> bool {
        // Allow reentrancy by checking thread local state
        IS_LOCKED.with(|l| {
            if l.get() {
                // CS already acquired in the current thread.
                return true;
            }

            // Note: it is fine to set this flag *before* acquiring the mutex because it's thread local.
            // No other thread can see its value, there's no potential for races.
            // This way, we hold the mutex for slightly less time.
            l.set(true);

            // Not acquired in the current thread, acquire it.
            let guard = match GLOBAL_MUTEX.lock() {
                Ok(guard) => guard,
                Err(err) => {
                    // Ignore poison on the global mutex in case a panic occurred
                    // while the mutex was held.
                    err.into_inner()
                }
            };
            unsafe {
                GLOBAL_GUARD.write(guard);
            }

            false
        })
    }

    unsafe fn release(nested_cs: bool) {
        if !nested_cs {
            // SAFETY: As per the acquire/release safety contract, release can only be called
            // if the critical section is acquired in the current thread,
            // in which case we know the GLOBAL_GUARD is initialized.
            unsafe {
                GLOBAL_GUARD.as_mut_ptr().drop_in_place();
            }

            // Note: it is fine to clear this flag *after* releasing the mutex because it's thread local.
            // No other thread can see its value, there's no potential for races.
            // This way, we hold the mutex for slightly less time.
            IS_LOCKED.with(|l| l.set(false));
        }
    }
}