library/std/src/sys/thread_local/guard/key.rs

//! A lot of UNIX platforms don't have a specialized way to register TLS
//! destructors for native TLS. Instead, we use one TLS key with a destructor
//! that will run all native TLS destructors in the destructor list.

use crate::ptr;
use crate::sys::thread_local::key::{LazyKey, set};

#[cfg(target_thread_local)]
pub fn enable() {
    use crate::sys::thread_local::destructors;

    static DTORS: LazyKey = LazyKey::new(Some(run));

    // Setting the key value to something other than NULL will result in the
    // destructor being run at thread exit.
    unsafe {
        set(DTORS.force(), ptr::without_provenance_mut(1));
    }

    unsafe extern "C" fn run(_: *mut u8) {
        unsafe {
            destructors::run();
            // On platforms with `__cxa_thread_atexit_impl`, `destructors::run`
            // does nothing on newer systems as the TLS destructors are
            // registered with the system. But because all of those platforms
            // call the destructors of TLS keys after the registered ones, this
            // function will still be run last (at the time of writing).
            crate::rt::thread_cleanup();
        }
    }
}

/// On platforms with key-based TLS, the system runs the destructors for us.
/// We still have to make sure that [`crate::rt::thread_cleanup`] is called,
/// however. This is done by deferring the execution of a TLS destructor to
/// the next round of destruction inside the TLS destructors.
#[cfg(not(target_thread_local))]
pub fn enable() {
    const DEFER: *mut u8 = ptr::without_provenance_mut(1);
    const RUN: *mut u8 = ptr::without_provenance_mut(2);

    static CLEANUP: LazyKey = LazyKey::new(Some(run));

    unsafe { set(CLEANUP.force(), DEFER) }

    unsafe extern "C" fn run(state: *mut u8) {
        if state == DEFER {
            // Make sure that this function is run again in the next round of
            // TLS destruction. If there is no further round, there will be leaks,
            // but that's okay, `thread_cleanup` is not guaranteed to be called.
            unsafe { set(CLEANUP.force(), RUN) }
        } else {
            debug_assert_eq!(state, RUN);
            // If the state is still RUN in the next round of TLS destruction,
            // it means that no other TLS destructors defined by this runtime
            // have been run, as they would have set the state to DEFER.
            crate::rt::thread_cleanup();
        }
    }
}