rust/src/libstd/sys/unix/fast_thread_local.rs

// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#![cfg(target_thread_local)]
#![unstable(feature = "thread_local_internals", issue = "0")]

use cell::{Cell, UnsafeCell};
use fmt;
use intrinsics;
use ptr;

pub struct Key<T> {
    inner: UnsafeCell<Option<T>>,

    // Metadata to keep track of the state of the destructor. Remember that
    // these variables are thread-local, not global.
    dtor_registered: Cell<bool>,
    dtor_running: Cell<bool>,
}

impl<T> fmt::Debug for Key<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.pad("Key { .. }")
    }
}

unsafe impl<T> ::marker::Sync for Key<T> { }

impl<T> Key<T> {
    pub const fn new() -> Key<T> {
        Key {
            inner: UnsafeCell::new(None),
            dtor_registered: Cell::new(false),
            dtor_running: Cell::new(false)
        }
    }

    pub fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> {
        unsafe {
            if intrinsics::needs_drop::<T>() && self.dtor_running.get() {
                return None
            }
            self.register_dtor();
        }
        Some(&self.inner)
    }

    unsafe fn register_dtor(&self) {
        if !intrinsics::needs_drop::<T>() || self.dtor_registered.get() {
            return
        }

        register_dtor(self as *const _ as *mut u8,
                      destroy_value::<T>);
        self.dtor_registered.set(true);
    }
}

#[cfg(any(target_os = "linux", target_os = "fuchsia"))]
unsafe fn register_dtor_fallback(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
    // The fallback implementation uses a vanilla OS-based TLS key to track
    // the list of destructors that need to be run for this thread. The key
    // then has its own destructor which runs all the other destructors.
    //
    // The destructor for DTORS is a little special in that it has a `while`
    // loop to continuously drain the list of registered destructors. It
    // *should* be the case that this loop always terminates because we
    // provide the guarantee that a TLS key cannot be set after it is
    // flagged for destruction.
    use sys_common::thread_local as os;

    static DTORS: os::StaticKey = os::StaticKey::new(Some(run_dtors));
    type List = Vec<(*mut u8, unsafe extern fn(*mut u8))>;
    if DTORS.get().is_null() {
        let v: Box<List> = box Vec::new();
        DTORS.set(Box::into_raw(v) as *mut u8);
    }
    let list: &mut List = &mut *(DTORS.get() as *mut List);
    list.push((t, dtor));

    unsafe extern fn run_dtors(mut ptr: *mut u8) {
        while !ptr.is_null() {
            let list: Box<List> = Box::from_raw(ptr as *mut List);
            for &(ptr, dtor) in list.iter() {
                dtor(ptr);
            }
            ptr = DTORS.get();
            DTORS.set(ptr::null_mut());
        }
    }
}

// Since what appears to be glibc 2.18 this symbol has been shipped which
// GCC and clang both use to invoke destructors in thread_local globals, so
// let's do the same!
//
// Note, however, that we run on lots older linuxes, as well as cross
// compiling from a newer linux to an older linux, so we also have a
// fallback implementation to use as well.
//
// Due to rust-lang/rust#18804, make sure this is not generic!
#[cfg(target_os = "linux")]
unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
    use mem;
    use libc;

    extern {
        #[linkage = "extern_weak"]
        static __dso_handle: *mut u8;
        #[linkage = "extern_weak"]
        static __cxa_thread_atexit_impl: *const libc::c_void;
    }
    if !__cxa_thread_atexit_impl.is_null() {
        type F = unsafe extern fn(dtor: unsafe extern fn(*mut u8),
                                  arg: *mut u8,
                                  dso_handle: *mut u8) -> libc::c_int;
        mem::transmute::<*const libc::c_void, F>(__cxa_thread_atexit_impl)
            (dtor, t, &__dso_handle as *const _ as *mut _);
        return
    }
    register_dtor_fallback(t, dtor);
}

// macOS's analog of the above linux function is this _tlv_atexit function.
// The disassembly of thread_local globals in C++ (at least produced by
// clang) will have this show up in the output.
#[cfg(target_os = "macos")]
unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
    extern {
        fn _tlv_atexit(dtor: unsafe extern fn(*mut u8),
                       arg: *mut u8);
    }
    _tlv_atexit(dtor, t);
}

// Just use the thread_local fallback implementation, at least until there's
// a more direct implementation.
#[cfg(target_os = "fuchsia")]
unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
    register_dtor_fallback(t, dtor);
}

pub unsafe extern fn destroy_value<T>(ptr: *mut u8) {
    let ptr = ptr as *mut Key<T>;
    // Right before we run the user destructor be sure to flag the
    // destructor as running for this thread so calls to `get` will return
    // `None`.
    (*ptr).dtor_running.set(true);

    // The macOS implementation of TLS apparently had an odd aspect to it
    // where the pointer we have may be overwritten while this destructor
    // is running. Specifically if a TLS destructor re-accesses TLS it may
    // trigger a re-initialization of all TLS variables, paving over at
    // least some destroyed ones with initial values.
    //
    // This means that if we drop a TLS value in place on macOS that we could
    // revert the value to its original state halfway through the
    // destructor, which would be bad!
    //
    // Hence, we use `ptr::read` on macOS (to move to a "safe" location)
    // instead of drop_in_place.
    if cfg!(target_os = "macos") {
        ptr::read((*ptr).inner.get());
    } else {
        ptr::drop_in_place((*ptr).inner.get());
    }
}