std/sys/thread_local/native/
lazy.rs

1use crate::cell::{Cell, UnsafeCell};
2use crate::mem::MaybeUninit;
3use crate::ptr;
4use crate::sys::thread_local::{abort_on_dtor_unwind, destructors};
5
6pub unsafe trait DestroyedState: Sized + Copy {
7    fn register_dtor<T>(s: &Storage<T, Self>);
8}
9
10unsafe impl DestroyedState for ! {
11    fn register_dtor<T>(_: &Storage<T, !>) {}
12}
13
14unsafe impl DestroyedState for () {
15    fn register_dtor<T>(s: &Storage<T, ()>) {
16        unsafe {
17            destructors::register(ptr::from_ref(s).cast_mut().cast(), destroy::<T>);
18        }
19    }
20}
21
22#[derive(Copy, Clone)]
23enum State<D> {
24    Uninitialized,
25    Alive,
26    Destroyed(D),
27}
28
29#[allow(missing_debug_implementations)]
30pub struct Storage<T, D> {
31    state: Cell<State<D>>,
32    value: UnsafeCell<MaybeUninit<T>>,
33}
34
35impl<T, D> Storage<T, D>
36where
37    D: DestroyedState,
38{
39    pub const fn new() -> Storage<T, D> {
40        Storage {
41            state: Cell::new(State::Uninitialized),
42            value: UnsafeCell::new(MaybeUninit::uninit()),
43        }
44    }
45
46    /// Gets a pointer to the TLS value, potentially initializing it with the
47    /// provided parameters. If the TLS variable has been destroyed, a null
48    /// pointer is returned.
49    ///
50    /// The resulting pointer may not be used after reentrant inialialization
51    /// or thread destruction has occurred.
52    ///
53    /// # Safety
54    /// The `self` reference must remain valid until the TLS destructor is run.
55    #[inline]
56    pub unsafe fn get_or_init(&self, i: Option<&mut Option<T>>, f: impl FnOnce() -> T) -> *const T {
57        if let State::Alive = self.state.get() {
58            self.value.get().cast()
59        } else {
60            unsafe { self.get_or_init_slow(i, f) }
61        }
62    }
63
64    /// # Safety
65    /// The `self` reference must remain valid until the TLS destructor is run.
66    #[cold]
67    unsafe fn get_or_init_slow(
68        &self,
69        i: Option<&mut Option<T>>,
70        f: impl FnOnce() -> T,
71    ) -> *const T {
72        match self.state.get() {
73            State::Uninitialized => {}
74            State::Alive => return self.value.get().cast(),
75            State::Destroyed(_) => return ptr::null(),
76        }
77
78        let v = i.and_then(Option::take).unwrap_or_else(f);
79
80        // SAFETY: we cannot be inside a `LocalKey::with` scope, as the initializer
81        // has already returned and the next scope only starts after we return
82        // the pointer. Therefore, there can be no references to the old value,
83        // even if it was initialized. Thus because we are !Sync we have exclusive
84        // access to self.value and may replace it.
85        let mut old_value = unsafe { self.value.get().replace(MaybeUninit::new(v)) };
86        match self.state.replace(State::Alive) {
87            // If the variable is not being recursively initialized, register
88            // the destructor. This might be a noop if the value does not need
89            // destruction.
90            State::Uninitialized => D::register_dtor(self),
91
92            // Recursive initialization, we only need to drop the old value
93            // as we've already registered the destructor.
94            State::Alive => unsafe { old_value.assume_init_drop() },
95
96            State::Destroyed(_) => unreachable!(),
97        }
98
99        self.value.get().cast()
100    }
101}
102
103/// Transition an `Alive` TLS variable into the `Destroyed` state, dropping its
104/// value.
105///
106/// # Safety
107/// * Must only be called at thread destruction.
108/// * `ptr` must point to an instance of `Storage<T, ()>` and be valid for
109///   accessing that instance.
110unsafe extern "C" fn destroy<T>(ptr: *mut u8) {
111    // Print a nice abort message if a panic occurs.
112    abort_on_dtor_unwind(|| {
113        let storage = unsafe { &*(ptr as *const Storage<T, ()>) };
114        if let State::Alive = storage.state.replace(State::Destroyed(())) {
115            // SAFETY: we ensured the state was Alive so the value was initialized.
116            // We also updated the state to Destroyed to prevent the destructor
117            // from accessing the thread-local variable, as this would violate
118            // the exclusive access provided by &mut T in Drop::drop.
119            unsafe { (*storage.value.get()).assume_init_drop() }
120        }
121    })
122}