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Diffstat (limited to '')
-rw-r--r-- | vendor/tracing-core/src/spin/once.rs | 158 |
1 files changed, 158 insertions, 0 deletions
diff --git a/vendor/tracing-core/src/spin/once.rs b/vendor/tracing-core/src/spin/once.rs new file mode 100644 index 000000000..27c99e56e --- /dev/null +++ b/vendor/tracing-core/src/spin/once.rs @@ -0,0 +1,158 @@ +use core::cell::UnsafeCell; +use core::fmt; +use core::hint::spin_loop; +use core::sync::atomic::{AtomicUsize, Ordering}; + +/// A synchronization primitive which can be used to run a one-time global +/// initialization. Unlike its std equivalent, this is generalized so that the +/// closure returns a value and it is stored. Once therefore acts something like +/// a future, too. +pub struct Once<T> { + state: AtomicUsize, + data: UnsafeCell<Option<T>>, // TODO remove option and use mem::uninitialized +} + +impl<T: fmt::Debug> fmt::Debug for Once<T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + match self.r#try() { + Some(s) => write!(f, "Once {{ data: ") + .and_then(|()| s.fmt(f)) + .and_then(|()| write!(f, "}}")), + None => write!(f, "Once {{ <uninitialized> }}"), + } + } +} + +// Same unsafe impls as `std::sync::RwLock`, because this also allows for +// concurrent reads. +unsafe impl<T: Send + Sync> Sync for Once<T> {} +unsafe impl<T: Send> Send for Once<T> {} + +// Four states that a Once can be in, encoded into the lower bits of `state` in +// the Once structure. +const INCOMPLETE: usize = 0x0; +const RUNNING: usize = 0x1; +const COMPLETE: usize = 0x2; +const PANICKED: usize = 0x3; + +use core::hint::unreachable_unchecked as unreachable; + +impl<T> Once<T> { + /// Initialization constant of `Once`. + pub const INIT: Self = Once { + state: AtomicUsize::new(INCOMPLETE), + data: UnsafeCell::new(None), + }; + + /// Creates a new `Once` value. + pub const fn new() -> Once<T> { + Self::INIT + } + + fn force_get<'a>(&'a self) -> &'a T { + match unsafe { &*self.data.get() }.as_ref() { + None => unsafe { unreachable() }, + Some(p) => p, + } + } + + /// Performs an initialization routine once and only once. The given closure + /// will be executed if this is the first time `call_once` has been called, + /// and otherwise the routine will *not* be invoked. + /// + /// This method will block the calling thread if another initialization + /// routine is currently running. + /// + /// When this function returns, it is guaranteed that some initialization + /// has run and completed (it may not be the closure specified). The + /// returned pointer will point to the result from the closure that was + /// run. + pub fn call_once<'a, F>(&'a self, builder: F) -> &'a T + where + F: FnOnce() -> T, + { + let mut status = self.state.load(Ordering::SeqCst); + + if status == INCOMPLETE { + status = match self.state.compare_exchange( + INCOMPLETE, + RUNNING, + Ordering::SeqCst, + Ordering::SeqCst, + ) { + Ok(status) => { + debug_assert_eq!( + status, INCOMPLETE, + "if compare_exchange succeeded, previous status must be incomplete", + ); + // We init + // We use a guard (Finish) to catch panics caused by builder + let mut finish = Finish { + state: &self.state, + panicked: true, + }; + unsafe { *self.data.get() = Some(builder()) }; + finish.panicked = false; + + self.state.store(COMPLETE, Ordering::SeqCst); + + // This next line is strictly an optimization + return self.force_get(); + } + Err(status) => status, + } + } + + loop { + match status { + INCOMPLETE => unreachable!(), + RUNNING => { + // We spin + spin_loop(); + status = self.state.load(Ordering::SeqCst) + } + PANICKED => panic!("Once has panicked"), + COMPLETE => return self.force_get(), + _ => unsafe { unreachable() }, + } + } + } + + /// Returns a pointer iff the `Once` was previously initialized + pub fn r#try<'a>(&'a self) -> Option<&'a T> { + match self.state.load(Ordering::SeqCst) { + COMPLETE => Some(self.force_get()), + _ => None, + } + } + + /// Like try, but will spin if the `Once` is in the process of being + /// initialized + pub fn wait<'a>(&'a self) -> Option<&'a T> { + loop { + match self.state.load(Ordering::SeqCst) { + INCOMPLETE => return None, + + RUNNING => { + spin_loop() // We spin + } + COMPLETE => return Some(self.force_get()), + PANICKED => panic!("Once has panicked"), + _ => unsafe { unreachable() }, + } + } + } +} + +struct Finish<'a> { + state: &'a AtomicUsize, + panicked: bool, +} + +impl<'a> Drop for Finish<'a> { + fn drop(&mut self) { + if self.panicked { + self.state.store(PANICKED, Ordering::SeqCst); + } + } +} |