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);
+        }
+    }
+}