extra: Re-add the Once primitve to extra::sync

This originally lived in std::unstable::mutex, but now it has a new home (and a
more proper one).

Author: alexcrichton

src/libextra/sync/one.rs

@@ -0,0 +1,168 @@
+// Copyright 2014 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.
+
+//! A "once initialization" primitive
+//!
+//! This primitive is meant to be used to run one-time initialization. An
+//! example use case would be for initializing an FFI library.
+
+use std::int;
+use std::sync::atomics;
+use sync::mutex::{StaticMutex, MUTEX_INIT};
+
+/// A type which can be used to run a one-time global initialization. This type
+/// is *unsafe* to use because it is built on top of the `Mutex` in this module.
+/// It does not know whether the currently running task is in a green or native
+/// context, and a blocking mutex should *not* be used under normal
+/// circumstances on a green task.
+///
+/// Despite its unsafety, it is often useful to have a one-time initialization
+/// routine run for FFI bindings or related external functionality. This type
+/// can only be statically constructed with the `ONCE_INIT` value.
+///
+/// # Example
+///
+/// ```rust
+/// use std::unstable::mutex::{Once, ONCE_INIT};
+///
+/// static mut START: Once = ONCE_INIT;
+/// unsafe {
+///     START.doit(|| {
+///         // run initialization here
+///     });
+/// }
+/// ```
+pub struct Once {
+    priv mutex: StaticMutex,
+    priv cnt: atomics::AtomicInt,
+    priv lock_cnt: atomics::AtomicInt,
+}
+
+/// Initialization value for static `Once` values.
+pub static ONCE_INIT: Once = Once {
+    mutex: MUTEX_INIT,
+    cnt: atomics::INIT_ATOMIC_INT,
+    lock_cnt: atomics::INIT_ATOMIC_INT,
+};
+
+impl Once {
+    /// Perform an initialization routine once and only once. The given closure
+    /// will be executed if this is the first time `doit` has been called, and
+    /// otherwise the routine will *not* be invoked.
+    ///
+    /// This method will block the calling *os 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).
+    pub fn doit(&mut self, f: ||) {
+        // Implementation-wise, this would seem like a fairly trivial primitive.
+        // The stickler part is where our mutexes currently require an
+        // allocation, and usage of a `Once` should't leak this allocation.
+        //
+        // This means that there must be a deterministic destroyer of the mutex
+        // contained within (because it's not needed after the initialization
+        // has run).
+        //
+        // The general scheme here is to gate all future threads once
+        // initialization has completed with a "very negative" count, and to
+        // allow through threads to lock the mutex if they see a non negative
+        // count. For all threads grabbing the mutex, exactly one of them should
+        // be responsible for unlocking the mutex, and this should only be done
+        // once everyone else is done with the mutex.
+        //
+        // This atomicity is achieved by swapping a very negative value into the
+        // shared count when the initialization routine has completed. This will
+        // read the number of threads which will at some point attempt to
+        // acquire the mutex. This count is then squirreled away in a separate
+        // variable, and the last person on the way out of the mutex is then
+        // responsible for destroying the mutex.
+        //
+        // It is crucial that the negative value is swapped in *after* the
+        // initialization routine has completed because otherwise new threads
+        // calling `doit` will return immediately before the initialization has
+        // completed.
+
+        let prev = self.cnt.fetch_add(1, atomics::SeqCst);
+        if prev < 0 {
+            // Make sure we never overflow, we'll never have int::MIN
+            // simultaneous calls to `doit` to make this value go back to 0
+            self.cnt.store(int::MIN, atomics::SeqCst);
+            return
+        }
+
+        // If the count is negative, then someone else finished the job,
+        // otherwise we run the job and record how many people will try to grab
+        // this lock
+        {
+            let _guard = self.mutex.lock();
+            if self.cnt.load(atomics::SeqCst) > 0 {
+                f();
+                let prev = self.cnt.swap(int::MIN, atomics::SeqCst);
+                self.lock_cnt.store(prev, atomics::SeqCst);
+            }
+        }
+
+        // Last one out cleans up after everyone else, no leaks!
+        if self.lock_cnt.fetch_add(-1, atomics::SeqCst) == 1 {
+            unsafe { self.mutex.destroy() }
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{ONCE_INIT, Once};
+    use std::task;
+
+    #[test]
+    fn smoke_once() {
+        static mut o: Once = ONCE_INIT;
+        let mut a = 0;
+        unsafe { o.doit(|| a += 1); }
+        assert_eq!(a, 1);
+        unsafe { o.doit(|| a += 1); }
+        assert_eq!(a, 1);
+    }
+
+    #[test]
+    fn stampede_once() {
+        static mut o: Once = ONCE_INIT;
+        static mut run: bool = false;
+
+        let (p, c) = SharedChan::new();
+        for _ in range(0, 10) {
+            let c = c.clone();
+            do spawn {
+                for _ in range(0, 4) { task::deschedule() }
+                unsafe {
+                    o.doit(|| {
+                        assert!(!run);
+                        run = true;
+                    });
+                    assert!(run);
+                }
+                c.send(());
+            }
+        }
+
+        unsafe {
+            o.doit(|| {
+                assert!(!run);
+                run = true;
+            });
+            assert!(run);
+        }
+
+        for _ in range(0, 10) {
+            p.recv();
+        }
+    }
+}

src/libnative/io/net.rs

@@ -201,14 +201,20 @@ pub fn init() {
     }
 
     unsafe {
-        use std::unstable::mutex::{Once, ONCE_INIT};
-        static mut INIT: Once = ONCE_INIT;
-        INIT.doit(|| {
-            let mut data: WSADATA = intrinsics::init();
-            let ret = WSAStartup(0x202,      // version 2.2
-                                 &mut data);
-            assert_eq!(ret, 0);
-        });
+        use std::unstable::mutex::{Mutex, MUTEX_INIT};
+        static mut INITIALIZED: bool = false;
+        static mut LOCK: Mutex = MUTEX_INIT;
+        unsafe {
+            LOCK.lock();
+            if !INITIALIZED {
+                let mut data: WSADATA = intrinsics::init();
+                let ret = WSAStartup(0x202,      // version 2.2
+                                     &mut data);
+                assert_eq!(ret, 0);
+                INITIALIZED = true;
+            }
+            LOCK.unlock();
+        }
     }
 }
 

src/librustc/back/link.rs

@@ -332,7 +332,7 @@ pub mod write {
     }
 
     unsafe fn configure_llvm(sess: Session) {
-        use std::unstable::mutex::{Once, ONCE_INIT};
+        use extra::sync::one::{Once, ONCE_INIT};
         static mut INIT: Once = ONCE_INIT;
 
         // Copy what clang does by turning on loop vectorization at O2 and

src/librustc/middle/trans/base.rs

@@ -2660,7 +2660,7 @@ pub fn trans_crate(sess: session::Session,
                    output: &Path) -> CrateTranslation {
     // Before we touch LLVM, make sure that multithreading is enabled.
     unsafe {
-        use std::unstable::mutex::{Once, ONCE_INIT};
+        use extra::sync::one::{Once, ONCE_INIT};
         static mut INIT: Once = ONCE_INIT;
         static mut POISONED: bool = false;
         INIT.doit(|| {