Imported Upstream version 2011.03.07.1upstream/2011.03.07.1

1 files changed, 90 insertions, 0 deletions

diff --git a/src/pkg/sync/cond.go b/src/pkg/sync/cond.go
new file mode 100644
index 000000000..ea48f2e7a
--- /dev/null
+++ b/src/pkg/sync/cond.go
@@ -0,0 +1,90 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync
+
+import "runtime"
+
+// Cond implements a condition variable, a rendezvous point
+// for goroutines waiting for or announcing the occurrence
+// of an event.
+//
+// Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
+// which must be held when changing the condition and
+// when calling the Wait method.
+type Cond struct {
+	L       Locker // held while observing or changing the condition
+	m       Mutex  // held to avoid internal races
+	waiters int    // number of goroutines blocked on Wait
+	sema    *uint32
+}
+
+// NewCond returns a new Cond with Locker l.
+func NewCond(l Locker) *Cond {
+	return &Cond{L: l}
+}
+
+// Wait atomically unlocks c.L and suspends execution
+// of the calling goroutine.  After later resuming execution,
+// Wait locks c.L before returning.
+//
+// Because L is not locked when Wait first resumes, the caller
+// typically cannot assume that the condition is true when
+// Wait returns.  Instead, the caller should Wait in a loop:
+//
+//    c.L.Lock()
+//    for !condition() {
+//        c.Wait()
+//    }
+//    ... make use of condition ...
+//    c.L.Unlock()
+//
+func (c *Cond) Wait() {
+	c.m.Lock()
+	if c.sema == nil {
+		c.sema = new(uint32)
+	}
+	s := c.sema
+	c.waiters++
+	c.m.Unlock()
+	c.L.Unlock()
+	runtime.Semacquire(s)
+	c.L.Lock()
+}
+
+// Signal wakes one goroutine waiting on c, if there is any.
+//
+// It is allowed but not required for the caller to hold c.L
+// during the call.
+func (c *Cond) Signal() {
+	c.m.Lock()
+	if c.waiters > 0 {
+		c.waiters--
+		runtime.Semrelease(c.sema)
+	}
+	c.m.Unlock()
+}
+
+// Broadcast wakes all goroutines waiting on c.
+//
+// It is allowed but not required for the caller to hold c.L
+// during the call.
+func (c *Cond) Broadcast() {
+	c.m.Lock()
+	if c.waiters > 0 {
+		s := c.sema
+		n := c.waiters
+		for i := 0; i < n; i++ {
+			runtime.Semrelease(s)
+		}
+		// We just issued n wakeups via the semaphore s.
+		// To ensure that they wake up the existing waiters
+		// and not waiters that arrive after Broadcast returns,
+		// clear c.sema.  The next operation will allocate
+		// a new one.
+		c.sema = nil
+		c.waiters = 0
+	}
+	c.m.Unlock()
+}