1 files changed, 118 insertions, 0 deletions
diff --git a/src/sync/cond.go b/src/sync/cond.go
new file mode 100644
index 000000000..9e6bc170f
--- /dev/null
+++ b/src/sync/cond.go
@@ -0,0 +1,118 @@
+// 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 (
+	"sync/atomic"
+	"unsafe"
+)
+
+// 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.
+//
+// A Cond can be created as part of other structures.
+// A Cond must not be copied after first use.
+type Cond struct {
+	// L is held while observing or changing the condition
+	L Locker
+
+	sema    syncSema
+	waiters uint32 // number of waiters
+	checker copyChecker
+}
+
+// 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.  Unlike in other systems,
+// Wait cannot return unless awoken by Broadcast or Signal.
+//
+// Because c.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.checker.check()
+	if raceenabled {
+		raceDisable()
+	}
+	atomic.AddUint32(&c.waiters, 1)
+	if raceenabled {
+		raceEnable()
+	}
+	c.L.Unlock()
+	runtime_Syncsemacquire(&c.sema)
+	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.signalImpl(false)
+}
+
+// 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.signalImpl(true)
+}
+
+func (c *Cond) signalImpl(all bool) {
+	c.checker.check()
+	if raceenabled {
+		raceDisable()
+	}
+	for {
+		old := atomic.LoadUint32(&c.waiters)
+		if old == 0 {
+			if raceenabled {
+				raceEnable()
+			}
+			return
+		}
+		new := old - 1
+		if all {
+			new = 0
+		}
+		if atomic.CompareAndSwapUint32(&c.waiters, old, new) {
+			if raceenabled {
+				raceEnable()
+			}
+			runtime_Syncsemrelease(&c.sema, old-new)
+			return
+		}
+	}
+}
+
+// copyChecker holds back pointer to itself to detect object copying.
+type copyChecker uintptr
+
+func (c *copyChecker) check() {
+	if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
+		!atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
+		uintptr(*c) != uintptr(unsafe.Pointer(c)) {
+		panic("sync.Cond is copied")
+	}
+}