sync: add sync.Once, a proposed replacement for package once.

This implementation is simpler and more useful, since it can work
with methods.

R=rsc
CC=golang-dev
https://golang.org/cl/1864051

src/pkg/sync/Makefile

@@ -7,6 +7,7 @@ include ../../Make.$(GOARCH)
 TARG=sync
 GOFILES=\
 	mutex.go\
+	once.go \
 	rwmutex.go\
 
 # 386-specific object files

src/pkg/sync/mutex.go

@@ -3,9 +3,10 @@
 // license that can be found in the LICENSE file.
 
 // The sync package provides basic synchronization primitives
-// such as mutual exclusion locks.  These are intended for use
-// by low-level library routines.  Higher-level synchronization
-// is better done via channels and communication.
+// such as mutual exclusion locks.  Other than the Once type,
+// most are intended for use by low-level library routines.
+// Higher-level synchronization  is better done via channels
+// and communication.
 package sync
 
 import "runtime"

src/pkg/sync/once.go

+// Copyright 2009 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
+
+// Once is an object that will perform exactly one action.
+type Once struct {
+	m    Mutex
+	done bool
+}
+
+// Do calls the function f if and only if the method is being called for the
+// first time with this receiver.  In other words, given
+// 	var once Once
+// if once.Do(f) is called multiple times, only the first call will invoke f,
+// even if f has a different value in each invocation.  A new instance of
+// Once is required for each function to execute.
+//
+// Do is intended for initialization that must be run exactly once.  Since f
+// is niladic, it may be necessary to use a function literal to capture the
+// arguments to a function to be invoked by Do:
+// 	config.once.Do(func() { config.init(filename) })
+//
+func (o *Once) Do(f func()) {
+	o.m.Lock()
+	defer o.m.Unlock()
+	if !o.done {
+		o.done = true
+		f()
+	}
+}

src/pkg/sync/once_test.go

+// Copyright 2009 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_test
+
+import (
+	. "sync"
+	"testing"
+)
+
+type one int
+
+func (o *one) Increment() {
+	*o++
+}
+
+func run(once *Once, o *one, c chan bool) {
+	once.Do(func() { o.Increment() })
+	c <- true
+}
+
+func TestOnce(t *testing.T) {
+	o := new(one)
+	once := new(Once)
+	c := make(chan bool)
+	const N = 10
+	for i := 0; i < N; i++ {
+		go run(once, o, c)
+	}
+	for i := 0; i < N; i++ {
+		<-c
+	}
+	if *o != 1 {
+		t.Errorf("once failed: %d is not 1", *o)
+	}
+}