From 5ff4c17907d5b19510a62e08fd8d3b11e62b431d Mon Sep 17 00:00:00 2001
From: Ondřej Surý <ondrej@sury.org>
Date: Tue, 13 Sep 2011 13:13:40 +0200
Subject: Imported Upstream version 60

---
 src/pkg/sync/once.go | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 src/pkg/sync/once.go

(limited to 'src/pkg/sync/once.go')

diff --git a/src/pkg/sync/once.go b/src/pkg/sync/once.go
new file mode 100644
index 000000000..04b714a3e
--- /dev/null
+++ b/src/pkg/sync/once.go
@@ -0,0 +1,43 @@
+// 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
+
+import (
+	"sync/atomic"
+)
+
+// Once is an object that will perform exactly one action.
+type Once struct {
+	m    Mutex
+	done uint32
+}
+
+// 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) })
+//
+// Because no call to Do returns until the one call to f returns, if f causes
+// Do to be called, it will deadlock.
+//
+func (o *Once) Do(f func()) {
+	if atomic.LoadUint32(&o.done) == 1 {
+		return
+	}
+	// Slow-path.
+	o.m.Lock()
+	defer o.m.Unlock()
+	if o.done == 0 {
+		f()
+		atomic.CompareAndSwapUint32(&o.done, 0, 1)
+	}
+}
-- 
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