47 lines
1.3 KiB
Go
47 lines
1.3 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package sync
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import (
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"sync/atomic"
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)
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// Once is an object that will perform exactly one action.
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type Once struct {
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m Mutex
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done uint32
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}
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// Do calls the function f if and only if Do is being called for the
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// first time for this instance of Once. In other words, given
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// var once Once
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// if once.Do(f) is called multiple times, only the first call will invoke f,
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// even if f has a different value in each invocation. A new instance of
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// Once is required for each function to execute.
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//
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// Do is intended for initialization that must be run exactly once. Since f
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// is niladic, it may be necessary to use a function literal to capture the
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// arguments to a function to be invoked by Do:
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// config.once.Do(func() { config.init(filename) })
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//
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// Because no call to Do returns until the one call to f returns, if f causes
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// Do to be called, it will deadlock.
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//
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// If f panics, Do considers it to have returned; future calls of Do return
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// without calling f.
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//
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func (o *Once) Do(f func()) {
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if atomic.LoadUint32(&o.done) == 1 {
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return
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}
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// Slow-path.
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o.m.Lock()
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defer o.m.Unlock()
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if o.done == 0 {
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defer atomic.StoreUint32(&o.done, 1)
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f()
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}
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}
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