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95611c0eb4
Updated the use of atomic.Uint32 to atomic.Bool for sync package. Change-Id: Ib8da66fea86ef06e1427ac5118016b96fbcda6b1 GitHub-Last-Rev: d36e0f431fcde988f90badf86bbf04a18a411947 GitHub-Pull-Request: golang/go#73447 Reviewed-on: https://go-review.googlesource.com/c/go/+/666895 Reviewed-by: Junyang Shao <shaojunyang@google.com> Reviewed-by: Keith Randall <khr@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
81 lines
2.5 KiB
Go
81 lines
2.5 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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//
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// A Once must not be copied after first use.
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//
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// In the terminology of [the Go memory model],
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// the return from f “synchronizes before”
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// the return from any call of once.Do(f).
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//
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// [the Go memory model]: https://go.dev/ref/mem
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type Once struct {
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_ noCopy
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// done indicates whether the action has been performed.
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// It is first in the struct because it is used in the hot path.
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// The hot path is inlined at every call site.
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// Placing done first allows more compact instructions on some architectures (amd64/386),
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// and fewer instructions (to calculate offset) on other architectures.
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done atomic.Bool
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m Mutex
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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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//
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// var once Once
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//
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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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//
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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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func (o *Once) Do(f func()) {
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// Note: Here is an incorrect implementation of Do:
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//
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// if o.done.CompareAndSwap(0, 1) {
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// f()
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// }
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//
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// Do guarantees that when it returns, f has finished.
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// This implementation would not implement that guarantee:
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// given two simultaneous calls, the winner of the cas would
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// call f, and the second would return immediately, without
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// waiting for the first's call to f to complete.
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// This is why the slow path falls back to a mutex, and why
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// the o.done.Store must be delayed until after f returns.
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if !o.done.Load() {
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// Outlined slow-path to allow inlining of the fast-path.
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o.doSlow(f)
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}
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}
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func (o *Once) doSlow(f func()) {
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o.m.Lock()
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defer o.m.Unlock()
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if !o.done.Load() {
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defer o.done.Store(true)
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f()
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}
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}
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