From f6ad4dcef54c5ce997a4bad5a6d86de229015700 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Tue, 16 Apr 2024 21:25:22 +0200 Subject: Adding upstream version 1.22.1. Signed-off-by: Daniel Baumann --- src/sync/once.go | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 76 insertions(+) create mode 100644 src/sync/once.go (limited to 'src/sync/once.go') diff --git a/src/sync/once.go b/src/sync/once.go new file mode 100644 index 0000000..3f58707 --- /dev/null +++ b/src/sync/once.go @@ -0,0 +1,76 @@ +// 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. +// +// A Once must not be copied after first use. +// +// In the terminology of the Go memory model, +// the return from f “synchronizes before” +// the return from any call of once.Do(f). +type Once struct { + // done indicates whether the action has been performed. + // It is first in the struct because it is used in the hot path. + // The hot path is inlined at every call site. + // Placing done first allows more compact instructions on some architectures (amd64/386), + // and fewer instructions (to calculate offset) on other architectures. + done atomic.Uint32 + m Mutex +} + +// Do calls the function f if and only if Do is being called for the +// first time for this instance of Once. 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. +// +// If f panics, Do considers it to have returned; future calls of Do return +// without calling f. +func (o *Once) Do(f func()) { + // Note: Here is an incorrect implementation of Do: + // + // if o.done.CompareAndSwap(0, 1) { + // f() + // } + // + // Do guarantees that when it returns, f has finished. + // This implementation would not implement that guarantee: + // given two simultaneous calls, the winner of the cas would + // call f, and the second would return immediately, without + // waiting for the first's call to f to complete. + // This is why the slow path falls back to a mutex, and why + // the o.done.Store must be delayed until after f returns. + + if o.done.Load() == 0 { + // Outlined slow-path to allow inlining of the fast-path. + o.doSlow(f) + } +} + +func (o *Once) doSlow(f func()) { + o.m.Lock() + defer o.m.Unlock() + if o.done.Load() == 0 { + defer o.done.Store(1) + f() + } +} -- cgit v1.2.3