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Source file src/sync/once.go

Documentation: sync

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package sync
     6  
     7  import (
     8  	"sync/atomic"
     9  )
    10  
    11  // Once is an object that will perform exactly one action.
    12  //
    13  // A Once must not be copied after first use.
    14  //
    15  // In the terminology of the Go memory model,
    16  // the return from f “synchronizes before”
    17  // the return from any call of once.Do(f).
    18  type Once struct {
    19  	// done indicates whether the action has been performed.
    20  	// It is first in the struct because it is used in the hot path.
    21  	// The hot path is inlined at every call site.
    22  	// Placing done first allows more compact instructions on some architectures (amd64/386),
    23  	// and fewer instructions (to calculate offset) on other architectures.
    24  	done atomic.Uint32
    25  	m    Mutex
    26  }
    27  
    28  // Do calls the function f if and only if Do is being called for the
    29  // first time for this instance of Once. In other words, given
    30  //
    31  //	var once Once
    32  //
    33  // if once.Do(f) is called multiple times, only the first call will invoke f,
    34  // even if f has a different value in each invocation. A new instance of
    35  // Once is required for each function to execute.
    36  //
    37  // Do is intended for initialization that must be run exactly once. Since f
    38  // is niladic, it may be necessary to use a function literal to capture the
    39  // arguments to a function to be invoked by Do:
    40  //
    41  //	config.once.Do(func() { config.init(filename) })
    42  //
    43  // Because no call to Do returns until the one call to f returns, if f causes
    44  // Do to be called, it will deadlock.
    45  //
    46  // If f panics, Do considers it to have returned; future calls of Do return
    47  // without calling f.
    48  func (o *Once) Do(f func()) {
    49  	// Note: Here is an incorrect implementation of Do:
    50  	//
    51  	//	if o.done.CompareAndSwap(0, 1) {
    52  	//		f()
    53  	//	}
    54  	//
    55  	// Do guarantees that when it returns, f has finished.
    56  	// This implementation would not implement that guarantee:
    57  	// given two simultaneous calls, the winner of the cas would
    58  	// call f, and the second would return immediately, without
    59  	// waiting for the first's call to f to complete.
    60  	// This is why the slow path falls back to a mutex, and why
    61  	// the o.done.Store must be delayed until after f returns.
    62  
    63  	if o.done.Load() == 0 {
    64  		// Outlined slow-path to allow inlining of the fast-path.
    65  		o.doSlow(f)
    66  	}
    67  }
    68  
    69  func (o *Once) doSlow(f func()) {
    70  	o.m.Lock()
    71  	defer o.m.Unlock()
    72  	if o.done.Load() == 0 {
    73  		defer o.done.Store(1)
    74  		f()
    75  	}
    76  }
    77  

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