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  type Once struct {
    13  	// done indicates whether the action has been performed.
    14  	// It is first in the struct because it is used in the hot path.
    15  	// The hot path is inlined at every call site.
    16  	// Placing done first allows more compact instructions on some architectures (amd64/x86),
    17  	// and fewer instructions (to calculate offset) on other architectures.
    18  	done uint32
    19  	m    Mutex
    20  }
    21  
    22  // Do calls the function f if and only if Do is being called for the
    23  // first time for this instance of Once. In other words, given
    24  // 	var once Once
    25  // if once.Do(f) is called multiple times, only the first call will invoke f,
    26  // even if f has a different value in each invocation. A new instance of
    27  // Once is required for each function to execute.
    28  //
    29  // Do is intended for initialization that must be run exactly once. Since f
    30  // is niladic, it may be necessary to use a function literal to capture the
    31  // arguments to a function to be invoked by Do:
    32  // 	config.once.Do(func() { config.init(filename) })
    33  //
    34  // Because no call to Do returns until the one call to f returns, if f causes
    35  // Do to be called, it will deadlock.
    36  //
    37  // If f panics, Do considers it to have returned; future calls of Do return
    38  // without calling f.
    39  //
    40  func (o *Once) Do(f func()) {
    41  	// Note: Here is an incorrect implementation of Do:
    42  	//
    43  	//	if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
    44  	//		f()
    45  	//	}
    46  	//
    47  	// Do guarantees that when it returns, f has finished.
    48  	// This implementation would not implement that guarantee:
    49  	// given two simultaneous calls, the winner of the cas would
    50  	// call f, and the second would return immediately, without
    51  	// waiting for the first's call to f to complete.
    52  	// This is why the slow path falls back to a mutex, and why
    53  	// the atomic.StoreUint32 must be delayed until after f returns.
    54  
    55  	if atomic.LoadUint32(&o.done) == 0 {
    56  		// Outlined slow-path to allow inlining of the fast-path.
    57  		o.doSlow(f)
    58  	}
    59  }
    60  
    61  func (o *Once) doSlow(f func()) {
    62  	o.m.Lock()
    63  	defer o.m.Unlock()
    64  	if o.done == 0 {
    65  		defer atomic.StoreUint32(&o.done, 1)
    66  		f()
    67  	}
    68  }
    69  

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