github.com/shijuvar/go@v0.0.0-20141209052335-e8f13700b70c/src/net/singleflight.go (about)

     1  // Copyright 2013 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 net
     6  
     7  import "sync"
     8  
     9  // call is an in-flight or completed singleflight.Do call
    10  type call struct {
    11  	wg sync.WaitGroup
    12  
    13  	// These fields are written once before the WaitGroup is done
    14  	// and are only read after the WaitGroup is done.
    15  	val interface{}
    16  	err error
    17  
    18  	// These fields are read and written with the singleflight
    19  	// mutex held before the WaitGroup is done, and are read but
    20  	// not written after the WaitGroup is done.
    21  	dups  int
    22  	chans []chan<- singleflightResult
    23  }
    24  
    25  // singleflight represents a class of work and forms a namespace in
    26  // which units of work can be executed with duplicate suppression.
    27  type singleflight struct {
    28  	mu sync.Mutex       // protects m
    29  	m  map[string]*call // lazily initialized
    30  }
    31  
    32  // singleflightResult holds the results of Do, so they can be passed
    33  // on a channel.
    34  type singleflightResult struct {
    35  	v      interface{}
    36  	err    error
    37  	shared bool
    38  }
    39  
    40  // Do executes and returns the results of the given function, making
    41  // sure that only one execution is in-flight for a given key at a
    42  // time. If a duplicate comes in, the duplicate caller waits for the
    43  // original to complete and receives the same results.
    44  // The return value shared indicates whether v was given to multiple callers.
    45  func (g *singleflight) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
    46  	g.mu.Lock()
    47  	if g.m == nil {
    48  		g.m = make(map[string]*call)
    49  	}
    50  	if c, ok := g.m[key]; ok {
    51  		c.dups++
    52  		g.mu.Unlock()
    53  		c.wg.Wait()
    54  		return c.val, c.err, true
    55  	}
    56  	c := new(call)
    57  	c.wg.Add(1)
    58  	g.m[key] = c
    59  	g.mu.Unlock()
    60  
    61  	g.doCall(c, key, fn)
    62  	return c.val, c.err, c.dups > 0
    63  }
    64  
    65  // DoChan is like Do but returns a channel that will receive the
    66  // results when they are ready.
    67  func (g *singleflight) DoChan(key string, fn func() (interface{}, error)) <-chan singleflightResult {
    68  	ch := make(chan singleflightResult, 1)
    69  	g.mu.Lock()
    70  	if g.m == nil {
    71  		g.m = make(map[string]*call)
    72  	}
    73  	if c, ok := g.m[key]; ok {
    74  		c.dups++
    75  		c.chans = append(c.chans, ch)
    76  		g.mu.Unlock()
    77  		return ch
    78  	}
    79  	c := &call{chans: []chan<- singleflightResult{ch}}
    80  	c.wg.Add(1)
    81  	g.m[key] = c
    82  	g.mu.Unlock()
    83  
    84  	go g.doCall(c, key, fn)
    85  
    86  	return ch
    87  }
    88  
    89  // doCall handles the single call for a key.
    90  func (g *singleflight) doCall(c *call, key string, fn func() (interface{}, error)) {
    91  	c.val, c.err = fn()
    92  	c.wg.Done()
    93  
    94  	g.mu.Lock()
    95  	delete(g.m, key)
    96  	for _, ch := range c.chans {
    97  		ch <- singleflightResult{c.val, c.err, c.dups > 0}
    98  	}
    99  	g.mu.Unlock()
   100  }
   101  
   102  // Forget tells the singleflight to forget about a key.  Future calls
   103  // to Do for this key will call the function rather than waiting for
   104  // an earlier call to complete.
   105  func (g *singleflight) Forget(key string) {
   106  	g.mu.Lock()
   107  	delete(g.m, key)
   108  	g.mu.Unlock()
   109  }