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Source file src/internal/singleflight/singleflight.go

Documentation: internal/singleflight

  // Copyright 2013 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 singleflight provides a duplicate function call suppression
  // mechanism.
  package singleflight
  
  import "sync"
  
  // call is an in-flight or completed singleflight.Do call
  type call struct {
  	wg sync.WaitGroup
  
  	// These fields are written once before the WaitGroup is done
  	// and are only read after the WaitGroup is done.
  	val interface{}
  	err error
  
  	// These fields are read and written with the singleflight
  	// mutex held before the WaitGroup is done, and are read but
  	// not written after the WaitGroup is done.
  	dups  int
  	chans []chan<- Result
  }
  
  // Group represents a class of work and forms a namespace in
  // which units of work can be executed with duplicate suppression.
  type Group struct {
  	mu sync.Mutex       // protects m
  	m  map[string]*call // lazily initialized
  }
  
  // Result holds the results of Do, so they can be passed
  // on a channel.
  type Result struct {
  	Val    interface{}
  	Err    error
  	Shared bool
  }
  
  // Do executes and returns the results of the given function, making
  // sure that only one execution is in-flight for a given key at a
  // time. If a duplicate comes in, the duplicate caller waits for the
  // original to complete and receives the same results.
  // The return value shared indicates whether v was given to multiple callers.
  func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
  	g.mu.Lock()
  	if g.m == nil {
  		g.m = make(map[string]*call)
  	}
  	if c, ok := g.m[key]; ok {
  		c.dups++
  		g.mu.Unlock()
  		c.wg.Wait()
  		return c.val, c.err, true
  	}
  	c := new(call)
  	c.wg.Add(1)
  	g.m[key] = c
  	g.mu.Unlock()
  
  	g.doCall(c, key, fn)
  	return c.val, c.err, c.dups > 0
  }
  
  // DoChan is like Do but returns a channel that will receive the
  // results when they are ready.
  func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
  	ch := make(chan Result, 1)
  	g.mu.Lock()
  	if g.m == nil {
  		g.m = make(map[string]*call)
  	}
  	if c, ok := g.m[key]; ok {
  		c.dups++
  		c.chans = append(c.chans, ch)
  		g.mu.Unlock()
  		return ch
  	}
  	c := &call{chans: []chan<- Result{ch}}
  	c.wg.Add(1)
  	g.m[key] = c
  	g.mu.Unlock()
  
  	go g.doCall(c, key, fn)
  
  	return ch
  }
  
  // doCall handles the single call for a key.
  func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
  	c.val, c.err = fn()
  	c.wg.Done()
  
  	g.mu.Lock()
  	delete(g.m, key)
  	for _, ch := range c.chans {
  		ch <- Result{c.val, c.err, c.dups > 0}
  	}
  	g.mu.Unlock()
  }
  
  // Forget tells the singleflight to forget about a key.  Future calls
  // to Do for this key will call the function rather than waiting for
  // an earlier call to complete.
  func (g *Group) Forget(key string) {
  	g.mu.Lock()
  	delete(g.m, key)
  	g.mu.Unlock()
  }
  

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