Source: singleflight.go in package internal/singleflight

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. The second result is true if the function will eventually be called, false if it will not (because there is a pending request with this key).

func (g *Group) DoChan(key string, fn func() (interface{}, error)) (<-chan Result, bool) {
	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, false
	}
	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, true
}

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()
}

ForgetUnshared tells the singleflight to forget about a key if it is not shared with any other goroutines. Future calls to Do for a forgotten key will call the function rather than waiting for an earlier call to complete. Returns whether the key was forgotten or unknown--that is, whether no other goroutines are waiting for the result.

func (g *Group) ForgetUnshared(key string) bool {
	g.mu.Lock()
	defer g.mu.Unlock()
	c, ok := g.m[key]
	if !ok {
		return true
	}
	if c.dups == 0 {
		delete(g.m, key)
		return true
	}
	return false