#singleflight

singleflight.go

package main

import "sync"


type call struct {
	wg sync.WaitGroup
	val interface{}
	err error
	result []chan
	dups int
}

type Group struct {
	lock sync.Mutex
	m map[string]*call
}

func (g *Group)Do (key string, fn func() (interface{}, error))(v interface{}, err error, shared bool){
	g.lock.Lock()

	if g.m == nil {
		g.m = make(map[string]*call)
	}

	if c, ok := g.m[key];ok {
		c.dups ++
		g.lock.Unlock()
		c.wg.Wait()
		return c.val, c.err, true
	}

	c := new(call)
	g.m[key] = c
	c.wg.Add(1)
	g.lock.Unlock()
	g.doCall(c, key, fn)
	return c.val, c.err, c.dups > 0
}

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()
}