groupcache使用singleflight实现资源访问合并

/ Package singleflight provides a duplicate function call suppression
// mechanism.
package singleflight

import "sync"

// call is an in-flight or completed Do call
type call struct {
    wg  sync.WaitGroup
    val interface{}
    err error
}

// 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
}

// 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.
func (g *Group) Do(key string, fn func() (interface{}, error)) (interface{}, error) {
    g.mu.Lock()
    if g.m == nil {
        g.m = make(map[string]*call)
    }
    if c, ok := g.m[key]; ok {
        g.mu.Unlock()
        c.wg.Wait()
        return c.val, c.err
    }
    c := new(call)
    c.wg.Add(1)
    g.m[key] = c
    g.mu.Unlock()

    c.val, c.err = fn()
    c.wg.Done()

    g.mu.Lock()
    delete(g.m, key)
    g.mu.Unlock()

    return c.val, c.err
}