golang中cache组件的使用及groupcache源码解析

type call struct {
	wg  sync.WaitGroup
	val interface{}
	err error
}

type Group struct {
	mu sync.Mutex       // protects m
	m  map[string]*call // lazily initialized
}

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()

type Cache struct {
	// MaxEntries is the maximum number of cache entries before
	// an item is evicted. Zero means no limit.
	MaxEntries int

	// OnEvicted optionally specifies a callback function to be
	// executed when an entry is purged from the cache.
	OnEvicted func(key Key, value interface{})

	ll    *list.List
	cache map[interface{}]*list.Element
}

func (c *Cache) Add(key Key, value interface{}) {
	if c.cache == nil {
		c.cache = make(map[interface{}]*list.Element)
		c.ll = list.New()
	}
	if ee, ok := c.cache[key]; ok {
		c.ll.MoveToFront(ee)
		ee.Value.(*entry).value = value
		return
	}
	ele := c.ll.PushFront(entry{key, value})
	c.cache[key] = ele
	if c.MaxEntries != 0  c.ll.Len() > c.MaxEntries {
		c.RemoveOldest()
	}
}

func (c *Cache) RemoveOldest() {
	if c.cache == nil {
		return
	}
	ele := c.ll.Back()
	if ele != nil {
		c.removeElement(ele)
	}
}