// mutexCache implements statCache, backed by a map and sync.RWMutex
type mutexCache struct {
	src    StatSource
	mtx    sync.RWMutex
	lookup map[string]*statSet
}

func (mc *mutexCache) get(name string) *statSet {
	// take a read lock to see if the set already exists
	mc.mtx.RLock()
	set, ok := mc.lookup[name]
	mc.mtx.RUnlock()

	if ok { // the set exists, return it
		return set
	}

	// need to take a write lock to update the map
	mc.mtx.Lock()
	// While waiting for the write lock, another goroutine may have created the
	// set. Here, we check again after obtaining the lock before making a new one
	if set, ok = mc.lookup[name]; !ok {
		set = newStatSet(mc.src, name)
		mc.lookup[name] = set
	}
	mc.mtx.Unlock()

	return set
}