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Commit e5cdaf92 authored by Colin Cross's avatar Colin Cross
Browse files

Make OncePer.Once reentrant 

The value constructor passed to OncePer.Once may call back into
Once.  Replace the global mutex with a temporary channel stored
in the map using the key, and have callers for the same key
wait on the channel if they find it instead of a real value.

Test: TestOncePerReentrant
Change-Id: Ifa88a3c48981b5076b83f47fbdee5d26311725c6
parent e48ff5bb

android/onceper.go

+25 −11
Original line number Diff line number Diff line
@@ -21,7 +21,22 @@ import ( 


type OncePer struct {

	values sync.Map

	valuesLock sync.Mutex

}



type onceValueWaiter chan bool



func (once *OncePer) maybeWaitFor(key OnceKey, value interface{}) interface{} {

	if wait, isWaiter := value.(onceValueWaiter); isWaiter {

		// The entry in the map is a placeholder waiter because something else is constructing the value

		// wait until the waiter is signalled, then load the real value.

		<-wait

		value, _ = once.values.Load(key)

		if _, isWaiter := value.(onceValueWaiter); isWaiter {

			panic(fmt.Errorf("Once() waiter completed but key is still not valid"))

		}

	}



	return value

}



// Once computes a value the first time it is called with a given key per OncePer, and returns the
@@ -29,21 +44,20 @@ type OncePer struct { 
func (once *OncePer) Once(key OnceKey, value func() interface{}) interface{} {

	// Fast path: check if the key is already in the map

	if v, ok := once.values.Load(key); ok {

		return v

		return once.maybeWaitFor(key, v)

	}



	// Slow path: lock so that we don't call the value function twice concurrently

	once.valuesLock.Lock()

	defer once.valuesLock.Unlock()



	// Check again with the lock held

	if v, ok := once.values.Load(key); ok {

		return v

	// Slow path: create a OnceValueWrapper and attempt to insert it

	waiter := make(onceValueWaiter)

	if v, loaded := once.values.LoadOrStore(key, waiter); loaded {

		// Got a value, something else inserted its own waiter or a constructed value

		return once.maybeWaitFor(key, v)

	}



	// Still not in the map, call the value function and store it

	// The waiter is inserted, call the value constructor, store it, and signal the waiter

	v := value()

	once.values.Store(key, v)

	close(waiter)



	return v

}

android/onceper_test.go

+11 −0
Original line number Diff line number Diff line
@@ -133,3 +133,14 @@ func TestNewCustomOnceKey(t *testing.T) { 
		t.Errorf(`second call to Once with the NewCustomOnceKey from equal key should return "a": %q`, b)

	}

}



func TestOncePerReentrant(t *testing.T) {

	once := OncePer{}

	key1 := NewOnceKey("key")

	key2 := NewOnceKey("key")



	a := once.Once(key1, func() interface{} { return once.Once(key2, func() interface{} { return "a" }) })

	if a != "a" {

		t.Errorf(`reentrant Once should return "a": %q`, a)

	}

}