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

android/onceper.go

@@ -20,8 +20,23 @@ import (
 )
 
 type OncePer struct {
-	values     sync.Map
-	valuesLock sync.Mutex
+	values sync.Map
+}
+
+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
 }