Fix for potential userCount race in changeWaiter

Previously calls to waiter.CurrentUserCount() were going straight through to the listener (i.e. the global user count), instead of using the last known user count for the waiter.

This has the potential for missed user updates in changes.go, when user changes happen between changes waking up, and changes calling CurrentUserCount().

Author: adamcfraser

db/change_listener.go

@@ -183,6 +183,7 @@ type changeWaiter struct {
 	userKeys                  []string
 	lastCounter               uint64
 	lastTerminateCheckCounter uint64
+	lastUserCount             uint64
 }
 
 // Creates a new changeWaiter that will wait for changes for the given document keys.
@@ -210,6 +211,9 @@ func (listener *changeListener) NewWaiterWithChannels(chans base.Set, user auth.
 	}
 	waiter := listener.NewWaiter(waitKeys)
 	waiter.userKeys = userKeys
+	if userKeys != nil {
+		waiter.lastUserCount = listener.CurrentCount(userKeys)
+	}
 	return waiter
 }
 
@@ -219,6 +223,9 @@ func (waiter *changeWaiter) Wait() uint32 {
 	lastTerminateCheckCounter := waiter.lastTerminateCheckCounter
 	lastCounter := waiter.lastCounter
 	waiter.lastCounter, waiter.lastTerminateCheckCounter = waiter.listener.Wait(waiter.keys, waiter.lastCounter, waiter.lastTerminateCheckCounter)
+	if waiter.userKeys != nil {
+		waiter.lastUserCount = waiter.listener.CurrentCount(waiter.userKeys)
+	}
 	countChanged := waiter.lastCounter > lastCounter
 
 	//Uses != to compare as value can cycle back through 0
@@ -236,10 +243,7 @@ func (waiter *changeWaiter) Wait() uint32 {
 // Returns the current counter value for the waiter's user (and roles).
 // If this value changes, it means the user or roles have been updated.
 func (waiter *changeWaiter) CurrentUserCount() uint64 {
-	if waiter.userKeys == nil {
-		return 0
-	}
-	return waiter.listener.CurrentCount(waiter.userKeys)
+	return waiter.lastUserCount
 }
 
 // Updates the set of channel keys in the ChangeWaiter (maintains the existing set of user keys)