Fix tests so they don't race

src/github.com/matrix-org/dendrite/syncapi/sync/notifier_test.go

@@ -80,7 +80,9 @@ func TestNewEventAndJoinedToRoom(t *testing.T) {
 		}
 		wg.Done()
 	}()
-	time.Sleep(1 * time.Millisecond)
+
+	stream := n.fetchUserStream("@bob:localhost", true)
+	waitForBlocking(stream, 1)
 
 	n.OnNewEvent(&randomMessageEvent, types.StreamPosition(12))
 
@@ -125,6 +127,14 @@ func waitForEvents(n *Notifier, req syncRequest) (types.StreamPosition, error) {
 	}
 }
 
+// Wait until something is Wait()ing on the user stream.
+func waitForBlocking(s *UserStream, numBlocking int) {
+	for numBlocking != s.NumWaiting() {
+		// This is horrible but I don't want to add a signalling mechanism JUST for testing.
+		time.Sleep(1 * time.Microsecond)
+	}
+}
+
 func newTestSyncRequest(userID string, since types.StreamPosition) syncRequest {
 	return syncRequest{
 		userID:        userID,

src/github.com/matrix-org/dendrite/syncapi/sync/userstream.go

@@ -11,6 +11,7 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
+
 package sync
 
 import (
@@ -25,10 +26,12 @@ import (
 type UserStream struct {
 	UserID string
 	// Because this is a Cond, we can notify all waiting goroutines so this works
-	// across devices. Protects pos.
+	// across devices for the same user. Protects pos.
 	cond *sync.Cond
 	// The position to broadcast to callers of Wait().
 	pos types.StreamPosition
+	// The number of goroutines blocked on Wait() - used for testing and metrics
+	numWaiting int
 }
 
 // NewUserStream creates a new user stream
@@ -42,8 +45,10 @@ func NewUserStream(userID string) *UserStream {
 // Wait blocks until there is a new stream position for this user, which is then returned.
 func (s *UserStream) Wait() (pos types.StreamPosition) {
 	s.cond.L.Lock()
+	s.numWaiting += 1
 	s.cond.Wait()
 	pos = s.pos
+	s.numWaiting -= 1
 	s.cond.L.Unlock()
 	return
 }
@@ -55,3 +60,10 @@ func (s *UserStream) Broadcast(pos types.StreamPosition) {
 	s.cond.L.Unlock()
 	s.cond.Broadcast()
 }
+
+// NumWaiting returns the number of goroutines waiting for Wait() to return. Used for metrics and testing.
+func (s *UserStream) NumWaiting() int {
+	s.cond.L.Lock()
+	defer s.cond.L.Unlock()
+	return s.numWaiting
+}