don't defer unlocking the mutex when getting window updates

internal/flowcontrol/stream_flow_controller.go

@@ -113,11 +113,11 @@ func (c *streamFlowController) SendWindowSize() protocol.ByteCount {
 }
 
 func (c *streamFlowController) GetWindowUpdate() protocol.ByteCount {
+	// don't use defer for unlocking the mutex here, GetWindowUpdate() is called frequently and defer shows up in the profiler
 	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	// if we already received the final offset for this stream, the peer won't need any additional flow control credit
 	if c.receivedFinalOffset {
+		c.mutex.Unlock()
 		return 0
 	}
 
@@ -129,5 +129,6 @@ func (c *streamFlowController) GetWindowUpdate() protocol.ByteCount {
 			c.connection.EnsureMinimumWindowIncrement(protocol.ByteCount(float64(c.receiveWindowIncrement) * protocol.ConnectionFlowControlMultiplier))
 		}
 	}
+	c.mutex.Unlock()
 	return offset
 }