remove flow control mutexes for the sending data

Receiving MAX_{STREAM}_DATA frames and sending data is all done
sequentially, so we don't need a mutex there.

internal/flowcontrol/base_flow_controller.go

@@ -10,35 +10,28 @@ import (
 )
 
 type baseFlowController struct {
-	mutex sync.RWMutex
-
-	rttStats *congestion.RTTStats
-
+	// for sending data
 	bytesSent  protocol.ByteCount
 	sendWindow protocol.ByteCount
 
-	lastWindowUpdateTime time.Time
-
+	// for receiving data
+	mutex                     sync.RWMutex
 	bytesRead                 protocol.ByteCount
 	highestReceived           protocol.ByteCount
 	receiveWindow             protocol.ByteCount
 	receiveWindowIncrement    protocol.ByteCount
 	maxReceiveWindowIncrement protocol.ByteCount
+	lastWindowUpdateTime      time.Time
+	rttStats                  *congestion.RTTStats
 }
 
 func (c *baseFlowController) AddBytesSent(n protocol.ByteCount) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	c.bytesSent += n
 }
 
 // UpdateSendWindow should be called after receiving a WindowUpdateFrame
 // it returns true if the window was actually updated
 func (c *baseFlowController) UpdateSendWindow(offset protocol.ByteCount) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	if offset > c.sendWindow {
 		c.sendWindow = offset
 	}
@@ -82,9 +75,6 @@ func (c *baseFlowController) getWindowUpdate() protocol.ByteCount {
 // IsBlocked says if it is blocked by flow control.
 // If it is blocked, the offset is returned.
 func (c *baseFlowController) IsBlocked() (bool, protocol.ByteCount) {
-	c.mutex.RLock()
-	defer c.mutex.RUnlock()
-
 	if c.sendWindowSize() != 0 {
 		return false, 0
 	}