flowcontrol: make it possible to call Abandon multiple times (#4459)

Abandon is called when a RESET_STREAM frame is received, and marks the
bytes between the highest read position and the final offset as
consumed. Making it possible to call Abandon multiple times makes using
this API a bit easier, since the stream doesn't need to track if it
already called it.

internal/flowcontrol/stream_flow_controller.go

@@ -111,6 +111,7 @@ func (c *streamFlowController) AddBytesRead(n protocol.ByteCount) {
 func (c *streamFlowController) Abandon() {
 	c.mutex.Lock()
 	unread := c.highestReceived - c.bytesRead
+	c.bytesRead = c.highestReceived
 	c.mutex.Unlock()
 	if unread > 0 {
 		c.connection.AddBytesRead(unread)

internal/flowcontrol/stream_flow_controller_test.go

@@ -159,6 +159,15 @@ var _ = Describe("Stream Flow controller", func() {
 				controller.Abandon()
 				Expect(controller.connection.(*connectionFlowController).bytesRead).To(Equal(protocol.ByteCount(100)))
 			})
+
+			It("tolerates repeated calls to Abandon", func() {
+				controller.AddBytesRead(5)
+				Expect(controller.UpdateHighestReceived(100, true)).To(Succeed())
+				controller.Abandon()
+				controller.Abandon()
+				controller.Abandon()
+				Expect(controller.connection.(*connectionFlowController).bytesRead).To(Equal(protocol.ByteCount(100)))
+			})
 		})
 
 		It("saves when data is read", func() {