add FlowController method to determine if we need to send a Blocked frame

ref #100

flow_controller.go

@@ -10,6 +10,7 @@ type flowController struct {
 
 	bytesSent                protocol.ByteCount
 	sendFlowControlWindow    protocol.ByteCount
+	lastBlockedSentForOffset protocol.ByteCount
 
 	bytesRead                         protocol.ByteCount
 	receiveWindowUpdateThreshold      protocol.ByteCount
@@ -51,6 +52,21 @@ func (c *flowController) AddBytesRead(n protocol.ByteCount) {
 	c.bytesRead += n
 }
 
+// MaybeTriggerBlocked determines if it is necessary to send a Blocked for this stream
+// it makes sure that only one Blocked is sent for each offset
+func (c *flowController) MaybeTriggerBlocked() bool {
+	if c.SendWindowSize() != 0 {
+		return false
+	}
+
+	if c.lastBlockedSentForOffset == c.sendFlowControlWindow {
+		return false
+	}
+
+	c.lastBlockedSentForOffset = c.sendFlowControlWindow
+	return true
+}
+
 // MaybeTriggerWindowUpdate determines if it is necessary to send a WindowUpdate
 // if so, it returns true and the offset of the window
 func (c *flowController) MaybeTriggerWindowUpdate() (bool, protocol.ByteCount) {

flow_controller_test.go

@@ -41,6 +41,34 @@ var _ = Describe("Flow controller", func() {
 			Expect(updateSuccessful).To(BeFalse())
 			Expect(controller.SendWindowSize()).To(Equal(protocol.ByteCount(20)))
 		})
+
+		Context("Blocked", func() {
+			var sendFlowControlWindow protocol.ByteCount = 20
+
+			BeforeEach(func() {
+				controller.sendFlowControlWindow = sendFlowControlWindow
+			})
+
+			It("sends a Blocked when there's no space left in the window", func() {
+				controller.bytesSent = sendFlowControlWindow
+				Expect(controller.MaybeTriggerBlocked()).To(BeTrue())
+			})
+
+			It("does not send a Blocked when there's still space in the window", func() {
+				controller.bytesSent = sendFlowControlWindow - 1
+				Expect(controller.MaybeTriggerBlocked()).To(BeFalse())
+			})
+
+			It("only sends one Blocked for one offset", func() {
+				controller.bytesSent = sendFlowControlWindow
+				Expect(controller.MaybeTriggerBlocked()).To(BeTrue())
+				Expect(controller.MaybeTriggerBlocked()).To(BeFalse())
+				updateSuccessfull := controller.UpdateSendWindow(sendFlowControlWindow + 1)
+				Expect(updateSuccessfull).To(BeTrue())
+				controller.bytesSent = sendFlowControlWindow + 1
+				Expect(controller.MaybeTriggerBlocked()).To(BeTrue())
+			})
+		})
 	})
 
 	Context("receive flow control", func() {