pack WindowUpdate frames into two packets if they don't fit into one

work towards #96

packet_packer.go

@@ -132,17 +132,15 @@ func (p *packetPacker) composeNextPacket(stopWaitingFrame *frames.StopWaitingFra
 
 	maxFrameSize := protocol.MaxFrameAndPublicHeaderSize - publicHeaderLength
 
-	// TODO: handle the extremely unlikely case that there are more windowUpdateFrames than we can fit into one packet
 	for len(p.windowUpdateFrames) > 0 {
 		frame := p.windowUpdateFrames[0]
 		minLength, _ := frame.MinLength() // windowUpdateFrames.MinLength() *never* returns an error
+		if payloadLength+minLength > maxFrameSize {
+			break
+		}
 		payloadLength += minLength
 		payloadFrames = append(payloadFrames, frame)
 		p.windowUpdateFrames = p.windowUpdateFrames[1:]
-
-		if payloadLength > maxFrameSize {
-			panic("internal inconsistency: packet payload too large")
-		}
 	}
 
 	if stopWaitingFrame != nil {

packet_packer_test.go

@@ -217,6 +217,25 @@ var _ = Describe("Packet packer", func() {
 			Expect(payloadFrames).To(ContainElement(f2))
 		})
 
+		It("packs a lot of window update frames into 2 packets if they don't fit into one", func() {
+			windowUpdateFrame := &frames.WindowUpdateFrame{
+				StreamID: 0x1337,
+			}
+			minLength, _ := windowUpdateFrame.MinLength()
+			maxFramesPerPacket := int(protocol.MaxFrameAndPublicHeaderSize-publicHeaderLen) / int(minLength)
+			var controlFrames []frames.Frame
+			for i := 0; i < maxFramesPerPacket+10; i++ {
+				controlFrames = append(controlFrames, windowUpdateFrame)
+			}
+			packer.controlFrames = controlFrames
+			payloadFrames, err := packer.composeNextPacket(nil, publicHeaderLen, true)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len(payloadFrames)).To(Equal(maxFramesPerPacket))
+			payloadFrames, err = packer.composeNextPacket(nil, publicHeaderLen, true)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len(payloadFrames)).To(Equal(10))
+		})
+
 		It("only packs a WindowUpdateFrame once", func() {
 			f := &frames.WindowUpdateFrame{
 				StreamID:   0x1337,