fix packing of probe packets when retransmissions are canceled

packet_packer.go

@@ -565,7 +565,7 @@ func (p *packetPacker) MaybePackProbePacket(encLevel protocol.EncryptionLevel) (
 	default:
 		panic("unknown encryption level")
 	}
-	if err != nil {
+	if err != nil || contents == nil {
 		return nil, err
 	}
 	if p.perspective == protocol.PerspectiveClient && encLevel == protocol.EncryptionInitial {

packet_packer_test.go

@@ -1127,6 +1127,18 @@ var _ = Describe("Packet packer", func() {
 				Expect(packet.frames).To(HaveLen(1))
 				Expect(packet.frames[0].Frame).To(Equal(f))
 			})
+
+			It("returns nil if there's no probe data to send", func() {
+				sealingManager.EXPECT().Get1RTTSealer().Return(getSealer(), nil)
+				ackFramer.EXPECT().GetAckFrame(protocol.Encryption1RTT)
+				pnManager.EXPECT().PeekPacketNumber(protocol.Encryption1RTT).Return(protocol.PacketNumber(0x42), protocol.PacketNumberLen2)
+				expectAppendControlFrames()
+				expectAppendStreamFrames()
+
+				packet, err := packer.MaybePackProbePacket(protocol.Encryption1RTT)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(packet).To(BeNil())
+			})
 		})
 	})
 })

session.go

@@ -1404,7 +1404,7 @@ func (s *session) sendProbePacket(encLevel protocol.EncryptionLevel) error {
 			return err
 		}
 	}
-	if packet == nil {
+	if packet == nil || packet.packetContents == nil {
 		return fmt.Errorf("session BUG: couldn't pack %s probe packet", encLevel)
 	}
 	s.sendPackedPacket(packet)