patch 7.4.1493
Problem:    Wrong callback invoked for zero-id messages.
Solution:   Don't use the first one-time callback when the sequence number
            doesn't match.
diff --git a/src/testdir/test_channel.py b/src/testdir/test_channel.py
index 02e6ba3..a465d65 100644
--- a/src/testdir/test_channel.py
+++ b/src/testdir/test_channel.py
@@ -143,6 +143,11 @@
                         print("sending: {}".format(cmd))
                         self.request.sendall(cmd.encode('utf-8'))
                         response = ""
+                    elif decoded[1] == 'send zero':
+                        cmd = '[0,"zero index"]'
+                        print("sending: {}".format(cmd))
+                        self.request.sendall(cmd.encode('utf-8'))
+                        response = "sent zero"
                     elif decoded[1] == 'close me':
                         print("closing")
                         self.request.close()