Add at CANCELLATION back-to-front ticket kind

This addresses issue #752.

src/python/src/grpc/_adapter/rear.py

@@ -191,8 +191,7 @@ class RearLink(ticket_interfaces.RearLink, activated.Activated):
     if event.status.code is _low.Code.OK:
       kind = tickets.BackToFrontPacket.Kind.COMPLETION
     elif event.status.code is _low.Code.CANCELLED:
-      # TODO(issue 752): Use a CANCELLATION ticket kind here.
+      kind = tickets.BackToFrontPacket.Kind.CANCELLATION
-      kind = tickets.BackToFrontPacket.Kind.SERVICER_FAILURE
     elif event.status.code is _low.Code.EXPIRED:
       kind = tickets.BackToFrontPacket.Kind.EXPIRATION
     else:

src/python/src/grpc/framework/base/packets/_reception.py

@@ -244,7 +244,9 @@ class _FrontReceiver(_Receiver):
       A base_interfaces.Outcome value describing operation abortion if the
         packet is abortive or None if the packet is not abortive.
     """
-    if packet.kind is packets.BackToFrontPacket.Kind.EXPIRATION:
+    if packet.kind is packets.BackToFrontPacket.Kind.CANCELLATION:
+      return base_interfaces.Outcome.CANCELLED
+    elif packet.kind is packets.BackToFrontPacket.Kind.EXPIRATION:
       return base_interfaces.Outcome.EXPIRED
     elif packet.kind is packets.BackToFrontPacket.Kind.SERVICER_FAILURE:
       return base_interfaces.Outcome.SERVICER_FAILURE

src/python/src/grpc/framework/base/packets/_transmission.py

@@ -63,6 +63,8 @@ _ABORTION_OUTCOME_TO_FRONT_TO_BACK_PACKET_KIND = {
 }
 
 _ABORTION_OUTCOME_TO_BACK_TO_FRONT_PACKET_KIND = {
+    interfaces.Outcome.CANCELLED:
+        packets.BackToFrontPacket.Kind.CANCELLATION,
     interfaces.Outcome.EXPIRED:
         packets.BackToFrontPacket.Kind.EXPIRATION,
     interfaces.Outcome.RECEPTION_FAILURE:

src/python/src/grpc/framework/base/packets/packets.py

@@ -108,9 +108,9 @@ class BackToFrontPacket(
   class Kind(enum.Enum):
     """Identifies the overall kind of a BackToFrontPacket."""
 
-    # TODO(issue 752): Add CANCELLATION.
     CONTINUATION = 'continuation'
     COMPLETION = 'completion'
+    CANCELLATION = 'cancellation'
     EXPIRATION = 'expiration'
     SERVICER_FAILURE = 'servicer failure'
     SERVICED_FAILURE = 'serviced failure'