Break FindGroupCoordinator into request/response methods

This splits the FindGroupCoordinator function (which is blocking) into
request generation / response parsing methods.

The public API does not change. However, this allows power users who are
willing to deal with risk of private methods changing under their feet
to decouple generating the message futures from processing their
responses. In other words, you can use these to fire a bunch of requests
at once and delay processing the responses until all requests are fired.

This is modeled on the work done in #1845.

Additionally, I removed the code that tried to leverage the error
checking from `cluster.add_group_coordinator()`. That code had changed
in #1822, removing most of the error checking... so it no longer adds
any value, but instead merely increases complexity and coupling.

kafka/admin/client.py

@@ -271,6 +271,47 @@ def _refresh_controller_id(self):
                 "Kafka Admin interface cannot determine the controller using MetadataRequest_v{}."
                 .format(version))
 
+    def _find_group_coordinator_id_send_request(self, group_id):
+        """Send a FindCoordinatorRequest to a broker.
+
+        :param group_id: The consumer group ID. This is typically the group
+            name as a string.
+        :return: A message future
+        """
+        # TODO add support for dynamically picking version of
+        # GroupCoordinatorRequest which was renamed to FindCoordinatorRequest.
+        # When I experimented with this, the coordinator value returned in
+        # GroupCoordinatorResponse_v1 didn't match the value returned by
+        # GroupCoordinatorResponse_v0 and I couldn't figure out why.
+        version = 0  # version = self._matching_api_version(GroupCoordinatorRequest)
+        if version <= 0:
+            request = GroupCoordinatorRequest[version](group_id)
+        else:
+            raise NotImplementedError(
+                "Support for GroupCoordinatorRequest_v{} has not yet been added to KafkaAdminClient."
+                .format(version))
+        return self._send_request_to_node(self._client.least_loaded_node(), request)
+
+    def _find_group_coordinator_id_process_response(self, response):
+        """Process a FindCoordinatorResponse.
+
+        :param response: a FindCoordinatorResponse.
+        :return: The node_id of the broker that is the coordinator.
+        """
+        if response.API_VERSION <= 0:
+            error_type = Errors.for_code(response.error_code)
+            if error_type is not Errors.NoError:
+                # Note: When error_type.retriable, Java will retry... see
+                # KafkaAdminClient's handleFindCoordinatorError method
+                raise error_type(
+                    "FindCoordinatorRequest failed with response '{}'."
+                    .format(response))
+        else:
+            raise NotImplementedError(
+                "Support for FindCoordinatorRequest_v{} has not yet been added to KafkaAdminClient."
+                .format(response.API_VERSION))
+        return response.coordinator_id
+
     def _find_group_coordinator_id(self, group_id):
         """Find the broker node_id of the coordinator of the given group.
 
@@ -283,35 +324,10 @@ def _find_group_coordinator_id(self, group_id):
         :return: The node_id of the broker that is the coordinator.
         """
         # Note: Java may change how this is implemented in KAFKA-6791.
-        #
-        # TODO add support for dynamically picking version of
-        # GroupCoordinatorRequest which was renamed to FindCoordinatorRequest.
-        # When I experimented with this, GroupCoordinatorResponse_v1 didn't
-        # match GroupCoordinatorResponse_v0 and I couldn't figure out why.
-        gc_request = GroupCoordinatorRequest[0](group_id)
-        future = self._send_request_to_node(self._client.least_loaded_node(), gc_request)
-
+        future = self._find_group_coordinator_id_send_request(group_id)
         self._wait_for_futures([future])
-
-        gc_response = future.value
-        # use the extra error checking in add_group_coordinator() rather than
-        # immediately returning the group coordinator.
-        success = self._client.cluster.add_group_coordinator(group_id, gc_response)
-        if not success:
-            error_type = Errors.for_code(gc_response.error_code)
-            assert error_type is not Errors.NoError
-            # Note: When error_type.retriable, Java will retry... see
-            # KafkaAdminClient's handleFindCoordinatorError method
-            raise error_type(
-                "Could not identify group coordinator for group_id '{}' from response '{}'."
-                .format(group_id, gc_response))
-        group_coordinator = self._client.cluster.coordinator_for_group(group_id)
-        # will be None if the coordinator was never populated, which should never happen here
-        assert group_coordinator is not None
-        # will be -1 if add_group_coordinator() failed... but by this point the
-        # error should have been raised.
-        assert group_coordinator != -1
-        return group_coordinator
+        response = future.value
+        return self._find_group_coordinator_id_process_response(response)
 
     def _send_request_to_node(self, node_id, request):
         """Send a Kafka protocol message to a specific broker.
@@ -329,7 +345,6 @@ def _send_request_to_node(self, node_id, request):
             self._client.poll()
         return self._client.send(node_id, request)
 
-
     def _send_request_to_controller(self, request):
         """Send a Kafka protocol message to the cluster controller.