catch noproc instead of doing alive check

the alive check does not work because a race condition still exists.

src/brod_group_coordinator.erl

@@ -75,6 +75,13 @@
         {lo_cmd_stabilize, AttemptCount, Reason}).
 
 -define(INITIAL_MEMBER_ID, <<>>).
+-define(CALL_MEMBER(MemberPid, EXPR),
+  try
+    EXPR
+  catch
+    exit:{noproc, {gen_server, call, [MemberPid | _]}} ->
+      exit({shutdown, member_down})
+  end).
 
 -type config() :: brod:group_config().
 -type ts() :: erlang:timestamp().
@@ -462,15 +469,6 @@ terminate(Reason, #state{ connection = Connection
 
 %%%_* Internal Functions =======================================================
 
-% Exit the process if MemberPid is set to pid but it's not
-% currently alive
--spec ensure_member_pid_alive(pid()) -> ok.
-ensure_member_pid_alive(MemberPid) ->
-  case brod_utils:is_pid_alive(MemberPid) of
-    true -> ok;
-    false -> exit(member_pid_shutdown)
-end.
-
 -spec discover_coordinator(state()) -> {ok, state()}.
 discover_coordinator(#state{ client     = Client
                            , connection = Connection0
@@ -508,8 +506,7 @@ stabilize(#state{ rejoin_delay_seconds = RejoinDelaySeconds
     log(State0, info, "re-joining group, reason:~p", [Reason]),
 
   %% 1. unsubscribe all currently assigned partitions
-  ok = ensure_member_pid_alive(MemberPid),
-  ok = MemberModule:assignments_revoked(MemberPid),
+  ?CALL_MEMBER(MemberPid, MemberModule:assignments_revoked(MemberPid)),
 
   %% 2. some brod_group_member implementations may wait for messages
   %%    to finish processing when assignments_revoked is called.
@@ -684,9 +681,8 @@ sync_group(#state{ groupId       = GroupId
   %% get my partition assignments
   Assignment = kpro:find(assignment, RspBody),
   TopicAssignments = get_topic_assignments(State, Assignment),
-  ok = ensure_member_pid_alive(MemberPid),
-  ok = MemberModule:assignments_received(MemberPid, MemberId,
-                                         GenerationId, TopicAssignments),
+  ?CALL_MEMBER(MemberPid,
+    MemberModule:assignments_received(MemberPid, MemberId, GenerationId, TopicAssignments)),
   NewState = State#state{is_in_group = true},
   log(NewState, info, "assignments received:~s",
       [format_assignments(TopicAssignments)]),
@@ -845,8 +841,7 @@ assign_partitions(State) when ?IS_LEADER(State) ->
   Assignments =
     case Strategy =:= callback_implemented of
       true  ->
-        ok = ensure_member_pid_alive(MemberPid),
-        MemberModule:assign_partitions(MemberPid, Members, AllPartitions);
+        ?CALL_MEMBER(MemberPid, MemberModule:assign_partitions(MemberPid, Members, AllPartitions));
       false ->
         do_assign_partitions(Strategy, Members, AllPartitions)
     end,