Don't prioritise DOWNs from the left. Also explain why we prioritise in various ways.

1 files changed, 14 insertions, 1 deletions

diff --git a/src/gm.erl b/src/gm.erl
index fb59b9cb0b..7a2f5835b0 100644
--- a/src/gm.erl
+++ b/src/gm.erl
@@ -732,8 +732,21 @@ code_change(_OldVsn, State, _Extra) ->
 
 prioritise_info(flush, _Len, _State) ->
     1;
+%% DOWN messages should not overtake initial catchups; if they do we
+%% will receive a DOWN we do not know what to do with.
 prioritise_info({'DOWN', _MRef, process, _Pid, _Reason}, _Len,
-                #state { members_state = MS }) when MS /= undefined ->
+                #state { members_state = undefined }) ->
+    0;
+%% We should not prioritise DOWN messages from our left since
+%% otherwise the DOWN can overtake any last activity from the left,
+%% causing that activity to be lost.
+prioritise_info({'DOWN', _MRef, process, LeftPid, _Reason}, _Len,
+                #state { left = {{_LeftVer, LeftPid}, _MRef2} }) ->
+    0;
+%% But prioritise all other DOWNs - we want to make sure we are not
+%% sending activity into the void for too long because our right is
+%% down but we don't know it.
+prioritise_info({'DOWN', MRef, process, Pid, _Reason}, _Len, _State) ->
     1;
 prioritise_info(_, _Len, _State) ->
     0.