Exclude witnesses from handover candidates

Summary: Exclude witness peers when determining good handover candidates as witnesses cannot become leader.

Reviewed By: jaher

Differential Revision: D70976290

fbshipit-source-id: 64d8ade85f5a40c15e94c925d0d07187264b2e14

src/wa_raft_server.erl

@@ -2029,6 +2029,14 @@ config_membership(#{membership := Membership}) when Membership =/= [] ->
 config_membership(_Config) ->
     error(membership_not_set).
 
+-spec config_replicas(Config :: config()) -> Replicas :: membership().
+config_replicas(#{membership := Membership, witness := Witnesses}) ->
+    Membership -- Witnesses;
+config_replicas(#{membership := Membership}) ->
+    Membership;
+config_replicas(_Config) ->
+    error(membership_not_set).
+
 -spec config_witnesses(Config :: config()) -> Witnesses :: [peer()].
 config_witnesses(#{witness := Witnesses}) ->
     Witnesses;
@@ -2474,10 +2482,10 @@ heartbeat(?IDENTITY_REQUIRES_MIGRATION(_, FollowerId) = Sender,
 
 -spec compute_handover_candidates(State :: #raft_state{}) -> [node()].
 compute_handover_candidates(#raft_state{application = App, log_view = View, match_index = MatchIndex} = State) ->
-    Membership = config_membership(config(State)),
+    Replicas = config_replicas(config(State)),
     LastLogIndex = wa_raft_log:last_index(View),
     MaxHandoverLogEntries = ?RAFT_HANDOVER_MAX_ENTRIES(App),
-    [Peer || {_Name, Peer} <- Membership, Peer =/= node(), LastLogIndex - maps:get(Peer, MatchIndex, 0) =< MaxHandoverLogEntries].
+    [Peer || {_Name, Peer} <- Replicas, Peer =/= node(), LastLogIndex - maps:get(Peer, MatchIndex, 0) =< MaxHandoverLogEntries].
 
 -spec adjust_config(Action :: {add, peer()} | {remove, peer()} | {add_witness, peer()} | {remove_witness, peer()} | {refresh, undefined},
                     Config :: config(), State :: #raft_state{}) -> {ok, NewConfig :: config()} | {error, Reason :: atom()}.