Make leader more tolerant w.r.t. incoming AppendEntriesRPC responses.

arangod/Agency/Agent.cpp

@@ -804,7 +804,20 @@ bool Agent::challengeLeadership() {
   
   for (auto const& i : _lastAcked) {
     duration<double> m = system_clock::now() - i.second;
-    if (0.9 * _config.minPing() * _config.timeoutMult() > m.count()) {
+    // This is rather arbitrary here: We used to have 0.9 here to absolutely
+    // ensure that a leader resigns before another one even starts an election.
+    // However, the Raft paper does not mention this at all. Rather, in the
+    // paper it is written that the leader should resign immediately if it
+    // sees a higher term from another server. Currently we have not
+    // implemented to return the follower's term with a response to
+    // AppendEntriesRPC, so the leader cannot find out a higher term this
+    // way. The leader can, however, see a higher term in the incoming
+    // AppendEntriesRPC a new leader sends out, and it will immediately
+    // resign if it sees that. For the moment, this value here can stay.
+    // We should soon implement sending the follower's term back with
+    // each response and probably get rid of this method altogether,
+    // but this requires a bit more thought.
+    if (_config.maxPing() * _config.timeoutMult() > m.count()) {
       ++good;
     }
   }