raft: activate pre-vote, add intermediate state check before real vote

CHANGELOG.md

@@ -6,14 +6,14 @@
 - The mechanism meant to provide an early warning for potential ``MANIFEST``
 corruption was flaky, and would sometimes report a problem where none existed.
 
-Many thanks to Franck Eyraud (JRC) for the bug report concerning erroneous ``MANIFEST``-related
-warning.
-
 ### Improvements
 - Implementation of an optional part of raft, pre-vote. This should prevent partitioned,
-or otherwise flaky rejoining servers from triggering needless elections. A node will first issue
-an experimental voting round before advancing its term, and start campaigning
-for real only if it has a good chance of winning.
+or otherwise flaky rejoining servers from triggering unnecessary and disruptive elections.
+A node will first issue an experimental voting round before advancing its term, and start campaigning
+for earnest only if it has a good chance of winning.
+
+Many thanks to Franck Eyraud (JRC) for the bug report concerning erroneous ``MANIFEST``-related
+warning.
 
 ## 0.4.2 (2020-03-12)
 

src/raft/RaftDirector.cc

@@ -136,7 +136,26 @@ void RaftDirector::runForLeader() {
     return;
   }
 
-  // advance the term by one, become a candidate.
+  //----------------------------------------------------------------------------
+  // vvvvvvvvvvvvvvvvvvvvvvv OPTIONAL PART OF RAFT vvvvvvvvvvvvvvvvvvvvvvvvvvvvv
+  //----------------------------------------------------------------------------
+  ElectionOutcome prevoteOutcome = RaftElection::performPreVote(votereq, state, contactDetails);
+  if(prevoteOutcome == ElectionOutcome::kVetoed) {
+    lastHeartbeatBeforeVeto = lastHeartbeat;
+    qdb_info("Pre-vote round for term " << snapshot->term + 1 << " resulted in a veto. This means, the next leader of this cluster cannot be me. Stopping election attempts until I receive a heartbeat.");
+  }
+
+  if(prevoteOutcome != ElectionOutcome::kElected) return;
+  if(!state.isSnapshotCurrent(snapshot.get())) {
+    qdb_info("Raft state has progressed since successful pre-vote round, retrying from scratch.");
+    return;
+  }
+
+  //----------------------------------------------------------------------------
+  // ^^^^^^^^^^^^^^^^^^^^^^^ OPTIONAL PART OF RAFT ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  //----------------------------------------------------------------------------
+
+  // pre-vote succeeded, advance the term by one, become a candidate.
   if(!state.observed(snapshot->term+1, {})) return;
   if(!state.becomeCandidate(snapshot->term+1)) return;
 

test/raft.cc

@@ -811,7 +811,7 @@ TEST_F(Raft_Director, late_consensus) {
 
   // verify the node tried to ascend, and failed
   RaftStateSnapshotPtr snapshot = state(0)->getSnapshot();
-  ASSERT_GE(snapshot->term, 1);
+  ASSERT_EQ(snapshot->term, 0);
   ASSERT_TRUE(snapshot->leader.empty());
   ASSERT_TRUE( (snapshot->status == RaftStatus::FOLLOWER) || (snapshot->status == RaftStatus::CANDIDATE) );