Fix missing onChange event when following a leader with greater term

tinyraft.js

@@ -182,9 +182,14 @@ class TinyRaft extends EventEmitter
         if (msg.term > this.term)
         {
             this.term = msg.term;
+            this.state = FOLLOWER;
+            this.followers = null;
             this.leader = msg.leader;
+            this.votes = {};
+            this.emit('change', { state: this.state, term: this.term, leader: this.leader });
             // Repeat VOTE to the leader to join it
             this.send(this.leader, { type: VOTE, term: this.term, leader: this.leader, priority: msg.priority });
+            return;
         }
         // add <from> as voter for <msg.leader>
         this.votes[msg.leader] = this.votes[msg.leader] || [];

tinyraft.spec.js

@@ -311,6 +311,40 @@ async function testLeaderPriority()
     console.log('testLeaderPriority: OK');
 }
 
+async function testPartition1_3()
+{
+    console.log('testPartition1_3');
+    const partitions = { '1-3': true, '3-1': true };
+    const nodes = newNodes(3, partitions, cfg => cfg.nodes = [ 1, 2, 3 ]);
+    let leaderChanges = 0, prevLeader = null;
+    nodes[2].on('change', (st) =>
+    {
+        if (st.leader && st.leader != prevLeader)
+        {
+            prevLeader = st.leader;
+            leaderChanges++;
+        }
+    });
+    // Check that 3 nodes are in quorum after 2000ms
+    // 2 is always elected as leader because it's the first node that can determine
+    // that the first voting round has failed: it receives a vote for 1 from 1 and
+    // a vote for 3 from 3. So it knows that no node can get 2 votes. 1 and 3, at
+    // the same time, don't know it for sure because they don't receive failures
+    // from each other due to the network partition.
+    await new Promise(ok => setTimeout(ok, 2000));
+    checkQuorum(nodes, 3);
+    if (leaderChanges >= 3)
+    {
+        throw new Error('More than 3 leader changes in 2000ms: '+leaderChanges);
+    }
+    // Clean up
+    for (const id in nodes)
+    {
+        nodes[id].stop();
+    }
+    console.log('testLeadershipExpiration: OK');
+}
+
 async function run()
 {
     await testStartThenRemoveNode();
@@ -319,6 +353,7 @@ async function run()
     await testRestart();
     await testChangeNodes();
     await testLeaderPriority();
+    await testPartition1_3();
     process.exit(0);
 }