NRG: Do not revert term on truncate WAL

Signed-off-by: Neil Twigg <neil@nats.io>
Co-authored-by: Maurice van Veen <github@mauricevanveen.com>

server/raft.go

@@ -3215,7 +3215,7 @@ func (n *raft) truncateWAL(term, index uint64) {
 		}
 	}
 	// Set after we know we have truncated properly.
-	n.term, n.pterm, n.pindex = term, term, index
+	n.pterm, n.pindex = term, index
 }
 
 // Reset our WAL. This is equivalent to truncating all data from the log.

server/raft_test.go

@@ -1007,3 +1007,42 @@ func TestNRGWALEntryWithoutQuorumMustTruncate(t *testing.T) {
 		return nil
 	})
 }
+
+func TestNRGTermNoDecreaseAfterWALReset(t *testing.T) {
+	c := createJetStreamClusterExplicit(t, "R3S", 3)
+	defer c.shutdown()
+
+	rg := c.createRaftGroup("TEST", 3, newStateAdder)
+	rg.waitOnLeader()
+
+	l := rg.leader().node().(*raft)
+	l.term = 20
+
+	esm := encodeStreamMsgAllowCompress("foo", "_INBOX.foo", nil, nil, 0, 0, true)
+	entries := []*Entry{newEntry(EntryNormal, esm)}
+	l.Lock()
+	ae := l.buildAppendEntry(entries)
+	l.Unlock()
+
+	for _, f := range rg {
+		if f.node().ID() != l.ID() {
+			fn := f.node().(*raft)
+			fn.processAppendEntry(ae, fn.aesub)
+			require_Equal(t, fn.term, 20) // Follower's term gets upped as expected.
+		}
+	}
+
+	// Lower the term, simulating the followers receiving a message from an old term/leader.
+	ae.term = 3
+	for _, f := range rg {
+		if f.node().ID() != l.ID() {
+			fn := f.node().(*raft)
+			fn.processAppendEntry(ae, fn.aesub)
+			require_Equal(t, fn.term, 20) // Follower should reject and the term stays the same.
+
+			fn.resetWAL()
+			fn.processAppendEntry(ae, fn.aesub)
+			require_Equal(t, fn.term, 20) // Follower should reject again, even after reset, term stays the same.
+		}
+	}
+}