Add a test for prevote.

test_configs/src/raft/config.rs

@@ -267,6 +267,16 @@ impl Config {
         }
     }
 
+    pub fn connect_pair(&self, one: usize, two: usize) {
+        let mut state = self.state.lock();
+        state.connected[one] = true;
+        state.connected[two] = true;
+
+        let mut network = self.network.lock();
+        network.set_enable_client(Self::client_name(one, two), true);
+        network.set_enable_client(Self::client_name(two, one), true);
+    }
+
     pub fn crash1(&self, index: usize) {
         self.disconnect(index);
 

tests/prevote_tests.rs

@@ -0,0 +1,45 @@
+use scopeguard::defer;
+
+use test_configs::make_config;
+use test_configs::raft::config;
+use test_configs::utils::sleep_election_timeouts;
+
+#[test]
+fn no_disruptive_rejoin() -> config::Result<()> {
+    const SERVERS: usize = 5;
+    let cfg = make_config!(SERVERS, false);
+    defer!(cfg.cleanup());
+    cfg.one(100, SERVERS, false)?;
+    let initial_term =
+        cfg.check_terms()?.expect("Servers should agree on term");
+
+    //  0 <--> 1 <--> 3
+    //  ^      ^
+    //  |      |
+    //  |      v
+    //  -----> 2 <--> 4
+
+    cfg.disconnect(4);
+    cfg.disconnect(3);
+
+    // The disconnected servers (3 & 4) should not disrupt the exist leader.
+    cfg.connect_pair(2, 4);
+    cfg.connect_pair(1, 3);
+
+    let leader = cfg.check_one_leader()?;
+    cfg.one(200, 3, false)?;
+
+    let (first_term, _) = cfg.leader_start(leader, 300).unwrap();
+    assert!(first_term >= initial_term);
+    assert!(first_term <= initial_term + 1);
+
+    sleep_election_timeouts(2);
+
+    let (second_term, _) = cfg.leader_start(leader, 400).unwrap();
+    assert_eq!(first_term, second_term);
+
+    let index = cfg.one(500, 3, false)?;
+    assert_eq!(index, 5);
+
+    Ok(())
+}