engine: forget a split server's users on SQUIT

A netsplit is signalled once via SQUIT, not as a QUIT per user, so every
user behind the departed server lingered in services' state — stale
sessions, session-limit slots, and channel memberships. The parser now
surfaces ServerSplit and the engine forgets every uid carrying that
server's SID prefix, leaving other servers' users untouched.

src/engine/mod.rs

@@ -1057,6 +1057,14 @@ impl Engine {
                 self.forget_user(&uid);
                 Vec::new()
             }
+            NetEvent::ServerSplit { server } => {
+                // Every user behind the departed server is gone in one message;
+                // forget them so their sessions, slots and memberships don't linger.
+                for uid in self.network.uids_on_server(&server) {
+                    self.forget_user(&uid);
+                }
+                Vec::new()
+            }
             NetEvent::Privmsg { from, to, text } => self.dispatch(&from, &to, &text),
             NetEvent::AccountRequest { reqid, origin, kind, account, p2, p3 } => {
                 self.account_request(reqid, origin, kind, account, p2, p3)

src/engine/state.rs

@@ -180,6 +180,12 @@ impl Network {
         self.users.get(uid).map(|u| u.host.as_str())
     }
 
+    // Every known user whose uid carries `sid` as its prefix — i.e. those behind
+    // a server, used to forget them all when it splits (SQUIT).
+    pub fn uids_on_server(&self, sid: &str) -> Vec<String> {
+        self.users.keys().filter(|u| u.starts_with(sid)).cloned().collect()
+    }
+
     pub fn user_nick_change(&mut self, uid: &str, nick: String) {
         if let Some(user) = self.users.get_mut(uid) {
             self.seen.insert(lc(&nick), Seen { nick: nick.clone(), ts: now(), what: format!("changing nick from {}", user.nick) });

src/engine/tests.rs

@@ -432,6 +432,25 @@
         assert!(parted, "the assigned bot parts the released channel");
     }
 
+    // A server split (SQUIT) forgets exactly the users behind it (uids carrying
+    // that SID), leaving users on other servers untouched.
+    #[test]
+    fn squit_forgets_users_behind_the_split() {
+        let mut e = engine_with("squit", "alice", "sesame");
+        e.db.register("bob", "hunter2", None).unwrap();
+        e.handle(NetEvent::UserConnect { uid: "0IRAAAAAB".into(), nick: "alice".into(), host: "h".into() , ip: "0.0.0.0".into() });
+        e.handle(NetEvent::UserConnect { uid: "0XYAAAAAB".into(), nick: "bob".into(), host: "h".into() , ip: "0.0.0.0".into() });
+        e.handle(NetEvent::Privmsg { from: "0IRAAAAAB".into(), to: "42SAAAAAA".into(), text: "IDENTIFY sesame".into() });
+        e.handle(NetEvent::Privmsg { from: "0XYAAAAAB".into(), to: "42SAAAAAA".into(), text: "IDENTIFY hunter2".into() });
+        assert_eq!(e.network.account_of("0IRAAAAAB"), Some("alice"));
+        assert_eq!(e.network.account_of("0XYAAAAAB"), Some("bob"));
+
+        // Server 0IR splits away.
+        e.handle(NetEvent::ServerSplit { server: "0IR".into() });
+        assert!(e.network.account_of("0IRAAAAAB").is_none(), "users behind the split are forgotten");
+        assert_eq!(e.network.account_of("0XYAAAAAB"), Some("bob"), "users on other servers are kept");
+    }
+
     // A netburst that replays a user's accountname restores their services login
     // (so a services restart doesn't silently log everyone out); empty = logout.
     #[test]