Allocate guest nicks that skip online and registered names

api/src/lib.rs

@@ -2277,6 +2277,25 @@ pub fn valid_email(addr: &str) -> bool {
     )
 }
 
+/// The next free guest nick: `base` + an incrementing counter, skipping any nick
+/// that is currently online or registered. An enforced/logged-out rename must not
+/// land on a real user or a registered nick — that would either defeat the rename
+/// (the target stays put) or collide with and kill an innocent occupant. Advances
+/// `seq` past whatever it returns.
+pub fn next_guest_nick(base: &str, seq: &mut u32, net: &dyn NetView, store: &dyn Store) -> String {
+    for _ in 0..100_000 {
+        let candidate = format!("{base}{seq}");
+        *seq = seq.wrapping_add(1);
+        if net.uid_by_nick(&candidate).is_none() && !store.exists(&candidate) {
+            return candidate;
+        }
+    }
+    // Pathological: the whole range is taken. Fall back rather than loop forever.
+    let candidate = format!("{base}{seq}");
+    *seq = seq.wrapping_add(1);
+    candidate
+}
+
 // Reject a look-alike / mixed-script registration name (nick or channel): the
 // impersonation trick that a message-content filter never sees at the registration
 // seam. Returns a rejection reason, or None if the name is fine. Genuine

modules/nickserv/src/ghost.rs

@@ -48,8 +48,7 @@ pub fn handle(me: &str, guest_nick: &str, guest_seq: &mut u32, from: &Sender, ar
             return;
         }
         Some(ghost) => {
-            let guest = format!("{guest_nick}{guest_seq}");
-            *guest_seq = guest_seq.wrapping_add(1);
+            let guest = echo_api::next_guest_nick(guest_nick, guest_seq, net, &*db);
             ctx.force_nick(&ghost, &guest);
             ctx.notice(me, from.uid, format!("\x02{target}\x02 has been freed."));
         }

modules/nickserv/src/lib.rs

@@ -112,7 +112,7 @@ impl Service for NickServ {
         match cmd.as_deref() {
             Some("REGISTER") => register::handle(me, from, args, ctx, db),
             Some("IDENTIFY") | Some("ID") => identify::handle(me, from, args, ctx, db),
-            Some("LOGOUT") | Some("LOGOFF") => logout::handle(me, &self.guest_nick, &mut self.guest_seq, from, ctx),
+            Some("LOGOUT") | Some("LOGOFF") => logout::handle(me, &self.guest_nick, &mut self.guest_seq, from, ctx, net, db),
             Some("CERT") => cert::handle(me, from, args, ctx, db),
             Some("INFO") => info::handle(me, from, args, ctx, net, db),
             Some("ALIST") => alist::handle(me, from, ctx, db),

modules/nickserv/src/logout.rs

@@ -1,13 +1,12 @@
-use echo_api::{Sender, ServiceCtx};
+use echo_api::{NetView, Sender, ServiceCtx, Store};
 
 // LOGOUT: log out and rename to a guest nick (prefix + a per-logout sequence).
-pub fn handle(me: &str, guest_nick: &str, guest_seq: &mut u32, from: &Sender, ctx: &mut ServiceCtx) {
+pub fn handle(me: &str, guest_nick: &str, guest_seq: &mut u32, from: &Sender, ctx: &mut ServiceCtx, net: &dyn NetView, db: &dyn Store) {
     if from.account.is_none() {
         ctx.notice(me, from.uid, "You're not logged in.");
         return;
     }
-    let guest = format!("{guest_nick}{guest_seq}");
-    *guest_seq = guest_seq.wrapping_add(1);
+    let guest = echo_api::next_guest_nick(guest_nick, guest_seq, net, db);
     ctx.logout(from.uid);
     ctx.force_nick(from.uid, &guest);
     ctx.notice(me, from.uid, format!("You're now logged out. Your nick is now \x02{}\x02.", guest));

src/engine/tests.rs

@@ -418,6 +418,22 @@
         e.handle(NetEvent::Privmsg { from: uid.into(), to: "42SAAAAAA".into(), text: "LOGOUT".into() })
     }
 
+    // A guest rename must skip a nick that's already online, or it would collide
+    // with (and get) an innocent user instead of freeing the name.
+    #[test]
+    fn logout_skips_an_occupied_guest_nick() {
+        let mut e = engine_with("guestskip", "foo", "sesame");
+        e.handle(NetEvent::UserConnect { uid: "000AAAAAB".into(), nick: "foo".into(), host: "h".into() , ip: "0.0.0.0".into() });
+        e.handle(NetEvent::Privmsg { from: "000AAAAAB".into(), to: "42SAAAAAA".into(), text: "IDENTIFY sesame".into() });
+        // Someone already holds Guest12345 (the first nick the counter would pick).
+        e.handle(NetEvent::UserConnect { uid: "000AAAAAC".into(), nick: "Guest12345".into(), host: "h".into() , ip: "0.0.0.0".into() });
+        let out = logout(&mut e, "000AAAAAB");
+        assert!(
+            out.iter().any(|a| matches!(a, NetAction::ForceNick { uid, nick } if uid == "000AAAAAB" && nick == "Guest12346")),
+            "logout skips the taken guest nick: {out:?}"
+        );
+    }
+
     // LOGOUT while not identified must not rename you or clear anything.
     #[test]
     fn logout_without_login_is_noop() {