Forget emptied channels so a stale key can't leak to a recreated name

src/engine/state.rs

@@ -341,11 +341,16 @@ impl Network {
         }
         self.users.remove(uid);
         self.accounts.remove(uid);
-        for c in self.channels.values_mut() {
+        // Drop the membership, and forget any channel it emptied — an emptied
+        // channel is destroyed by the ircd, so a lingering entry would both grow
+        // unbounded over the process lifetime and hand stale modes (e.g. a `+k`
+        // key) to whoever recreates the name later.
+        self.channels.retain(|_, c| {
             c.members.remove(uid);
             c.ops.remove(uid);
             c.voices.remove(uid);
-        }
+            !c.members.is_empty()
+        });
     }
 
     pub fn nick_of(&self, uid: &str) -> Option<&str> {
@@ -389,10 +394,19 @@ impl Network {
 
     // A user left a channel (part/kick): drop membership and record last-seen.
     pub fn channel_part(&mut self, channel: &str, uid: &str) {
-        if let Some(c) = self.channels.get_mut(&lc(channel)) {
+        let key = lc(channel);
+        let emptied = if let Some(c) = self.channels.get_mut(&key) {
             c.members.remove(uid);
             c.ops.remove(uid);
             c.voices.remove(uid);
+            c.members.is_empty()
+        } else {
+            false
+        };
+        // An emptied channel is gone on the ircd; drop it so a later channel of
+        // the same name can't inherit its stale modes (see user_quit).
+        if emptied {
+            self.channels.remove(&key);
         }
         if let Some(nick) = self.nick_of(uid).map(str::to_string) {
             self.record_seen(lc(&nick), Seen { nick, ts: now(), what: format!("leaving {channel}") });