nickserv: log out the loser of a registration conflict

When a peer's registration wins a name a local session was logged into,
that session authenticated against a credential that no longer exists.
The node now clears its accountname (the ircd emits RPL_LOGGEDOUT) and
tells the user why. A new outbound channel lets the engine push these
services-initiated actions to the uplink from the gossip path, drained
by the link loop alongside incoming traffic.

modules/nickserv/nickserv.rs

@@ -21,6 +21,9 @@ impl Service for NickServ {
     fn gecos(&self) -> &str {
         "Nickname Services"
     }
+    fn manages_accounts(&self) -> bool {
+        true
+    }
 
     fn on_command(&mut self, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, _net: &Network, db: &mut Db) {
         let me = self.uid.as_str();

src/engine/db.rs

@@ -206,6 +206,13 @@ pub struct LogEntry {
     event: Event,
 }
 
+#[cfg(test)]
+impl LogEntry {
+    pub(crate) fn for_test(origin: &str, seq: u64, lamport: u64, event: Event) -> Self {
+        LogEntry { origin: origin.to_string(), seq, lamport, event }
+    }
+}
+
 // Append-only log, the sole persistent source of truth: `open` replays it,
 // `append` stamps and writes a locally-authored entry, and `ingest` folds in an
 // entry authored by another node. The version vector (highest seq applied per
@@ -439,12 +446,25 @@ impl Db {
     }
 
     /// Fold an entry authored by another node into the store — the services-side
-    /// of the gossip seam. Idempotent (re-delivered entries are dropped).
-    pub fn ingest(&mut self, entry: LogEntry) -> std::io::Result<()> {
+    /// of the gossip seam. Idempotent (re-delivered entries are dropped). Returns
+    /// the account name if this ingest changed its owner (a registration conflict
+    /// resolved against the local claim), so the caller can log out stale sessions.
+    pub fn ingest(&mut self, entry: LogEntry) -> std::io::Result<Option<String>> {
+        // Record the current owner of an incoming account before applying, to see
+        // if this ingest transfers ownership away from us.
+        let contested = match &entry.event {
+            Event::AccountRegistered(a) => self.accounts.get(&key(&a.name)).map(|cur| (a.name.clone(), cur.home.clone())),
+            _ => None,
+        };
         if let Some(event) = self.log.ingest(entry)? {
             apply(&mut self.accounts, &mut self.channels, event);
         }
-        Ok(())
+        if let Some((name, prev_home)) = contested {
+            if self.accounts.get(&key(&name)).is_some_and(|cur| cur.home != prev_home) {
+                return Ok(Some(name)); // ownership moved to another node
+            }
+        }
+        Ok(None)
     }
 
     /// Rewrite the log to one entry per account and channel, reclaiming churn.

src/engine/mod.rs

@@ -7,6 +7,7 @@ use std::collections::HashMap;
 use std::time::{Duration, Instant};
 
 use base64::{engine::general_purpose::STANDARD, Engine as _};
+use tokio::sync::mpsc;
 
 use crate::proto::{NetAction, NetEvent, RegReply};
 use db::{Db, LogEntry, RegError};
@@ -74,11 +75,14 @@ pub struct Engine {
     sasl_sessions: HashMap<String, TimedSession>, // client uid -> in-progress exchange
     reg_limiter: RegLimiter,
     chan_service: Option<String>, // uid to source channel modes from (ChanServ)
+    nick_service: Option<String>, // uid of the account service (NickServ), for its notices
+    irc_out: Option<mpsc::UnboundedSender<NetAction>>, // services-initiated actions -> the uplink
 }
 
 impl Engine {
     pub fn new(services: Vec<Box<dyn Service>>, db: Db) -> Self {
         let chan_service = services.iter().find(|s| s.manages_channels()).map(|s| s.uid().to_string());
+        let nick_service = services.iter().find(|s| s.manages_accounts()).map(|s| s.uid().to_string());
         Self {
             services,
             network: Network::default(),
@@ -86,6 +90,21 @@ impl Engine {
             sasl_sessions: HashMap::new(),
             reg_limiter: RegLimiter::new(),
             chan_service,
+            nick_service,
+            irc_out: None,
+        }
+    }
+
+    // Wire the channel the link layer drains, so the engine can push actions to the
+    // uplink that no ircd event asked for (here: a forced logout after losing a
+    // registration conflict).
+    pub fn set_irc_out(&mut self, tx: mpsc::UnboundedSender<NetAction>) {
+        self.irc_out = Some(tx);
+    }
+
+    fn emit_irc(&self, action: NetAction) {
+        if let Some(tx) = &self.irc_out {
+            let _ = tx.send(action); // unbounded: never blocks; only fails if the link is down
         }
     }
 
@@ -104,7 +123,29 @@ impl Engine {
     }
 
     pub fn gossip_ingest(&mut self, entry: LogEntry) -> std::io::Result<()> {
-        self.db.ingest(entry)
+        // If ingesting a peer's registration took an account name away from a
+        // local session (it lost the conflict), log that session out.
+        if let Some(account) = self.db.ingest(entry)? {
+            self.logout_lost_sessions(&account);
+        }
+        Ok(())
+    }
+
+    // A registration conflict resolved against this node: the name `account` is now
+    // owned by another node. Any local session logged into it authenticated against
+    // the old owner's credential, which no longer exists — force them out and say why.
+    fn logout_lost_sessions(&mut self, account: &str) {
+        for uid in self.network.uids_logged_into(account) {
+            self.network.clear_account(&uid);
+            self.emit_irc(NetAction::Metadata { target: uid.clone(), key: "accountname".to_string(), value: String::new() });
+            if let Some(ns) = &self.nick_service {
+                self.emit_irc(NetAction::Notice {
+                    from: ns.clone(),
+                    to: uid,
+                    text: format!("Your registration of \x02{account}\x02 collided with another network and it now belongs elsewhere. You have been logged out; please register a different name."),
+                });
+            }
+        }
     }
 
     // Compact the log if it has grown past the live account count. Returns
@@ -988,6 +1029,42 @@ mod tests {
         assert!(out.iter().any(|a| matches!(a, NetAction::Metadata { key, value, .. } if key == "accountname" && value == "realnick")), "must log into the current nick's account: {out:?}");
     }
 
+    // Losing a registration conflict logs out the local session on that name and
+    // notifies them over the services-initiated outbound path.
+    #[test]
+    fn lost_conflict_logs_out_local_session() {
+        let mut e = engine_with("lostconf", "alice", "sesame");
+        let (tx, mut rx) = tokio::sync::mpsc::unbounded_channel();
+        e.set_irc_out(tx);
+        // A local user logs into alice.
+        e.handle(NetEvent::UserConnect { uid: "000AAAAAB".into(), nick: "alice".into(), host: "h".into() });
+        e.handle(NetEvent::Privmsg { from: "000AAAAAB".into(), to: "42SAAAAAA".into(), text: "IDENTIFY sesame".into() });
+        assert_eq!(e.network.account_of("000AAAAAB"), Some("alice"), "logged in before the conflict");
+
+        // An earlier claim from another node wins and takes the name over.
+        let winner = db::Account {
+            name: "alice".into(), password_hash: "OTHER".into(), email: None,
+            ts: 0, home: "peer".into(), scram256: None, scram512: None, certfps: vec![],
+        };
+        let entry = LogEntry::for_test("peer", 0, 1, db::Event::AccountRegistered(winner));
+        e.gossip_ingest(entry).unwrap();
+
+        // The local session is logged out...
+        assert!(e.network.account_of("000AAAAAB").is_none(), "session cleared after losing the name");
+        // ...and the outbound path got a logout (empty accountname) plus a notice.
+        let mut logout = false;
+        let mut notice = false;
+        while let Ok(a) = rx.try_recv() {
+            match a {
+                NetAction::Metadata { target, key, value } if target == "000AAAAAB" && key == "accountname" && value.is_empty() => logout = true,
+                NetAction::Notice { to, text, .. } if to == "000AAAAAB" && text.contains("collided") => notice = true,
+                _ => {}
+            }
+        }
+        assert!(logout, "a logout must be pushed to the uplink");
+        assert!(notice, "the user must be told why");
+    }
+
     // IDENTIFY accepts the two-arg account+password form: log into a named
     // account even when the current nick differs; a wrong password is refused.
     #[test]

src/engine/service.rs

@@ -24,6 +24,11 @@ pub trait Service: Send {
     fn manages_channels(&self) -> bool {
         false
     }
+    // Whether this is the account service (NickServ), so the engine can source
+    // account-related notices from it.
+    fn manages_accounts(&self) -> bool {
+        false
+    }
     fn on_command(&mut self, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &Network, db: &mut Db);
 }
 

src/engine/state.rs

@@ -81,6 +81,15 @@ impl Network {
         self.accounts.get(uid).map(String::as_str)
     }
 
+    // Uids of every live session currently identified to `account`.
+    pub fn uids_logged_into(&self, account: &str) -> Vec<String> {
+        self.accounts
+            .iter()
+            .filter(|(_, a)| a.eq_ignore_ascii_case(account))
+            .map(|(uid, _)| uid.clone())
+            .collect()
+    }
+
     pub fn set_account(&mut self, uid: &str, account: &str) {
         self.accounts.insert(uid.to_string(), account.to_string());
     }

src/link.rs

@@ -3,7 +3,7 @@ use std::sync::Arc;
 use anyhow::Result;
 use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
 use tokio::net::TcpStream;
-use tokio::sync::Mutex;
+use tokio::sync::{mpsc, Mutex};
 
 use crate::engine::db::Db;
 use crate::engine::Engine;
@@ -12,7 +12,7 @@ use crate::proto::{NetAction, Protocol};
 // One uplink session: connect, handshake + burst, then translate lines forever.
 // The engine is shared with the gossip layer, so it is locked per operation and
 // never held across the registration key-stretching await.
-pub async fn run(mut proto: Box<dyn Protocol>, engine: Arc<Mutex<Engine>>, addr: &str) -> Result<()> {
+pub async fn run(mut proto: Box<dyn Protocol>, engine: Arc<Mutex<Engine>>, addr: &str, mut irc_rx: mpsc::UnboundedReceiver<NetAction>) -> Result<()> {
     let stream = TcpStream::connect(addr).await?;
     let (read, mut write) = stream.into_split();
     let mut lines = BufReader::new(read).lines();
@@ -27,7 +27,11 @@ pub async fn run(mut proto: Box<dyn Protocol>, engine: Arc<Mutex<Engine>>, addr:
         }
     }
 
-    while let Some(line) = lines.next_line().await? {
+    loop {
+        tokio::select! {
+            // A line from the uplink: translate it and answer.
+            line = lines.next_line() => {
+                let Some(line) = line? else { break };
                 tracing::debug!(dir = "<<", %line);
                 for event in proto.parse(&line) {
                     let actions = engine.lock().await.handle(event);
@@ -60,6 +64,15 @@ pub async fn run(mut proto: Box<dyn Protocol>, engine: Arc<Mutex<Engine>>, addr:
                     }
                 }
             }
+            // A services-initiated action (e.g. a forced logout after a lost
+            // registration conflict), pushed by the engine from the gossip path.
+            Some(action) = irc_rx.recv() => {
+                for out in proto.serialize(&action) {
+                    send(&mut write, &out).await?;
+                }
+            }
+        }
+    }
 
     tracing::warn!("uplink closed the connection");
     Ok(())

src/main.rs

@@ -57,6 +57,10 @@ async fn main() -> Result<()> {
     db.set_outbound(gossip_tx.clone());
     let engine = Arc::new(Mutex::new(Engine::new(services, db)));
 
+    // Channel for services-initiated actions to reach the uplink (drained by the link loop).
+    let (irc_tx, irc_rx) = tokio::sync::mpsc::unbounded_channel();
+    engine.lock().await.set_irc_out(irc_tx);
+
     if let Some(gossip) = cfg.gossip.clone() {
         tracing::info!(peers = cfg.peer.len(), "starting gossip");
         tokio::spawn(gossip::run(engine.clone(), gossip, cfg.peer.clone(), cfg.server.sid.clone(), gossip_tx));
@@ -77,5 +81,5 @@ async fn main() -> Result<()> {
 
     let addr = format!("{}:{}", cfg.uplink.host, cfg.uplink.port);
     tracing::info!(server = %cfg.server.name, %addr, "linking to uplink");
-    link::run(proto, engine, &addr).await
+    link::run(proto, engine, &addr, irc_rx).await
 }