fix(watch-together): re-host an abandoned room code instead of joining it

A room whose peers have all left is a code nobody is using, but the relay
kept it bound to the creator's reconnect capability and rejected every
other create with room_exists. The app compounded it: enterRoom only
promoted to host on room_not_found, so tapping a recent code landed the
user in the retained room as a guest of a host that was never coming
back, until the cleanup sweep finally dropped the room.

Create now replaces a room with no connected peers, and enterRoom hosts
the code when its probe join finds an empty room. An occupied room still
rejects create, including from its previous owner, and a host that is
merely disconnected still reclaims its peer ID through join with the
matching token.

lib/watch_together/providers/watch_together_provider.dart

@@ -438,8 +438,8 @@ class WatchTogetherProvider with ChangeNotifier {
     }
   }
 
-  /// Enter a room by code — joins any reserved room and creates only when the
+  /// Enter a room by code — joins a room that still has someone in it and
-  /// relay reports that no room reservation exists.
+  /// hosts the code otherwise.
   ///
   /// Returns `true` if the user became the host.
   Future<bool> enterRoom(
@@ -453,13 +453,19 @@ class WatchTogetherProvider with ChangeNotifier {
     final probe = _peerServiceFactory(endpoint: relayEndpoint);
     var shouldBeHost = false;
     try {
+      var probeJoined = false;
       try {
         await probe.joinSession(sessionId);
+        probeJoined = true;
       } on PeerError catch (error) {
         if (error.serverCode != RelayProtocol.roomNotFoundCode) rethrow;
-        shouldBeHost = true;
       }
-      if (!shouldBeHost) {
+      // A room the relay still holds but nobody is connected to is an
+      // abandoned code, not a session: its declared host is gone and the
+      // reservation only survives until the next cleanup sweep. Take the code
+      // over instead of waiting on a host that is never coming back.
+      shouldBeHost = !probeJoined || probe.connectedPeers.isEmpty;
+      if (probeJoined) {
         await probe.releaseSession();
       }
     } finally {

server/main.go

@@ -1883,16 +1883,13 @@ func (s *Server) handleWS(w http.ResponseWriter, r *http.Request) {
 					}
 					continue
 				}
-				authorizedLegacyReplacement :=
+				// A room nobody is connected to is an abandoned code, not property.
-					len(existing.Peers) == 0 &&
+				// Whoever asks for it next takes it, so a host that restarted with a
-						!existing.closing &&
+				// fresh reconnect token can reuse its own code instead of waiting out
-						msg.ProtocolVersion == legacyRelayProtocolVersion &&
+				// the cleanup sweep. An occupied room still belongs to its peers.
-						existing.ProtocolVersion == legacyRelayProtocolVersion &&
+				reclaimable := len(existing.Peers) == 0 && !existing.closing
-						msg.PeerID == existing.HostPeerID &&
-						msg.ReconnectToken != "" &&
-						reconnectVerifierMatches(existing.hostVerifier, hostVerifier)
 				existing.mu.Unlock()
-				if !authorizedLegacyReplacement {
+				if !reclaimable {
 					rejection = &serverMsg{Type: relayTypeError, Code: relayErrorRoomExists, Message: "Room already exists"}
 				}
 			} else if len(s.rooms) >= maxRetainedRooms {

server/main_test.go

@@ -2273,12 +2273,13 @@ func TestIdempotentCreateReannouncesPreviouslyAbsentHost(t *testing.T) {
 	}
 }
 
-func TestCreateCannotReclaimReservedEmptyRoom(t *testing.T) {
+func TestCreateReclaimsAbandonedEmptyRoom(t *testing.T) {
 	h := newRelayHarness(t)
 	hostToken, hostVerifier := mustReconnectToken(t)
 	original := &Room{
 		SessionID:        "STALE",
 		HostPeerID:       "old-host",
+		ProtocolVersion:  relayProtocolVersion,
 		hostVerifier:     hostVerifier,
 		peerReservations: make(map[string]peerReservation),
 		Peers:            map[string]*Client{},
@@ -2289,29 +2290,82 @@ func TestCreateCannotReclaimReservedEmptyRoom(t *testing.T) {
 	h.srv.rooms["STALE"] = original
 	h.srv.mu.Unlock()
 
+	creatorToken, _ := mustReconnectToken(t)
 	creator := h.dial(t, "1.1.1.6")
-	creator.send(clientMsg{Type: relayTypeCreate, SessionID: "STALE", PeerID: "new-host"})
+	creator.send(clientMsg{
-	creator.expectError(relayErrorRoomExists)
+		Type:            relayTypeCreate,
-
-	unproved := h.dial(t, "1.1.1.60")
-	unproved.send(clientMsg{Type: relayTypeJoin, SessionID: "STALE", PeerID: "old-host"})
-	unproved.expectError(relayErrorPeerIdUnavailable)
-
-	reconnected := h.dial(t, "1.1.1.61")
-	reconnected.send(clientMsg{
-		Type:           relayTypeJoin,
 		SessionID:       "STALE",
-		PeerID:         "old-host",
+		PeerID:          "new-host",
-		ReconnectToken: hostToken,
+		ReconnectToken:  creatorToken,
+		ProtocolVersion: relayProtocolVersion,
 	})
-	reconnected.expectAuthority(relayTypeJoined, "old-host")
+	creator.expectAuthority(relayTypeCreated, "new-host")
 
 	h.srv.mu.RLock()
 	current := h.srv.rooms["STALE"]
 	h.srv.mu.RUnlock()
-	if current != original {
+	if current == original {
-		t.Fatal("reserved room identity was replaced")
+		t.Fatal("abandoned room identity survived the reclaim")
 	}
+
+	// The previous owner's capability died with the room it belonged to, and
+	// the live replacement is not reclaimable by anyone, owner included.
+	former := h.dial(t, "1.1.1.60")
+	former.send(clientMsg{
+		Type:            relayTypeCreate,
+		SessionID:       "STALE",
+		PeerID:          "old-host",
+		ReconnectToken:  hostToken,
+		ProtocolVersion: relayProtocolVersion,
+	})
+	former.expectError(relayErrorRoomExists)
+}
+
+// The recent-rooms flow: a host restarts its app, so it presents a fresh
+// reconnect capability for a code the relay still holds. The abandoned code
+// must come back as a hosted room instead of a ghost room with no host.
+func TestAbandonedCodeIsRecreatableByARestartedHost(t *testing.T) {
+	h := newRelayHarness(t)
+	firstToken, _ := mustReconnectToken(t)
+	host := h.dial(t, "6.4.0.1")
+	host.send(clientMsg{
+		Type:            relayTypeCreate,
+		SessionID:       "REUSE",
+		PeerID:          "H",
+		ReconnectToken:  firstToken,
+		ProtocolVersion: relayProtocolVersion,
+	})
+	host.expectAuthority(relayTypeCreated, "H")
+	host.conn.Close()
+	h.waitRoomPeers(t, "REUSE", 0)
+
+	// A restarted app mints a new capability, so it cannot prove the previous
+	// ownership even when it reuses its own peer ID.
+	restartToken, _ := mustReconnectToken(t)
+	restarted := h.dial(t, "6.4.0.2")
+	restarted.send(clientMsg{
+		Type:            relayTypeCreate,
+		SessionID:       "REUSE",
+		PeerID:          "H",
+		ReconnectToken:  restartToken,
+		ProtocolVersion: relayProtocolVersion,
+	})
+	recreated := restarted.expectAuthority(relayTypeCreated, "H")
+	if recreated.ReconnectToken != restartToken {
+		t.Fatalf("recreated room token=%q, want the presented capability", recreated.ReconnectToken)
+	}
+
+	guestToken, _ := mustReconnectToken(t)
+	guest := h.dial(t, "6.4.0.3")
+	guest.send(clientMsg{
+		Type:            relayTypeJoin,
+		SessionID:       "REUSE",
+		PeerID:          "G",
+		ReconnectToken:  guestToken,
+		ProtocolVersion: relayProtocolVersion,
+	})
+	guest.expectAuthority(relayTypeJoined, "H")
+	restarted.expect(relayTypePeerJoined)
 }
 
 func TestCreateReclaimsOwnedEmptyRoomWithoutDoubleCharging(t *testing.T) {

test/watch_together/watch_together_provider_test.dart

@@ -698,7 +698,7 @@ void main() {
   });
 
   group('WatchTogetherProvider — relay authority', () {
-    test('an empty successful probe joins the reserved room without creating', () async {
+    test('an occupied room is joined as a guest without creating', () async {
       late final _ProviderRelay relay;
       relay = await _ProviderRelay.start((socket, message) {
         if (message['type'] == 'join') {
@@ -708,6 +708,7 @@ void main() {
             'hostPeerId': _providerHostId,
             'reconnectToken': message['reconnectToken'],
             'protocolVersion': 2,
+            'peers': [_providerHostId],
           });
         } else if (message['type'] == 'leave') {
           relay.send(socket, {
@@ -726,7 +727,7 @@ void main() {
         provider.dispose();
       });
 
-      final becameHost = await provider.enterRoom('empty1', relayEndpoint: endpoint, displayName: 'Guest');
+      final becameHost = await provider.enterRoom('busy01', relayEndpoint: endpoint, displayName: 'Guest');
 
       expect(becameHost, isFalse);
       expect(provider.isHost, isFalse);
@@ -746,6 +747,60 @@ void main() {
       expect(relay.messages.where((message) => message['type'] == 'create'), isEmpty);
     });
 
+    test('an abandoned room with no peers is hosted instead of joined', () async {
+      late final _ProviderRelay relay;
+      relay = await _ProviderRelay.start((socket, message) {
+        switch (message['type']) {
+          case 'join':
+            relay.send(socket, {
+              'type': 'joined',
+              'sessionId': message['sessionId'],
+              'hostPeerId': _providerHostId,
+              'reconnectToken': message['reconnectToken'],
+              'protocolVersion': 2,
+            });
+          case 'leave':
+            relay.send(socket, {
+              'type': 'left',
+              'sessionId': message['sessionId'],
+              'peerId': message['peerId'],
+              'protocolVersion': 2,
+            });
+          case 'create':
+            relay.send(socket, {
+              'type': 'created',
+              'sessionId': message['sessionId'],
+              'hostPeerId': message['peerId'],
+              'reconnectToken': message['reconnectToken'],
+              'protocolVersion': 2,
+            });
+          case 'endSession':
+            relay.send(socket, {'type': 'ended', 'sessionId': message['sessionId'], 'protocolVersion': 2});
+        }
+      });
+      addTearDown(relay.close);
+      final endpoint = WatchTogetherRelayEndpoint.resolve(relay.baseUrl);
+      final provider = WatchTogetherProvider();
+      addTearDown(() async {
+        await provider.leaveSession();
+        provider.dispose();
+      });
+
+      final becameHost = await provider.enterRoom('empty1', relayEndpoint: endpoint, displayName: 'Host');
+
+      expect(becameHost, isTrue);
+      expect(provider.isHost, isTrue);
+      // The probe identity is released before the code is taken over, so the
+      // relay sees an empty room when the create lands.
+      expect(relay.messages.map((message) => message['type']).take(3), ['join', 'leave', 'create']);
+      final create = relay.messages.singleWhere((message) => message['type'] == 'create');
+      expect(create['sessionId'], 'EMPTY1');
+      expect(provider.session?.hostPeerId, create['peerId']);
+      final probeJoin = relay.messages.firstWhere((message) => message['type'] == 'join');
+      expect(create['peerId'], isNot(probeJoin['peerId']));
+      expect(provider.session?.hostPeerId, isNot(_providerHostId));
+    });
+
     test('a room-not-found probe creates with relay-declared host authority', () async {
       late final _ProviderRelay relay;
       relay = await _ProviderRelay.start((socket, message) {