fix(voice-limit-guard): two simultaneous joins can no longer both pass a full room (#25)

The limit was checked against participants LiveKit already had, so
tokens issued but not yet connected were not counted: two users
pressing Join together could both get into a 1-person channel.
Serialise the check per room and give each admitted user a 30 s
reservation, counted like a present user until LiveKit lists them or
it expires.

Refs #25

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

livekit/test_voice_limit_guard.py

@@ -389,3 +389,31 @@ class NormalizePermissionTest(unittest.TestCase):
         guard.livekit_update_participant = lambda *a: None
         participant = {"identity": "@a:x:DEV", "permission": {"canPublish": True, "canPublishSources": ["CAMERA"]}}
         self.assertFalse(guard.reconcile_participant("!r:x", participant, {"SCREEN_SHARE"}))
+
+
+class SlotReservationTests(unittest.TestCase):
+    """[matrix #25] Simultaneous joins can't both pass a full room."""
+
+    def test_second_simultaneous_join_is_blocked(self):
+        r = guard.SlotReservations(ttl=30)
+        present = set()
+        counted = present | r.reserved("room", present, now=0)
+        self.assertFalse(guard.should_block(1, counted, "@a:x"))
+        r.reserve("room", "@a:x", now=0)
+        # @b asks a moment later, before @a has connected to LiveKit.
+        counted = present | r.reserved("room", present, now=1)
+        self.assertTrue(guard.should_block(1, counted, "@b:x"))
+        # @a asking again (second device) is still allowed.
+        self.assertFalse(guard.should_block(1, counted, "@a:x"))
+
+    def test_reservation_expires_and_clears_once_present(self):
+        r = guard.SlotReservations(ttl=30)
+        r.reserve("room", "@a:x", now=0)
+        self.assertEqual(r.reserved("room", set(), now=31), set())
+        r.reserve("room", "@a:x", now=40)
+        self.assertEqual(r.reserved("room", {"@a:x"}, now=41), set())
+
+    def test_room_lock_is_per_room(self):
+        r = guard.SlotReservations()
+        self.assertIs(r.room_lock("a"), r.room_lock("a"))
+        self.assertIsNot(r.room_lock("a"), r.room_lock("b"))