fix: delegate !similar playback to music bot, not calling bot

When merlin ran !similar, music operations (fade, queue, play loop)
targeted merlin instead of derp, causing audio to play over derp's
stream. New _music_bot() helper resolves the DJ bot via active state
or plugin filter config, so playback always routes to derp.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/lastfm.py

@@ -105,26 +105,41 @@ def _parse_title(raw_title: str) -> tuple[str, str]:
     return ("", raw_title)
 
 
-def _current_meta(bot) -> tuple[str, str]:
-    """Extract artist and title from the currently playing track.
+def _music_bot(bot):
+    """Return the bot instance that owns music playback.
 
-    Returns (artist, title). Either or both may be empty.
-    Tries the music plugin's current track metadata on this bot first,
-    then checks peer bots (shared registry) so extra bots can see what
-    the music bot is playing.
+    Checks the calling bot first, then peer bots via the shared registry.
+    Returns the first bot with an active music state, or ``bot`` as fallback.
     """
-    # Check this bot first, then peers
     candidates = [bot]
     for peer in getattr(getattr(bot, "registry", None), "_bots", {}).values():
         if peer is not bot:
             candidates.append(peer)
     for b in candidates:
         music_ps = getattr(b, "_pstate", {}).get("music", {})
-        current = music_ps.get("current")
-        if current is not None:
-            raw_title = current.title or ""
-            if raw_title:
-                return _parse_title(raw_title)
+        if music_ps.get("current") is not None or music_ps.get("queue"):
+            return b
+    # No active music state -- prefer a bot that allows the music plugin
+    for b in candidates:
+        only = getattr(b, "_only_plugins", None)
+        if only is not None and "music" in only:
+            return b
+    return bot
+
+
+def _current_meta(bot) -> tuple[str, str]:
+    """Extract artist and title from the currently playing track.
+
+    Returns (artist, title). Either or both may be empty.
+    Checks the music bot (via ``_music_bot``) for now-playing metadata.
+    """
+    mb = _music_bot(bot)
+    music_ps = getattr(mb, "_pstate", {}).get("music", {})
+    current = music_ps.get("current")
+    if current is not None:
+        raw_title = current.title or ""
+        if raw_title:
+            return _parse_title(raw_title)
     return ("", "")
 
 
@@ -410,13 +425,14 @@ async def cmd_similar(bot, message):
         await bot.reply(message, "No playable tracks resolved")
         return
 
-    # Transition: fade out current, load new playlist
-    ps = music_mod._ps(bot)
-    await music_mod._fade_and_cancel(bot, duration=3.0)
+    # Transition on the music bot (derp), not the calling bot (may be merlin)
+    dj = _music_bot(bot)
+    ps = music_mod._ps(dj)
+    await music_mod._fade_and_cancel(dj, duration=3.0)
     ps["queue"].clear()
     ps["current"] = None
     ps["queue"] = list(tracks)
-    music_mod._ensure_loop(bot, fade_in=True)
+    music_mod._ensure_loop(dj, fade_in=True)
     await bot.reply(message, f"Playing {len(tracks)} similar tracks for {search_artist}")
 
 

tests/test_lastfm.py

@@ -2,7 +2,6 @@
 
 import asyncio
 import importlib.util
-import json
 import sys
 from dataclasses import dataclass
 from unittest.mock import AsyncMock, MagicMock, patch
@@ -21,14 +20,17 @@ _spec.loader.exec_module(_mod)
 class _FakeRegistry:
     def __init__(self):
         self._modules: dict = {}
+        self._bots: dict = {}
 
 
 class _FakeBot:
-    def __init__(self, *, api_key: str = "test-key"):
+    def __init__(self, *, api_key: str = "test-key", name: str = "derp"):
         self.replied: list[str] = []
         self.config: dict = {"lastfm": {"api_key": api_key}} if api_key else {}
         self._pstate: dict = {}
+        self._only_plugins: set[str] | None = None
         self.registry = _FakeRegistry()
+        self._username = name
 
     async def reply(self, message, text: str) -> None:
         self.replied.append(text)
@@ -615,6 +617,47 @@ class TestCmdSimilar:
             asyncio.run(_mod.cmd_similar(bot, msg))
         assert any("Playing 1 similar" in r for r in bot.replied)
 
+    def test_cross_bot_delegates_to_music_bot(self):
+        """When merlin runs !similar, playback starts on derp (music bot)."""
+        # derp = music bot (has active music state)
+        derp = _FakeBot(api_key="test-key", name="derp")
+        derp._only_plugins = {"music", "voice"}
+        derp._pstate["music"] = {
+            "current": _FakeTrack(title="Tool - Lateralus"),
+            "queue": [],
+        }
+        # merlin = calling bot (no music plugin)
+        merlin = _FakeBot(api_key="test-key", name="merlin")
+        shared_reg = _FakeRegistry()
+        shared_reg._bots = {"derp": derp, "merlin": merlin}
+        derp.registry = shared_reg
+        merlin.registry = shared_reg
+
+        artists = [{"name": "Deftones", "match": "0.8"}]
+        fake_tracks = [_FakeTrack(url="http://yt/1", title="Song 1")]
+
+        music_mod = MagicMock()
+        music_mod._ps.return_value = {
+            "queue": [], "current": None, "task": None,
+        }
+        music_mod._fade_and_cancel = AsyncMock()
+        music_mod._ensure_loop = MagicMock()
+        shared_reg._modules["music"] = music_mod
+
+        msg = _Msg(text="!similar Tool")
+        with patch.object(_mod, "_get_similar_tracks", return_value=[]), \
+             patch.object(_mod, "_get_similar_artists", return_value=artists), \
+             patch.object(_mod, "_resolve_playlist",
+                          return_value=fake_tracks):
+            asyncio.run(_mod.cmd_similar(merlin, msg))
+
+        # Music ops must target derp, not merlin
+        music_mod._fade_and_cancel.assert_called_once_with(derp, duration=3.0)
+        music_mod._ensure_loop.assert_called_once_with(derp, fade_in=True)
+        music_mod._ps.assert_called_with(derp)
+        # Reply still goes through merlin (the bot the user talked to)
+        assert any("Playing 1 similar" in r for r in merlin.replied)
+
 
 # ---------------------------------------------------------------------------
 # TestCmdTags
@@ -694,6 +737,46 @@ class TestCmdTags:
         assert any("Lateralus:" in r for r in bot.replied)
 
 
+# ---------------------------------------------------------------------------
+# TestMusicBot
+# ---------------------------------------------------------------------------
+
+
+class TestMusicBot:
+    def test_returns_self_when_active(self):
+        """Returns calling bot when it has active music state."""
+        bot = _FakeBot()
+        bot._pstate["music"] = {"current": _FakeTrack(title="X"), "queue": []}
+        assert _mod._music_bot(bot) is bot
+
+    def test_returns_peer_with_active_state(self):
+        """Returns peer bot that has music playing."""
+        derp = _FakeBot(name="derp")
+        derp._pstate["music"] = {"current": _FakeTrack(title="X"), "queue": []}
+        merlin = _FakeBot(name="merlin")
+        reg = _FakeRegistry()
+        reg._bots = {"derp": derp, "merlin": merlin}
+        derp.registry = reg
+        merlin.registry = reg
+        assert _mod._music_bot(merlin) is derp
+
+    def test_falls_back_to_only_plugins(self):
+        """Returns bot with only_plugins containing 'music' when no active state."""
+        derp = _FakeBot(name="derp")
+        derp._only_plugins = {"music", "voice"}
+        merlin = _FakeBot(name="merlin")
+        reg = _FakeRegistry()
+        reg._bots = {"derp": derp, "merlin": merlin}
+        derp.registry = reg
+        merlin.registry = reg
+        assert _mod._music_bot(merlin) is derp
+
+    def test_returns_self_as_last_resort(self):
+        """Returns calling bot when no peer has music state or filters."""
+        bot = _FakeBot()
+        assert _mod._music_bot(bot) is bot
+
+
 # ---------------------------------------------------------------------------
 # TestParseTitle
 # ---------------------------------------------------------------------------