Avoid repeated artwork miss fetches and refresh churn

Lowers CPU usage:
- Only call refresh() when real artwork is found
- Treating NoArtwork as a valid cached result (art is not None check).
- Adding in-flight dedupe (pending_tracks) so the same track doesn't spawn parallel fetches.

src/mpd_now_playable/mpd/artwork_cache.py

@@ -30,19 +30,28 @@ class MpdArtworkCache:
 	mpd: MpdStateHandler
 	album_cache: Cache[Artwork]
 	track_cache: Cache[Artwork]
+	pending_tracks: set[str]
 
 	def __init__(self, mpd: MpdStateHandler, cache_url: URL = MEMORY):
 		self.mpd = mpd
 		self.album_cache = make_cache(ArtworkSchema, cache_url, "album")
 		self.track_cache = make_cache(ArtworkSchema, cache_url, "track")
+		self.pending_tracks = set()
 
 	async def get_cached_artwork(self, song: CurrentSongResponse) -> bytes | None:
-		art = await self.track_cache.get(calc_track_key(song))
-		if art:
-			return art.data
+		track_key = calc_track_key(song)
+		art = await self.track_cache.get(track_key)
+		if art is not None:
+			# NoArtwork is a valid cached value too: returning None here avoids
+			# repeatedly re-querying MPD for files that have no embedded art.
+			if art:
+				return art.data
+			return None
 
 		# If we don't have track artwork cached, go find some.
-		run_background_task(self.cache_artwork(song))
+		if track_key not in self.pending_tracks:
+			self.pending_tracks.add(track_key)
+			run_background_task(self.cache_artwork(song))
 
 		# Even if we don't have cached track art, we can try looking for cached album art.
 		art = await self.album_cache.get(calc_album_key(song))
@@ -52,10 +61,16 @@ class MpdArtworkCache:
 		return None
 
 	async def cache_artwork(self, song: CurrentSongResponse) -> None:
-		art = to_artwork(await self.mpd.get_art(song["file"]))
+		track_key = calc_track_key(song)
 		try:
-			await self.album_cache.add(calc_album_key(song), art, ttl=CACHE_TTL)
-		except ValueError:
-			pass
-		await self.track_cache.set(calc_track_key(song), art, ttl=CACHE_TTL)
-		await self.mpd.refresh()
+			art = to_artwork(await self.mpd.get_art(song["file"]))
+			try:
+				await self.album_cache.add(calc_album_key(song), art, ttl=CACHE_TTL)
+			except ValueError:
+				pass
+			await self.track_cache.set(track_key, art, ttl=CACHE_TTL)
+			# Refresh receivers only when we discovered actual artwork.
+			if art:
+				await self.mpd.refresh()
+		finally:
+			self.pending_tracks.discard(track_key)