feat(artwork): promote worker concurrency and external rate to real configs

The artwork worker's drain speed was governed by two hidden Dev flags,
DevArtworkWorkerConcurrency and DevArtworkExternalRPS, both defaulting to 2.
On a large library's one-time backfill the external rate limiter is the real
ceiling: every art-less item waits on it before the (rate-limited) external
lookup, so the drain crawls at ~RPS items/sec while local-art items are
unaffected.

Promote both to documented, supported options: ArtworkWorkerConcurrency
(default 4) sets local-resolution parallelism, ArtworkExternalMaxRPS
(default 2, 0 = unlimited) caps external-agent lookups to stay polite to
Last.fm/Deezer/etc. Operators can now trade first-backfill speed against
external-API rate limits. The old Dev names still map for backward compat.

conf/configuration.go

@@ -57,6 +57,8 @@ type configOptions struct {
 	ImageCacheSize                  string
 	AlbumPlayCountMode              string
 	EnableArtworkPrecache           bool
+	ArtworkWorkerConcurrency        int
+	ArtworkExternalMaxRPS           int
 	AutoImportPlaylists             bool
 	DefaultPlaylistPublicVisibility bool
 	PlaylistsPath                   string
@@ -139,8 +141,6 @@ type configOptions struct {
 	DevArtworkThrottleBacklogLimit    int
 	DevArtworkThrottleBacklogTimeout  time.Duration
 	DevArtworkThrottleBuffered        bool
-	DevArtworkWorkerConcurrency       int
-	DevArtworkExternalRPS             int
 	DevArtistInfoTimeToLive           time.Duration
 	DevAlbumInfoTimeToLive            time.Duration
 	DevExternalScanner                bool
@@ -348,6 +348,8 @@ func Load(noConfigDump bool) {
 	mapDeprecatedOption("CoverJpegQuality", "CoverArtQuality")
 	mapDeprecatedOption("SimilarSongsMatchThreshold", "Matcher.FuzzyThreshold")
 	mapDeprecatedOption("EnableTranscodingCancellation", "Transcoding.EnableCancellation")
+	mapDeprecatedOption("DevArtworkWorkerConcurrency", "ArtworkWorkerConcurrency")
+	mapDeprecatedOption("DevArtworkExternalRPS", "ArtworkExternalMaxRPS")
 
 	err := viper.Unmarshal(&Server, viper.DecodeHook(
 		mapstructure.ComposeDecodeHookFunc(
@@ -902,8 +904,8 @@ func setViperDefaults() {
 	viper.SetDefault("devartworkthrottlebackloglimit", consts.RequestThrottleBacklogLimit)
 	viper.SetDefault("devartworkthrottlebacklogtimeout", consts.RequestThrottleBacklogTimeout)
 	viper.SetDefault("devartworkthrottlebuffered", true)
-	viper.SetDefault("devartworkworkerconcurrency", 2)
-	viper.SetDefault("devartworkexternalrps", 2)
+	viper.SetDefault("artworkworkerconcurrency", 4)
+	viper.SetDefault("artworkexternalmaxrps", 2)
 	viper.SetDefault("devartistinfotimetolive", consts.ArtistInfoTimeToLive)
 	viper.SetDefault("devalbuminfotimetolive", consts.AlbumInfoTimeToLive)
 	viper.SetDefault("devexternalscanner", true)

core/artwork/worker.go

@@ -43,8 +43,8 @@ type Worker struct {
 }
 
 func NewWorker(ds model.DataStore, store *ImageStore, prov external.Provider, ffmpeg ffmpeg.FFmpeg) *Worker {
-	rps := conf.Server.DevArtworkExternalRPS
-	limit := rate.Inf
+	rps := conf.Server.ArtworkExternalMaxRPS
+	limit := rate.Inf // 0 or negative disables the external throttle
 	if rps > 0 {
 		limit = rate.Limit(rps)
 	}
@@ -64,7 +64,7 @@ func NewWorker(ds model.DataStore, store *ImageStore, prov external.Provider, ff
 // leaked goroutines: each drain waits for its batch before the loop can return.
 func (w *Worker) Run(ctx context.Context) error {
 	w.runCtx = ctx
-	concurrency := max(1, conf.Server.DevArtworkWorkerConcurrency)
+	concurrency := max(1, conf.Server.ArtworkWorkerConcurrency)
 	ticker := time.NewTicker(workerPollInterval)
 	defer ticker.Stop()
 	for {

core/artwork/worker_test.go

@@ -85,7 +85,7 @@ var _ = Describe("Worker", func() {
 		ds.MockedAlbum = tests.CreateMockAlbumRepo()
 		store = NewImageStore(GinkgoT().TempDir())
 		conf.Server.CoverArtPriority = "cover.jpg, embedded"
-		conf.Server.DevArtworkExternalRPS = 1000 // keep the limiter out of the way of behavior tests
+		conf.Server.ArtworkExternalMaxRPS = 1000 // keep the limiter out of the way of behavior tests
 		w = NewWorker(ds, store, prov, ffm)
 	})