fix: dedupe and cap concurrent lyrics plugin fetches (#5792)

* fix: dedupe and cap concurrent lyrics plugin fetches

Clients like Finamp prefetch lyrics for several queue tracks at once. The
resulting burst of concurrent plugin calls can rate-limit the primary
lyrics provider into a timeout, making the plugin fall back to a lower
quality source and cache the bad result.

SimpleCache.GetWithLoader now deduplicates concurrent loads of the same
key via singleflight, with every waiter receiving the winner's result or
error. The Jellyfin lyrics loader is detached from the request context so
one cancelled request cannot fail the load for all waiters, and the
lyrics adapter caps in-flight plugin calls at 2 per plugin, queueing the
rest. As a side effect, the cached HTTP client used by the Last.fm,
Deezer and ListenBrainz agents also collapses identical concurrent
requests into a single upstream call.

* fix: harden lyrics concurrency fixes per review

Replace the stringified singleflight keys with a per-cache flight map
keyed by the cache key type itself, eliminating potential key collisions
for non-string keys, the nil-interface assertion panic, and the
stringification overhead. Release the lyrics semaphore slot via defer so
a panicking plugin call cannot leak it, and bound the detached lyrics
load with a one-minute timeout so a hung plugin cannot pin its
singleflight and semaphore slot indefinitely.

utils/cache/simple_cache.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"runtime"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -44,7 +45,8 @@ func NewSimpleCache[K comparable, V any](options ...Options) SimpleCache[K, V] {
 
 	c := ttlcache.New[K, V](opts...)
 	cache := &simpleCache[K, V]{
-		data: c,
+		data:  c,
+		loads: make(map[K]*flight[V]),
 	}
 	go cache.data.Start()
 
@@ -61,6 +63,23 @@ const evictionTimeout = 1 * time.Hour
 type simpleCache[K comparable, V any] struct {
 	data             *ttlcache.Cache[K, V]
 	evictionDeadline atomic.Pointer[time.Time]
+	loadsMu          sync.Mutex
+	loads            map[K]*flight[V]
+}
+
+// flight tracks an in-progress load so concurrent misses of the same key share it.
+type flight[V any] struct {
+	done chan struct{}
+	val  V
+	err  error
+}
+
+func (f *flight[V]) result() (V, error) {
+	if f.err != nil {
+		var zero V
+		return zero, fmt.Errorf("cache error: loader returned %w", f.err)
+	}
+	return f.val, nil
 }
 
 func (c *simpleCache[K, V]) Add(key K, value V) error {
@@ -90,31 +109,47 @@ func (c *simpleCache[K, V]) Get(key K) (V, error) {
 	return item.Value(), nil
 }
 
+// GetWithLoader loads misses via the loader, deduplicating concurrent loads of
+// the same key: one loader call runs, and every waiter shares its result (or error).
 func (c *simpleCache[K, V]) GetWithLoader(key K, loader func(key K) (V, time.Duration, error)) (V, error) {
-	var err error
-	loaderWrapper := ttlcache.LoaderFunc[K, V](
-		func(t *ttlcache.Cache[K, V], key K) *ttlcache.Item[K, V] {
-			c.evictExpired()
-			var value V
-			var ttl time.Duration
-			value, ttl, err = loader(key)
-			if err != nil {
-				return nil
-			}
-			return t.Set(key, value, ttl)
-		},
-	)
-	item := c.data.Get(key, ttlcache.WithLoader[K, V](loaderWrapper))
-	if item == nil {
-		var zero V
-		if err != nil {
-			return zero, fmt.Errorf("cache error: loader returned %w", err)
-		}
-		return zero, errors.New("item not found")
+	if item := c.data.Get(key); item != nil {
+		return item.Value(), nil
 	}
-	return item.Value(), nil
+
+	c.loadsMu.Lock()
+	if f, ok := c.loads[key]; ok {
+		c.loadsMu.Unlock()
+		<-f.done
+		return f.result()
+	}
+	f := &flight[V]{done: make(chan struct{}), err: errLoaderPanicked}
+	c.loads[key] = f
+	c.loadsMu.Unlock()
+
+	// Deregister even if the loader panics, so waiters get an error instead of
+	// blocking forever on a flight that will never complete.
+	defer func() {
+		close(f.done)
+		c.loadsMu.Lock()
+		delete(c.loads, key)
+		c.loadsMu.Unlock()
+	}()
+
+	if item := c.data.Get(key); item != nil { // a flight may have completed since the miss
+		f.val, f.err = item.Value(), nil
+	} else {
+		c.evictExpired()
+		var ttl time.Duration
+		f.val, ttl, f.err = loader(key)
+		if f.err == nil {
+			c.data.Set(key, f.val, ttl)
+		}
+	}
+	return f.result()
 }
 
+var errLoaderPanicked = errors.New("loader panicked")
+
 func (c *simpleCache[K, V]) evictExpired() {
 	if c.evictionDeadline.Load() == nil || c.evictionDeadline.Load().Before(time.Now()) {
 		c.data.DeleteExpired()