perf(network): mobile stability and data-usage fixes from network audit

Stability:
- transient timeouts no longer classified as ISP blocking; they fall
  through to retry backoff (hard blocks - DNS/RST/cert - still abort)
- connectivity change now closes idle Go connections in every network
  mode (debounced), so pooled sockets from the old interface are not
  reused after a wifi/cellular handoff
- download body reads get a 60s stall watchdog that cancels and
  surfaces a retryable network error instead of hanging; distinct
  from user cancellation
- ResponseHeaderTimeout 45s on all transports; IdleConnTimeout 90->60s;
  dial timeout 30->10s; retry backoff gains full jitter; Retry-After
  honored on 5xx and Deezer 429

Data usage / speed:
- cover downloads deduplicated with singleflight plus a 24MB/15min
  in-memory cache keyed by final URL (album batches fetched the same
  1800px cover once per track before)
- song.link availability cached (30min positive / 5min negative)
  in front of the 9-req/min rate limiter
- uTLS Cloudflare path now pools one HTTP/2 connection per host with
  a shared TLS session cache instead of a full handshake per request

Deezer artist track-count N+1 kept: counts feed the discography
download UI and are already amortized by the artist TTL cache.

go_backend/cover.go

@@ -6,6 +6,8 @@ import (
 	"net/http"
 	"regexp"
 	"strings"
+	"sync"
+	"time"
 )
 
 const (
@@ -52,6 +54,104 @@ func downloadCoverToMemory(coverURL string, maxQuality bool) ([]byte, error) {
 
 	GoLog("[Cover] Final URL: %s", downloadURL)
 
+	data, err := fetchCoverCached(downloadURL)
+	if err != nil {
+		return nil, err
+	}
+	// Cached bytes are shared across goroutines and must never be mutated;
+	// hand callers their own copy.
+	return append([]byte(nil), data...), nil
+}
+
+const (
+	coverCacheMaxBytes = 24 * 1024 * 1024
+	coverCacheTTL      = 15 * time.Minute
+)
+
+type coverCacheEntry struct {
+	data      []byte
+	expiresAt time.Time
+}
+
+type coverInflightCall struct {
+	wg   sync.WaitGroup
+	data []byte
+	err  error
+}
+
+var (
+	coverMu         sync.Mutex
+	coverCache      = map[string]*coverCacheEntry{}
+	coverCacheBytes int
+	coverInflight   = map[string]*coverInflightCall{}
+	coverFetch      = fetchCoverBytes
+)
+
+// fetchCoverCached returns cover bytes for a final URL, collapsing concurrent
+// requests for the same URL into a single fetch (singleflight) and caching
+// results in memory for the duration of an album batch. The returned slice is
+// shared; callers must copy before mutating.
+func fetchCoverCached(downloadURL string) ([]byte, error) {
+	coverMu.Lock()
+	if e, ok := coverCache[downloadURL]; ok {
+		if time.Now().Before(e.expiresAt) {
+			data := e.data
+			coverMu.Unlock()
+			return data, nil
+		}
+		delete(coverCache, downloadURL)
+		coverCacheBytes -= len(e.data)
+	}
+	if call, ok := coverInflight[downloadURL]; ok {
+		coverMu.Unlock()
+		call.wg.Wait()
+		return call.data, call.err
+	}
+	call := &coverInflightCall{}
+	call.wg.Add(1)
+	coverInflight[downloadURL] = call
+	coverMu.Unlock()
+
+	data, err := coverFetch(downloadURL)
+	call.data, call.err = data, err
+	if err == nil {
+		coverCachePut(downloadURL, data)
+	}
+	call.wg.Done()
+
+	coverMu.Lock()
+	delete(coverInflight, downloadURL)
+	coverMu.Unlock()
+
+	return data, err
+}
+
+func coverCachePut(downloadURL string, data []byte) {
+	if len(data) == 0 || len(data) > coverCacheMaxBytes {
+		return
+	}
+	coverMu.Lock()
+	defer coverMu.Unlock()
+	if e, ok := coverCache[downloadURL]; ok {
+		coverCacheBytes -= len(e.data)
+	}
+	coverCache[downloadURL] = &coverCacheEntry{data: data, expiresAt: time.Now().Add(coverCacheTTL)}
+	coverCacheBytes += len(data)
+	for coverCacheBytes > coverCacheMaxBytes && len(coverCache) > 1 {
+		var oldestKey string
+		var oldest time.Time
+		first := true
+		for k, e := range coverCache {
+			if first || e.expiresAt.Before(oldest) {
+				oldest, oldestKey, first = e.expiresAt, k, false
+			}
+		}
+		coverCacheBytes -= len(coverCache[oldestKey].data)
+		delete(coverCache, oldestKey)
+	}
+}
+
+func fetchCoverBytes(downloadURL string) ([]byte, error) {
 	client := NewHTTPClientWithTimeout(DefaultTimeout)
 
 	req, err := http.NewRequest("GET", downloadURL, nil)

go_backend/cover_test.go

@@ -0,0 +1,96 @@
+package gobackend
+
+import (
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func resetCoverCache() {
+	coverMu.Lock()
+	coverCache = map[string]*coverCacheEntry{}
+	coverInflight = map[string]*coverInflightCall{}
+	coverCacheBytes = 0
+	coverMu.Unlock()
+}
+
+func TestFetchCoverCachedSingleflight(t *testing.T) {
+	orig := coverFetch
+	defer func() { coverFetch = orig }()
+	resetCoverCache()
+
+	var calls int32
+	entered := make(chan struct{})
+	release := make(chan struct{})
+	coverFetch = func(string) ([]byte, error) {
+		if atomic.AddInt32(&calls, 1) == 1 {
+			close(entered)
+		}
+		<-release
+		return []byte("coverbytes"), nil
+	}
+
+	const url = "https://cdn.example/cover_max.jpg"
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		if _, err := fetchCoverCached(url); err != nil {
+			t.Errorf("leader fetch error: %v", err)
+		}
+	}()
+
+	<-entered // leader has registered inflight and is blocked in coverFetch
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		if _, err := fetchCoverCached(url); err != nil {
+			t.Errorf("follower fetch error: %v", err)
+		}
+	}()
+
+	close(release)
+	wg.Wait()
+
+	if got := atomic.LoadInt32(&calls); got != 1 {
+		t.Fatalf("expected 1 fetch for concurrent requests, got %d", got)
+	}
+}
+
+func TestFetchCoverCachedTTLExpiry(t *testing.T) {
+	orig := coverFetch
+	defer func() { coverFetch = orig }()
+	resetCoverCache()
+
+	var calls int32
+	coverFetch = func(string) ([]byte, error) {
+		atomic.AddInt32(&calls, 1)
+		return []byte("data"), nil
+	}
+
+	const url = "https://cdn.example/ttl.jpg"
+	if _, err := fetchCoverCached(url); err != nil {
+		t.Fatalf("first fetch error: %v", err)
+	}
+	// second call served from cache
+	if _, err := fetchCoverCached(url); err != nil {
+		t.Fatalf("second fetch error: %v", err)
+	}
+	if got := atomic.LoadInt32(&calls); got != 1 {
+		t.Fatalf("expected cache hit, got %d fetches", got)
+	}
+
+	// expire the entry and confirm a refetch
+	coverMu.Lock()
+	coverCache[url].expiresAt = time.Now().Add(-time.Minute)
+	coverMu.Unlock()
+
+	if _, err := fetchCoverCached(url); err != nil {
+		t.Fatalf("third fetch error: %v", err)
+	}
+	if got := atomic.LoadInt32(&calls); got != 2 {
+		t.Fatalf("expected refetch after TTL expiry, got %d fetches", got)
+	}
+}

go_backend/deezer.go

@@ -1269,6 +1269,10 @@ func (c *DeezerClient) getJSON(ctx context.Context, endpoint string, dst any) er
 	for attempt := 0; attempt <= deezerMaxRetries; attempt++ {
 		if attempt > 0 {
 			delay := deezerRetryDelay * time.Duration(1<<(attempt-1))
+			var apiErr *deezerAPIError
+			if errors.As(lastErr, &apiErr) && apiErr.RetryAfter > 0 {
+				delay = apiErr.RetryAfter
+			}
 			GoLog("[Deezer] Retry %d/%d after %v...\n", attempt, deezerMaxRetries, delay)
 			time.Sleep(delay)
 		}
@@ -1292,6 +1296,7 @@ func (c *DeezerClient) getJSON(ctx context.Context, endpoint string, dst any) er
 type deezerAPIError struct {
 	StatusCode int
 	Body       string
+	RetryAfter time.Duration
 }
 
 func (e *deezerAPIError) Error() string {
@@ -1329,7 +1334,7 @@ func (c *DeezerClient) doGetJSON(ctx context.Context, endpoint string, dst any)
 	}
 
 	if resp.StatusCode != http.StatusOK {
-		return &deezerAPIError{StatusCode: resp.StatusCode, Body: string(body)}
+		return &deezerAPIError{StatusCode: resp.StatusCode, Body: string(body), RetryAfter: getRetryAfterDuration(resp)}
 	}
 
 	return json.Unmarshal(body, dst)

go_backend/extension_runtime.go

@@ -1,6 +1,7 @@
 package gobackend
 
 import (
+	"context"
 	"fmt"
 	"net"
 	"net/http"
@@ -276,6 +277,49 @@ func (r *extensionRuntime) bindDownloadCancelContext(req *http.Request) *http.Re
 	return req.WithContext(initDownloadCancel(itemID))
 }
 
+// downloadStallTimeout is how long a download may go without receiving a single
+// byte before the stall watchdog aborts it. A dead radio mid-transfer otherwise
+// blocks on Body.Read until the 24h client timeout with no error and no retry.
+const downloadStallTimeout = 60 * time.Second
+
+// stallWatchdog cancels an in-flight download when no data arrives within
+// timeout. It wraps the request context in a child cancel so firing it does NOT
+// set the user-cancel flag (isDownloadCancelled stays false) — a stall is a
+// distinct, retryable condition. Call reset() after every successful Read and
+// stop() when the transfer ends.
+type stallWatchdog struct {
+	cancel  context.CancelFunc
+	timer   *time.Timer
+	timeout time.Duration
+	stalled atomic.Bool
+}
+
+func bindStallWatchdog(req *http.Request, timeout time.Duration) (*http.Request, *stallWatchdog) {
+	ctx, cancel := context.WithCancel(req.Context())
+	w := &stallWatchdog{cancel: cancel, timeout: timeout}
+	w.timer = time.AfterFunc(timeout, func() {
+		w.stalled.Store(true)
+		cancel()
+	})
+	return req.WithContext(ctx), w
+}
+
+func (w *stallWatchdog) reset() { w.timer.Reset(w.timeout) }
+
+// stop halts the timer and releases the child context so a completed download
+// leaks neither a pending timer nor a live cancel func.
+func (w *stallWatchdog) stop() {
+	w.timer.Stop()
+	w.cancel()
+}
+
+// stallError is returned when the watchdog fires. The message is deliberately
+// free of "cancel" and worded to classify as retryable network failure, so the
+// fallback layer retries instead of treating it as a user cancellation.
+func (r *extensionRuntime) stallError() goja.Value {
+	return r.jsError("download stalled: no data received for %ds (network timeout)", int(downloadStallTimeout.Seconds()))
+}
+
 func newExtensionHTTPClient(ext *loadedExtension, jar http.CookieJar, timeout time.Duration, compressResponses bool) *http.Client {
 	// Extension sandbox enforces HTTPS-only domains. Do not apply global
 	// allow_http scheme downgrade here, because some extension APIs (e.g.

go_backend/extension_runtime_file.go

@@ -203,6 +203,8 @@ func (r *extensionRuntime) fileDownload(call goja.FunctionCall) goja.Value {
 		return r.jsError("%s", err.Error())
 	}
 	req = r.bindDownloadCancelContext(req)
+	req, wd := bindStallWatchdog(req, downloadStallTimeout)
+	defer wd.stop()
 
 	for k, v := range headers {
 		req.Header.Set(k, v)
@@ -213,6 +215,9 @@ func (r *extensionRuntime) fileDownload(call goja.FunctionCall) goja.Value {
 
 	resp, err := client.Do(req)
 	if err != nil {
+		if wd.stalled.Load() {
+			return r.stallError()
+		}
 		return r.jsError("%s", err.Error())
 	}
 	defer resp.Body.Close()
@@ -259,6 +264,7 @@ func (r *extensionRuntime) fileDownload(call goja.FunctionCall) goja.Value {
 	for {
 		nr, er := resp.Body.Read(buf)
 		if nr > 0 {
+			wd.reset()
 			nw, ew := progressWriter.Write(buf[0:nr])
 			if nw < 0 || nr < nw {
 				nw = 0
@@ -283,6 +289,9 @@ func (r *extensionRuntime) fileDownload(call goja.FunctionCall) goja.Value {
 		}
 		if er != nil {
 			if er != io.EOF {
+				if wd.stalled.Load() {
+					return r.stallError()
+				}
 				return r.jsError("failed to read response: %v", er)
 			}
 			break
@@ -332,6 +341,9 @@ func (r *extensionRuntime) fileDownloadChunked(client *http.Client, urlStr, full
 		return r.jsError("chunked: probe request error: %v", err)
 	}
 	probeReq = r.bindDownloadCancelContext(probeReq)
+	probeReq, wd := bindStallWatchdog(probeReq, downloadStallTimeout)
+	defer wd.stop()
+	stallCtx := probeReq.Context()
 	probeReq.Header.Set("User-Agent", ua)
 	for k, v := range headers {
 		if k != "Range" { // Don't copy any existing Range header
@@ -342,6 +354,9 @@ func (r *extensionRuntime) fileDownloadChunked(client *http.Client, urlStr, full
 
 	probeResp, err := client.Do(probeReq)
 	if err != nil {
+		if wd.stalled.Load() {
+			return r.stallError()
+		}
 		return r.jsError("chunked: probe error: %v", err)
 	}
 	io.Copy(io.Discard, probeResp.Body)
@@ -420,7 +435,7 @@ func (r *extensionRuntime) fileDownloadChunked(client *http.Client, urlStr, full
 			if err != nil {
 				return r.jsError("chunked: request error at offset %d: %v", offset, err)
 			}
-			chunkReq = r.bindDownloadCancelContext(chunkReq)
+			chunkReq = chunkReq.WithContext(stallCtx)
 			chunkReq.Header.Set("User-Agent", ua)
 			for k, v := range headers {
 				if k != "Range" {
@@ -431,6 +446,9 @@ func (r *extensionRuntime) fileDownloadChunked(client *http.Client, urlStr, full
 
 			chunkResp, chunkErr = client.Do(chunkReq)
 			if chunkErr != nil {
+				if wd.stalled.Load() {
+					return r.stallError()
+				}
 				if retry < maxRetries-1 {
 					time.Sleep(time.Duration(retry+1) * time.Second)
 					continue
@@ -459,6 +477,7 @@ func (r *extensionRuntime) fileDownloadChunked(client *http.Client, urlStr, full
 		for {
 			nr, er := chunkResp.Body.Read(buf)
 			if nr > 0 {
+				wd.reset()
 				nw, ew := progressWriter.Write(buf[0:nr])
 				if nw < 0 || nr < nw {
 					nw = 0
@@ -487,6 +506,9 @@ func (r *extensionRuntime) fileDownloadChunked(client *http.Client, urlStr, full
 			if er != nil {
 				if er != io.EOF {
 					chunkResp.Body.Close()
+					if wd.stalled.Load() {
+						return r.stallError()
+					}
 					return r.jsError("failed to read chunk at offset %d: %v", offset, er)
 				}
 				break

go_backend/extension_runtime_stall_test.go

@@ -0,0 +1,54 @@
+package gobackend
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+// TestStallWatchdogCancelsOnNoData verifies the watchdog aborts a transfer that
+// stops sending bytes, marks itself stalled (not user-cancelled), and does so
+// after resetting on the initial byte.
+func TestStallWatchdogCancelsOnNoData(t *testing.T) {
+	block := make(chan struct{})
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Length", "1000")
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("x"))
+		w.(http.Flusher).Flush()
+		<-block // simulate a dead radio mid-transfer: stop sending
+	}))
+	defer srv.Close()
+	defer close(block)
+
+	req, _ := http.NewRequestWithContext(context.Background(), "GET", srv.URL, nil)
+	req, wd := bindStallWatchdog(req, 150*time.Millisecond)
+	defer wd.stop()
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer resp.Body.Close()
+
+	buf := make([]byte, 32)
+	var readErr error
+	for {
+		n, er := resp.Body.Read(buf)
+		if n > 0 {
+			wd.reset()
+		}
+		if er != nil {
+			readErr = er
+			break
+		}
+	}
+	if readErr == nil {
+		t.Fatal("expected read error from stall cancel")
+	}
+	if !wd.stalled.Load() {
+		t.Fatalf("watchdog did not mark stalled; err=%v", readErr)
+	}
+}

go_backend/httputil.go

@@ -65,14 +65,15 @@ var (
 
 var sharedTransport = &http.Transport{
 	DialContext: (&net.Dialer{
-		Timeout:   30 * time.Second,
+		Timeout:   10 * time.Second,
 		KeepAlive: 30 * time.Second,
 	}).DialContext,
 	MaxIdleConns:          100,
 	MaxIdleConnsPerHost:   10,
 	MaxConnsPerHost:       20,
-	IdleConnTimeout:       90 * time.Second,
+	IdleConnTimeout:       60 * time.Second,
 	TLSHandshakeTimeout:   10 * time.Second,
+	ResponseHeaderTimeout: 45 * time.Second,
 	ExpectContinueTimeout: 1 * time.Second,
 	DisableKeepAlives:     false,
 	ForceAttemptHTTP2:     true,
@@ -84,14 +85,15 @@ var sharedTransport = &http.Transport{
 
 var extensionAPITransport = &http.Transport{
 	DialContext: (&net.Dialer{
-		Timeout:   30 * time.Second,
+		Timeout:   10 * time.Second,
 		KeepAlive: 30 * time.Second,
 	}).DialContext,
 	MaxIdleConns:          100,
 	MaxIdleConnsPerHost:   10,
 	MaxConnsPerHost:       20,
-	IdleConnTimeout:       90 * time.Second,
+	IdleConnTimeout:       60 * time.Second,
 	TLSHandshakeTimeout:   10 * time.Second,
+	ResponseHeaderTimeout: 45 * time.Second,
 	ExpectContinueTimeout: 1 * time.Second,
 	DisableKeepAlives:     false,
 	ForceAttemptHTTP2:     true,
@@ -103,14 +105,15 @@ var extensionAPITransport = &http.Transport{
 
 var metadataTransport = &http.Transport{
 	DialContext: (&net.Dialer{
-		Timeout:   30 * time.Second,
+		Timeout:   10 * time.Second,
 		KeepAlive: 30 * time.Second,
 	}).DialContext,
 	MaxIdleConns:          30,
 	MaxIdleConnsPerHost:   5,
 	MaxConnsPerHost:       10,
-	IdleConnTimeout:       90 * time.Second,
+	IdleConnTimeout:       60 * time.Second,
 	TLSHandshakeTimeout:   10 * time.Second,
+	ResponseHeaderTimeout: 45 * time.Second,
 	ExpectContinueTimeout: 1 * time.Second,
 	DisableKeepAlives:     false,
 	ForceAttemptHTTP2:     true,
@@ -276,7 +279,7 @@ func DoRequestWithRetry(client *http.Client, req *http.Request, config RetryConf
 		if err != nil {
 			lastErr = err
 
-			if CheckAndLogISPBlocking(err, reqCopy.URL.String(), "HTTP") {
+			if isHardConnectivityBlock(err) {
 				return nil, WrapErrorWithISPCheck(err, reqCopy.URL.String(), "HTTP")
 			}
 
@@ -331,6 +334,9 @@ func DoRequestWithRetry(client *http.Client, req *http.Request, config RetryConf
 
 		if resp.StatusCode >= 500 {
 			resp.Body.Close()
+			if retryAfter := getRetryAfterDuration(resp); retryAfter > 0 {
+				delay = retryAfter
+			}
 			lastErr = fmt.Errorf("server error: HTTP %d", resp.StatusCode)
 			if attempt < config.MaxRetries {
 				GoLog("[HTTP] Server error %d, retrying in %v...\n", resp.StatusCode, delay)
@@ -346,9 +352,20 @@ func DoRequestWithRetry(client *http.Client, req *http.Request, config RetryConf
 	return nil, fmt.Errorf("request failed after %d retries: %w", config.MaxRetries+1, lastErr)
 }
 
+// jitterFloat returns a fraction in [0,1); overridable in tests for
+// deterministic backoff assertions.
+var jitterFloat = rand.Float64
+
 func calculateNextDelay(currentDelay time.Duration, config RetryConfig) time.Duration {
 	nextDelay := time.Duration(float64(currentDelay) * config.BackoffFactor)
-	return min(nextDelay, config.MaxDelay)
+	capped := min(nextDelay, config.MaxDelay)
+	// Full jitter: spread retries between InitialDelay and the capped
+	// exponential ceiling to avoid synchronized thundering-herd retries.
+	if capped <= config.InitialDelay {
+		return capped
+	}
+	span := capped - config.InitialDelay
+	return config.InitialDelay + time.Duration(jitterFloat()*float64(span))
 }
 
 // Returns 0 if the header is missing or invalid so callers can keep their
@@ -524,6 +541,42 @@ func isTLSHandshakeOrResetError(err error) bool {
 	return false
 }
 
+// isHardConnectivityBlock reports transport failures that signal an active
+// block (DNS not found, connection refused/reset, TLS/cert MITM) and should
+// abort retries immediately. Timeouts and deadline-exceeded are treated as
+// transient and excluded so they fall through to normal retry backoff.
+func isHardConnectivityBlock(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	var urlErr *url.Error
+	if errors.As(err, &urlErr) && urlErr.Timeout() {
+		return false
+	}
+
+	var dnsErr *net.DNSError
+	if errors.As(err, &dnsErr) {
+		return dnsErr.IsNotFound
+	}
+
+	var opErr *net.OpError
+	if errors.As(err, &opErr) {
+		if opErr.Timeout() {
+			return false
+		}
+		var errno syscall.Errno
+		if errors.As(opErr.Err, &errno) {
+			switch errno {
+			case syscall.ECONNREFUSED, syscall.ECONNRESET:
+				return true
+			}
+		}
+	}
+
+	return isTLSHandshakeOrResetError(err)
+}
+
 func IsISPBlocking(err error, requestURL string) *ISPBlockingError {
 	if err == nil {
 		return nil

go_backend/httputil_supplement_test.go

@@ -112,9 +112,12 @@ func TestHTTPUtilityHelpers(t *testing.T) {
 	if body, err := ReadResponseBody(&http.Response{Body: io.NopCloser(strings.NewReader("ok"))}); err != nil || string(body) != "ok" {
 		t.Fatalf("ReadResponseBody = %q/%v", body, err)
 	}
+	origJitter := jitterFloat
+	jitterFloat = func() float64 { return 1 }
 	if calculateNextDelay(10*time.Millisecond, RetryConfig{BackoffFactor: 3, MaxDelay: 20 * time.Millisecond}) != 20*time.Millisecond {
 		t.Fatal("calculateNextDelay mismatch")
 	}
+	jitterFloat = origJitter
 	if getRetryAfterDuration(&http.Response{Header: http.Header{"Retry-After": []string{"bad"}}}) != 0 {
 		t.Fatal("invalid retry-after should be zero")
 	}

go_backend/httputil_utls.go

@@ -4,27 +4,38 @@ package gobackend
 
 import (
 	"context"
-	"crypto/tls"
 	"io"
 	"net"
 	"net/http"
 	"net/url"
 	"strings"
+	"sync"
 
 	utls "github.com/refraction-networking/utls"
 	"golang.org/x/net/http2"
 )
 
+// utlsSessionCache is shared by every uTLS handshake so TLS 1.3 tickets enable
+// resumption (fewer round-trips) across requests and hosts.
+var utlsSessionCache = utls.NewLRUClientSessionCache(0)
+
+// utlsTransport dials with a Chrome TLS fingerprint and pools one healthy HTTP/2
+// connection per host, re-dialing when it dies (e.g. after a network switch).
 type utlsTransport struct {
 	dialer *net.Dialer
+	h2     *http2.Transport
+	mu     sync.Mutex
+	conns  map[string]*http2.ClientConn
 }
 
 func newUTLSTransport() *utlsTransport {
 	return &utlsTransport{
 		dialer: &net.Dialer{
-			Timeout:   30 * Second,
+			Timeout:   10 * Second,
 			KeepAlive: 30 * Second,
 		},
+		h2:    &http2.Transport{},
+		conns: make(map[string]*http2.ClientConn),
 	}
 }
 
@@ -34,48 +45,94 @@ func (t *utlsTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	}
 
 	host := req.URL.Hostname()
-	port := t.getPort(req.URL)
-	addr := net.JoinHostPort(host, port)
+	addr := net.JoinHostPort(host, t.getPort(req.URL))
 
-	conn, err := t.dialer.DialContext(req.Context(), "tcp", addr)
+	if cc := t.cachedConn(addr); cc != nil {
+		resp, err := cc.RoundTrip(req)
+		if err != nil {
+			t.invalidate(addr, cc)
+		}
+		return resp, err
+	}
+
+	tlsConn, proto, err := t.dial(req.Context(), host, addr)
 	if err != nil {
 		return nil, err
 	}
 
+	if proto != "h2" {
+		// HTTP/1.1: single-use conn closed once the body is drained.
+		transport := &http.Transport{
+			DialTLSContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+				return tlsConn, nil
+			},
+			DisableKeepAlives: true,
+		}
+		return transport.RoundTrip(req)
+	}
+
+	cc, err := t.h2.NewClientConn(tlsConn)
+	if err != nil {
+		tlsConn.Close()
+		return nil, err
+	}
+	cc = t.storeConn(addr, cc)
+	return cc.RoundTrip(req)
+}
+
+// dial opens a TCP connection and completes the Chrome-fingerprint TLS handshake,
+// returning the connection and negotiated ALPN protocol.
+func (t *utlsTransport) dial(ctx context.Context, host, addr string) (*utls.UConn, string, error) {
+	conn, err := t.dialer.DialContext(ctx, "tcp", addr)
+	if err != nil {
+		return nil, "", err
+	}
+
 	opts := GetNetworkCompatibilityOptions()
 	tlsConn := utls.UClient(conn, &utls.Config{
 		RootCAs:            supplementalRootCAs(),
 		InsecureSkipVerify: opts.InsecureTLS,
 		ServerName:         host,
 		NextProtos:         []string{"h2", "http/1.1"},
+		ClientSessionCache: utlsSessionCache,
 	}, utls.HelloChrome_Auto)
 
 	if err := tlsConn.Handshake(); err != nil {
 		conn.Close()
-		return nil, err
+		return nil, "", err
 	}
+	return tlsConn, tlsConn.ConnectionState().NegotiatedProtocol, nil
+}
 
-	negotiatedProto := tlsConn.ConnectionState().NegotiatedProtocol
-
-	if negotiatedProto == "h2" {
-		h2Transport := &http2.Transport{
-			DialTLSContext: func(ctx context.Context, network, addr string, cfg *tls.Config) (net.Conn, error) {
-				return tlsConn, nil
-			},
-			AllowHTTP:          false,
-			DisableCompression: false,
-		}
-		return h2Transport.RoundTrip(req)
+func (t *utlsTransport) cachedConn(addr string) *http2.ClientConn {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if cc := t.conns[addr]; cc != nil && cc.CanTakeNewRequest() {
+		return cc
 	}
+	return nil
+}
 
-	transport := &http.Transport{
-		DialTLSContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
-			return tlsConn, nil
-		},
-		DisableKeepAlives: true,
+func (t *utlsTransport) invalidate(addr string, cc *http2.ClientConn) {
+	t.mu.Lock()
+	if t.conns[addr] == cc {
+		delete(t.conns, addr)
 	}
+	t.mu.Unlock()
+}
 
-	return transport.RoundTrip(req)
+// storeConn caches cc, but if a concurrent dial already cached a healthy conn for
+// addr it discards the freshly built cc (no in-flight requests) and returns the
+// existing one, avoiding a leaked connection.
+func (t *utlsTransport) storeConn(addr string, cc *http2.ClientConn) *http2.ClientConn {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if existing := t.conns[addr]; existing != nil && existing.CanTakeNewRequest() {
+		cc.Close()
+		return existing
+	}
+	t.conns[addr] = cc
+	return cc
 }
 
 func (t *utlsTransport) getPort(u *url.URL) string {

go_backend/songlink.go

@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"strings"
 	"sync"
+	"time"
 )
 
 type SongLinkClient struct {
@@ -294,18 +295,111 @@ func (s *SongLinkClient) doSongLinkRequest(apiURL string) (map[string]songLinkPl
 	return songLinkResp.LinksByPlatform, nil
 }
 
+const (
+	trackAvailabilityCacheTTL    = 30 * time.Minute
+	trackAvailabilityNegCacheTTL = 5 * time.Minute
+	trackAvailabilityCacheMax    = 500
+)
+
+type trackAvailabilityCacheEntry struct {
+	availability *TrackAvailability
+	err          bool
+	expiresAt    time.Time
+}
+
+var (
+	trackAvailabilityCacheMu sync.Mutex
+	trackAvailabilityCache   = map[string]trackAvailabilityCacheEntry{}
+)
+
+// CheckTrackAvailability resolves platform availability for a track. Results are
+// cached in memory (keyed by region + spotifyID/ISRC) to spare the song.link
+// path its 9 req/min budget. This is an extra layer beneath the Dart-side
+// cached-invoke and is safe: lookups are idempotent. Negative results use a
+// shorter TTL so transient failures recover quickly.
 func (s *SongLinkClient) CheckTrackAvailability(spotifyTrackID string, isrc string) (*TrackAvailability, error) {
 	spotifyTrackID = strings.TrimSpace(spotifyTrackID)
 	isrc = strings.ToUpper(strings.TrimSpace(isrc))
 
+	var idKey string
 	switch {
 	case spotifyTrackID != "":
-		return s.checkTrackAvailabilityFromSpotify(spotifyTrackID)
+		idKey = "spotify:" + spotifyTrackID
 	case isrc != "":
-		return s.checkTrackAvailabilityFromISRC(isrc)
+		idKey = "isrc:" + isrc
 	default:
 		return nil, fmt.Errorf("spotify track ID and ISRC are empty")
 	}
+	key := GetSongLinkRegion() + "|" + idKey
+
+	if cached, hit, cachedErr := trackAvailabilityCacheLookup(key); hit {
+		if cachedErr {
+			return nil, fmt.Errorf("track availability unavailable (cached)")
+		}
+		return cloneTrackAvailability(cached), nil
+	}
+
+	var availability *TrackAvailability
+	var err error
+	switch {
+	case spotifyTrackID != "":
+		availability, err = s.checkTrackAvailabilityFromSpotify(spotifyTrackID)
+	default:
+		availability, err = s.checkTrackAvailabilityFromISRC(isrc)
+	}
+
+	trackAvailabilityCacheStore(key, availability, err)
+	if err != nil {
+		return nil, err
+	}
+	return cloneTrackAvailability(availability), nil
+}
+
+func cloneTrackAvailability(a *TrackAvailability) *TrackAvailability {
+	if a == nil {
+		return nil
+	}
+	c := *a
+	return &c
+}
+
+func trackAvailabilityCacheLookup(key string) (*TrackAvailability, bool, bool) {
+	trackAvailabilityCacheMu.Lock()
+	defer trackAvailabilityCacheMu.Unlock()
+	e, ok := trackAvailabilityCache[key]
+	if !ok {
+		return nil, false, false
+	}
+	if time.Now().After(e.expiresAt) {
+		delete(trackAvailabilityCache, key)
+		return nil, false, false
+	}
+	return e.availability, true, e.err
+}
+
+func trackAvailabilityCacheStore(key string, availability *TrackAvailability, err error) {
+	ttl := trackAvailabilityCacheTTL
+	if err != nil {
+		ttl = trackAvailabilityNegCacheTTL
+	}
+	trackAvailabilityCacheMu.Lock()
+	defer trackAvailabilityCacheMu.Unlock()
+	if _, exists := trackAvailabilityCache[key]; !exists && len(trackAvailabilityCache) >= trackAvailabilityCacheMax {
+		var oldestKey string
+		var oldest time.Time
+		first := true
+		for k, e := range trackAvailabilityCache {
+			if first || e.expiresAt.Before(oldest) {
+				oldest, oldestKey, first = e.expiresAt, k, false
+			}
+		}
+		delete(trackAvailabilityCache, oldestKey)
+	}
+	trackAvailabilityCache[key] = trackAvailabilityCacheEntry{
+		availability: availability,
+		err:          err != nil,
+		expiresAt:    time.Now().Add(ttl),
+	}
 }
 
 func (s *SongLinkClient) checkTrackAvailabilityFromSpotify(spotifyTrackID string) (*TrackAvailability, error) {

go_backend/songlink_test.go

@@ -4,7 +4,9 @@ import (
 	"io"
 	"net/http"
 	"strings"
+	"sync/atomic"
 	"testing"
+	"time"
 )
 
 type roundTripFunc func(*http.Request) (*http.Response, error)
@@ -23,7 +25,14 @@ func TestGetRetryAfterDurationMissingHeaderReturnsZero(t *testing.T) {
 	}
 }
 
+func resetTrackAvailabilityCache() {
+	trackAvailabilityCacheMu.Lock()
+	trackAvailabilityCache = map[string]trackAvailabilityCacheEntry{}
+	trackAvailabilityCacheMu.Unlock()
+}
+
 func TestCheckTrackAvailabilityFromSpotifyViaResolveAPI(t *testing.T) {
+	resetTrackAvailabilityCache()
 	origRetryConfig := songLinkRetryConfig
 	defer func() { songLinkRetryConfig = origRetryConfig }()
 
@@ -65,6 +74,7 @@ func TestCheckTrackAvailabilityFromSpotifyViaResolveAPI(t *testing.T) {
 }
 
 func TestCheckTrackAvailabilityFromSpotifyResolveAPIFailure(t *testing.T) {
+	resetTrackAvailabilityCache()
 	origRetryConfig := songLinkRetryConfig
 	songLinkRetryConfig = func() RetryConfig {
 		return RetryConfig{MaxRetries: 0, InitialDelay: 0, MaxDelay: 0, BackoffFactor: 1}
@@ -115,6 +125,7 @@ func TestCheckTrackAvailabilityFromSpotifyResolveAPIFailure(t *testing.T) {
 }
 
 func TestCheckTrackAvailabilityFromSpotifyViaResolveAPIMixedSongURLShapes(t *testing.T) {
+	resetTrackAvailabilityCache()
 	origRetryConfig := songLinkRetryConfig
 	defer func() { songLinkRetryConfig = origRetryConfig }()
 
@@ -155,6 +166,61 @@ func TestCheckTrackAvailabilityFromSpotifyViaResolveAPIMixedSongURLShapes(t *tes
 	}
 }
 
+func TestCheckTrackAvailabilityCachesResult(t *testing.T) {
+	resetTrackAvailabilityCache()
+	origRetryConfig := songLinkRetryConfig
+	defer func() { songLinkRetryConfig = origRetryConfig }()
+
+	var calls int32
+	client := &SongLinkClient{
+		client: &http.Client{
+			Transport: roundTripFunc(func(req *http.Request) (*http.Response, error) {
+				atomic.AddInt32(&calls, 1)
+				body := `{"success":true,"songUrls":{"Spotify":"https://open.spotify.com/track/cachedid","Deezer":"https://www.deezer.com/track/111"}}`
+				return &http.Response{
+					StatusCode: 200,
+					Header:     make(http.Header),
+					Body:       io.NopCloser(strings.NewReader(body)),
+					Request:    req,
+				}, nil
+			}),
+		},
+	}
+
+	first, err := client.CheckTrackAvailability("cachedid", "")
+	if err != nil {
+		t.Fatalf("first CheckTrackAvailability() error = %v", err)
+	}
+	second, err := client.CheckTrackAvailability("cachedid", "")
+	if err != nil {
+		t.Fatalf("second CheckTrackAvailability() error = %v", err)
+	}
+
+	if got := atomic.LoadInt32(&calls); got != 1 {
+		t.Fatalf("expected 1 network call with caching, got %d", got)
+	}
+	if first == second {
+		t.Fatal("expected cache to return a distinct clone, got same pointer")
+	}
+	if second.DeezerID != "111" {
+		t.Fatalf("cached DeezerID = %q, want 111", second.DeezerID)
+	}
+}
+
+func TestCheckTrackAvailabilityNegativeCacheTTL(t *testing.T) {
+	resetTrackAvailabilityCache()
+
+	entry := trackAvailabilityCacheEntry{err: true, expiresAt: time.Now().Add(-time.Second)}
+	key := GetSongLinkRegion() + "|spotify:expiredneg"
+	trackAvailabilityCacheMu.Lock()
+	trackAvailabilityCache[key] = entry
+	trackAvailabilityCacheMu.Unlock()
+
+	if _, hit, _ := trackAvailabilityCacheLookup(key); hit {
+		t.Fatal("expired negative entry should not be a cache hit")
+	}
+}
+
 func TestCheckAvailabilityFromDeezerUsesSongLink(t *testing.T) {
 	origRetryConfig := songLinkRetryConfig
 	songLinkRetryConfig = func() RetryConfig {

lib/providers/download_queue_provider.dart

@@ -196,6 +196,9 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
   final Set<String> _ensuredDirs = {};
   int _idleProgressPollTick = 0;
   bool _networkPausedByWifiOnly = false;
+  List<ConnectivityResult>? _lastConnectivityResults;
+  DateTime _lastConnectionCleanupAt = DateTime.fromMillisecondsSinceEpoch(0);
+  static const _connectionCleanupDebounce = Duration(seconds: 2);
   String? _lastServiceTrackName;
   String? _lastServiceArtistName;
   String? _lastServiceStatus;
@@ -1829,18 +1832,59 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
     }
 
     final shouldResume = _networkPausedByWifiOnly && state.isPaused;
-    _stopConnectivityMonitoring();
+    // Keep monitoring active in every mode while downloading so idle
+    // connections are still recycled when the network switches.
+    if (state.isProcessing) {
+      _networkPausedByWifiOnly = false;
+      _startConnectivityMonitoring();
+    } else {
+      _stopConnectivityMonitoring();
+    }
     if (shouldResume) {
       resumeQueue();
     }
   }
 
-  void _handleConnectivityResults(List<ConnectivityResult> results) {
-    final settings = ref.read(settingsProvider);
-    if (settings.downloadNetworkMode != 'wifi_only') {
-      _handleDownloadNetworkModeChanged(settings.downloadNetworkMode);
+  /// Closes idle backend HTTP connections when the connectivity set changes
+  /// (e.g. WiFi <-> cellular). Stale sockets bound to the old interface would
+  /// otherwise be reused, stalling the first request after a network switch.
+  /// Applies to every download network mode. Fire-and-forget with a light
+  /// debounce so network flapping does not spam the bridge.
+  void _maybeCleanupOnNetworkChange(List<ConnectivityResult> results) {
+    final previous = _lastConnectivityResults;
+    final unchanged =
+        previous != null && _connectivitySetEquals(previous, results);
+    _lastConnectivityResults = List<ConnectivityResult>.unmodifiable(results);
+    if (previous == null || unchanged) return;
+
+    final now = DateTime.now();
+    if (now.difference(_lastConnectionCleanupAt) < _connectionCleanupDebounce) {
       return;
     }
+    _lastConnectionCleanupAt = now;
+
+    _log.i('Network changed, closing idle backend connections');
+    unawaited(
+      PlatformBridge.cleanupConnections().catchError((Object e) {
+        _log.w('Failed to clean up connections after network change: $e');
+      }),
+    );
+  }
+
+  bool _connectivitySetEquals(
+    List<ConnectivityResult> a,
+    List<ConnectivityResult> b,
+  ) {
+    final setA = a.toSet();
+    final setB = b.toSet();
+    return setA.length == setB.length && setA.containsAll(setB);
+  }
+
+  void _handleConnectivityResults(List<ConnectivityResult> results) {
+    _maybeCleanupOnNetworkChange(results);
+
+    final settings = ref.read(settingsProvider);
+    if (settings.downloadNetworkMode != 'wifi_only') return;
 
     if (_hasWifiConnection(results)) {
       if (_networkPausedByWifiOnly && state.isPaused) {
@@ -1921,11 +1965,11 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
         state = state.copyWith(isProcessing: false, isPaused: true);
         return;
       }
-      _networkPausedByWifiOnly = false;
-      _startConnectivityMonitoring();
-    } else {
-      _stopConnectivityMonitoring();
     }
+    // Monitor in every mode so idle connections are recycled on a network
+    // switch, even though only wifi_only pauses the queue.
+    _networkPausedByWifiOnly = false;
+    _startConnectivityMonitoring();
 
     if (await _tryProcessQueueWithAndroidNativeWorker(settings)) {
       return;