fix(ytmusic): preserve playlist batch boundaries

external/ytmusic/cookie_provider.go

@@ -31,7 +31,7 @@ const (
 type cookieBase struct {
 	browser    string
 	fetchFn    func(browser string) ([]playlist.PlaylistInfo, error)
-	resolveFn  func(pageURL string, start, count int, browser ...string) ([]playlist.Track, error)
+	resolveFn  func(pageURL string, start, count int, browser ...string) ([]playlist.Track, int, error)
 	mu         sync.Mutex
 	playlists  []playlist.PlaylistInfo
 	trackCache map[string][]playlist.Track
@@ -86,19 +86,19 @@ func (b *cookieBase) fetchTracks(target string) ([]playlist.Track, error) {
 
 	resolveBatch := b.resolveFn
 	if resolveBatch == nil {
-		resolveBatch = resolve.ResolveYTDLBatch
+		resolveBatch = resolve.ResolveYTDLBatchPage
 	}
 	var tracks []playlist.Track
 	for start := 0; ; {
-		batch, err := resolveBatch(target, start, cookiePlaylistBatchSize, b.browser)
+		batch, entries, err := resolveBatch(target, start, cookiePlaylistBatchSize, b.browser)
 		if err != nil {
 			return nil, fmt.Errorf("ytmusic: resolve playlist tracks: %w", err)
 		}
 		tracks = append(tracks, batch...)
-		if len(batch) < cookiePlaylistBatchSize {
+		if entries < cookiePlaylistBatchSize {
 			break
 		}
-		start += len(batch)
+		start += cookiePlaylistBatchSize
 	}
 
 	b.mu.Lock()

external/ytmusic/cookie_provider_test.go

@@ -255,7 +255,7 @@ func TestCookieProviderTracksLoadsInBatches(t *testing.T) {
 	t.Setenv("HOME", t.TempDir())
 	base := newCookieBase("firefox")
 	var starts []int
-	base.resolveFn = func(_ string, start, count int, browser ...string) ([]playlist.Track, error) {
+	base.resolveFn = func(_ string, start, count int, browser ...string) ([]playlist.Track, int, error) {
 		starts = append(starts, start)
 		if count != cookiePlaylistBatchSize {
 			t.Fatalf("count = %d, want %d", count, cookiePlaylistBatchSize)
@@ -266,20 +266,26 @@ func TestCookieProviderTracksLoadsInBatches(t *testing.T) {
 		n := cookiePlaylistBatchSize
 		if start > 0 {
 			n = 20
+		} else {
+			n-- // One malformed source entry was omitted from the parsed tracks.
 		}
 		tracks := make([]playlist.Track, n)
 		for i := range tracks {
 			tracks[i].Title = fmt.Sprintf("Track %d", start+i)
 		}
-		return tracks, nil
+		entries := n
+		if start == 0 {
+			entries = cookiePlaylistBatchSize
+		}
+		return tracks, entries, nil
 	}
 
 	tracks, err := (&CookieProvider{base: base, kind: KindMusic}).Tracks("PL123")
 	if err != nil {
 		t.Fatalf("Tracks() error: %v", err)
 	}
-	if len(tracks) != 120 {
-		t.Fatalf("tracks = %d, want 120", len(tracks))
+	if len(tracks) != 119 {
+		t.Fatalf("tracks = %d, want 119", len(tracks))
 	}
 	if !slices.Equal(starts, []int{0, cookiePlaylistBatchSize}) {
 		t.Fatalf("batch starts = %v, want [0 %d]", starts, cookiePlaylistBatchSize)

resolve/resolve.go

@@ -616,6 +616,18 @@ func ResolveYTDLBatch(pageURL string, start, count int, browser ...string) ([]pl
 	return ResolveYTDLBatchContext(ctx, pageURL, start, count, browser...)
 }
 
+// ResolveYTDLBatchPage returns the valid tracks and number of source entries
+// emitted by yt-dlp for a playlist range.
+func ResolveYTDLBatchPage(pageURL string, start, count int, browser ...string) ([]playlist.Track, int, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	end := 0
+	if count > 0 {
+		end = start + count
+	}
+	return resolveYTDLRangePageContext(ctx, pageURL, start, end, browser...)
+}
+
 // ResolveYTDLBatchContext is ResolveYTDLBatch with caller-controlled
 // cancellation and timeout.
 func ResolveYTDLBatchContext(ctx context.Context, pageURL string, start, count int, browser ...string) ([]playlist.Track, error) {
@@ -644,8 +656,13 @@ func resolveYTDLRange(pageURL string, start, end int, browser ...string) ([]play
 }
 
 func resolveYTDLRangeContext(ctx context.Context, pageURL string, start, end int, browser ...string) ([]playlist.Track, error) {
+	tracks, _, err := resolveYTDLRangePageContext(ctx, pageURL, start, end, browser...)
+	return tracks, err
+}
+
+func resolveYTDLRangePageContext(ctx context.Context, pageURL string, start, end int, browser ...string) ([]playlist.Track, int, error) {
 	if _, err := exec.LookPath("yt-dlp"); err != nil {
-		return nil, fmt.Errorf("yt-dlp not found in PATH — see https://github.com/yt-dlp/yt-dlp#installation")
+		return nil, 0, fmt.Errorf("yt-dlp not found in PATH — see https://github.com/yt-dlp/yt-dlp#installation")
 	}
 
 	args := []string{"--flat-playlist", "-j", "--socket-timeout", "15"}
@@ -673,17 +690,21 @@ func resolveYTDLRangeContext(ctx context.Context, pageURL string, start, end int
 	stdout, err := cmd.Output()
 	if err != nil {
 		if ctxErr := ctx.Err(); ctxErr != nil {
-			return nil, fmt.Errorf("yt-dlp: resolve %s: %w", pageURL, ctxErr)
+			return nil, 0, fmt.Errorf("yt-dlp: resolve %s: %w", pageURL, ctxErr)
 		}
 		msg := strings.TrimSpace(stderr.String())
 		if msg != "" {
-			return nil, fmt.Errorf("yt-dlp: %s", msg)
+			return nil, 0, fmt.Errorf("yt-dlp: %s", msg)
 		}
-		return nil, fmt.Errorf("yt-dlp: %w", err)
+		return nil, 0, fmt.Errorf("yt-dlp: %w", err)
 	}
+	return parseYTDLTracks(bytes.NewReader(stdout))
+}
 
+func parseYTDLTracks(r io.Reader) ([]playlist.Track, int, error) {
 	var tracks []playlist.Track
-	scanner := bufio.NewScanner(bytes.NewReader(stdout))
+	entries := 0
+	scanner := bufio.NewScanner(r)
 	// yt-dlp JSON can exceed bufio.Scanner's default 64KB token limit
 	// (e.g. videos with very long descriptions).
 	scanner.Buffer(make([]byte, 0, scannerInitBufSize), scannerMaxLineSize)
@@ -692,6 +713,7 @@ func resolveYTDLRangeContext(ctx context.Context, pageURL string, start, end int
 		if line == "" {
 			continue
 		}
+		entries++
 		var e ytdlFlatEntry
 		if err := json.Unmarshal([]byte(line), &e); err != nil {
 			continue
@@ -722,7 +744,7 @@ func resolveYTDLRangeContext(ctx context.Context, pageURL string, start, end int
 			DurationSecs: int(e.Duration),
 		})
 	}
-	return tracks, scanner.Err()
+	return tracks, entries, scanner.Err()
 }
 
 // DownloadYTDL downloads a single track via yt-dlp to the given directory

resolve/resolve_test.go

@@ -311,3 +311,22 @@ func TestResolveYTDLBatchCookieSelection(t *testing.T) {
 		t.Errorf("did not expect fallback cookies 'firefox' in args, got: %s", string(logged))
 	}
 }
+
+func TestParseYTDLTracksCountsMalformedEntries(t *testing.T) {
+	input := strings.Join([]string{
+		`{"webpage_url":"https://example.com/one","title":"One"}`,
+		`{malformed}`,
+		`{"title":"Missing URL"}`,
+	}, "\n")
+
+	tracks, entries, err := parseYTDLTracks(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("parseYTDLTracks() error: %v", err)
+	}
+	if entries != 3 {
+		t.Fatalf("source entries = %d, want 3", entries)
+	}
+	if len(tracks) != 1 || tracks[0].Title != "One" {
+		t.Fatalf("tracks = %+v, want one valid track", tracks)
+	}
+}