fix: preserve fresh WEB subtitle tracks

Author: Priveetee

src/subtitle-content.ts

@@ -32,24 +32,44 @@ export async function fetchSubtitleContent(
 	fetchYoutube: typeof youtubeFetch = youtubeFetch,
 ): Promise<Uint8Array> {
 	const request = parseSubtitleRequest(rawUrl);
+	try {
+		return await fetchVttContent(
+			refreshedVttUrl(rawUrl, request.translatedLanguageTag),
+			fetchYoutube,
+		);
+	} catch (error) {
+		if (error instanceof SubtitleFetchError && error.code === "subtitle_upstream_throttled") {
+			throw error;
+		}
+	}
+
 	const { visitorData, poToken } = await fetchPoToken(request.videoId);
 	const tracks = await fetchCaptionTracks(request.videoId, visitorData, poToken);
 	const track = selectTrack(tracks, request);
 	if (!track?.baseUrl) {
 		throw new SubtitleFetchError("Subtitle track not found", "subtitle_track_not_found", 404);
 	}
-	const response = await fetchYoutube(
+	return fetchVttContent(
 		refreshedVttUrl(track.baseUrl, request.translatedLanguageTag),
-		{
-			headers: {
-				accept: "text/vtt,*/*;q=0.8",
-				origin: "https://m.youtube.com",
-				referer: "https://m.youtube.com/",
-				"user-agent": MWEB_USER_AGENT,
-				"x-goog-visitor-id": visitorData,
-			},
-		},
+		fetchYoutube,
+		visitorData,
 	);
+}
+
+async function fetchVttContent(
+	url: URL,
+	fetchYoutube: typeof youtubeFetch,
+	visitorData?: string,
+): Promise<Uint8Array> {
+	const response = await fetchYoutube(url, {
+		headers: {
+			accept: "text/vtt,*/*;q=0.8",
+			origin: "https://m.youtube.com",
+			referer: "https://m.youtube.com/",
+			"user-agent": MWEB_USER_AGENT,
+			...(visitorData ? { "x-goog-visitor-id": visitorData } : {}),
+		},
+	});
 	if (response.status === 429) {
 		throw new SubtitleFetchError(
 			"YouTube temporarily throttled subtitle retrieval",

tests/subtitles.test.ts

@@ -114,7 +114,8 @@ describe("fetchSubtitles", () => {
 				vssId: ".fr",
 			},
 		]);
-		mockYoutubeFetch.mockImplementation(
+		mockYoutubeFetch.mockImplementationOnce(async () => new Response("", { status: 403 }));
+		mockYoutubeFetch.mockImplementationOnce(
 			async () => new Response("WEBVTT\n\n00:00.000 --> 00:01.000\nBonjour"),
 		);
 		const { fetchSubtitleContent } = await import("../src/subtitle-content.ts");
@@ -126,6 +127,7 @@ describe("fetchSubtitles", () => {
 		const requestHeaders = new Headers(mockYoutubeFetch.mock.calls.at(-1)?.[1]?.headers);
 
 		expect(new TextDecoder().decode(bytes).startsWith("WEBVTT")).toBe(true);
+		expect(mockYoutubeFetch).toHaveBeenCalledTimes(2);
 		expect(requested.searchParams.get("expire")).toBe("999");
 		expect(requested.searchParams.get("fmt")).toBe("vtt");
 		expect(requested.searchParams.get("tlang")).toBe("es");
@@ -139,7 +141,8 @@ describe("fetchSubtitles", () => {
 			{ ...frTrack, languageCode: "en", vssId: ".en" },
 			asrTrack,
 		]);
-		mockYoutubeFetch.mockImplementation(async () => new Response("WEBVTT\n\nAutomatic"));
+		mockYoutubeFetch.mockImplementationOnce(async () => new Response("", { status: 403 }));
+		mockYoutubeFetch.mockImplementationOnce(async () => new Response("WEBVTT\n\nAutomatic"));
 		const { fetchSubtitleContent } = await import("../src/subtitle-content.ts");
 		await fetchSubtitleContent(
 			"https://www.youtube.com/api/timedtext?v=video&lang=en&kind=asr&expire=1",
@@ -151,6 +154,26 @@ describe("fetchSubtitles", () => {
 		expect(requested.searchParams.get("expire")).toBe("999");
 	});
 
+	it("uses a current WEB track without replacing it through MWEB", async () => {
+		mockFetchCaptionTracks.mockClear();
+		mockYoutubeFetch.mockClear();
+		mockYoutubeFetch.mockImplementationOnce(
+			async () => new Response("WEBVTT\n\n00:00.000 --> 00:01.000\nCurrent"),
+		);
+		const { fetchSubtitleContent } = await import("../src/subtitle-content.ts");
+		const bytes = await fetchSubtitleContent(
+			"https://www.youtube.com/api/timedtext?v=video&lang=en&expire=999&fmt=ttml",
+			mockYoutubeFetch,
+		);
+		const requested = new URL(String(mockYoutubeFetch.mock.calls[0]?.[0]));
+
+		expect(new TextDecoder().decode(bytes).startsWith("WEBVTT")).toBe(true);
+		expect(requested.hostname).toBe("www.youtube.com");
+		expect(requested.searchParams.get("fmt")).toBe("vtt");
+		expect(mockFetchCaptionTracks).not.toHaveBeenCalled();
+		expect(mockYoutubeFetch).toHaveBeenCalledTimes(1);
+	});
+
 	it("returns a typed throttling failure without retrying", async () => {
 		mockFetchCaptionTracks.mockImplementation(async () => [asrTrack]);
 		mockYoutubeFetch.mockClear();