Cache music as mp3 files on startup

src/lib/server/music.ts

@@ -9,7 +9,7 @@ const mp3Bitrate = '160k';
 
 const youtube = new Ytdl();
 
-const playlistVideoIds = playlist.videos.flatMap((video) => {
+export const playlistVideoIds = playlist.videos.flatMap((video) => {
 	const videoId = parseVideoId(video);
 	return videoId ? [videoId] : [];
 });
@@ -30,7 +30,8 @@ async function getTrack(videoId: string): Promise<Track> {
 		title: videoDetails.title,
 		artist: videoDetails.author.name,
 		durationSeconds: videoDetails.lengthSeconds,
-		cover: `https://i.ytimg.com/vi/${videoId}/hqdefault.jpg`
+		cover: `https://i.ytimg.com/vi/${videoId}/hqdefault.jpg`,
+		cached: false
 	};
 	trackCache.set(videoId, track);
 	return track;
@@ -45,7 +46,13 @@ export async function getPlaylistTracks(): Promise<Track[]> {
 	});
 }
 
-export function streamTrackAsMp3(videoId: string, startSeconds: number): ReadableStream<Uint8Array> {
+export interface Transcode {
+	output: Readable;
+	finished: Promise<void>;
+	stop: () => void;
+}
+
+export function startTranscode(videoId: string, startSeconds: number): Transcode {
 	const downloadController = new AbortController();
 	const youtubeAudio = youtube.download(videoId, {
 		quality: Quality.HighestAudio,
@@ -62,17 +69,28 @@ export function streamTrackAsMp3(videoId: string, startSeconds: number): Readabl
 		if (ffmpeg.exitCode === null) ffmpeg.kill('SIGKILL');
 	};
 
-	youtubeAudio.on('error', (error) => {
-		console.error(`Music download failed for ${videoId}:`, error);
-		stop();
-	});
-	ffmpeg.on('error', (error) => {
-		console.error('ffmpeg failed to start:', error);
-		stop();
+	const finished = new Promise<void>((resolve, reject) => {
+		youtubeAudio.on('error', reject);
+		ffmpeg.on('error', reject);
+		ffmpeg.on('close', (exitCode) => (exitCode === 0 ? resolve() : reject(new Error(`ffmpeg exited with code ${exitCode}`))));
 	});
+	finished.catch(stop);
+
 	ffmpeg.stdin.on('error', stop);
-	ffmpeg.stdout.on('close', stop);
 	youtubeAudio.pipe(ffmpeg.stdin);
 
-	return Readable.toWeb(ffmpeg.stdout) as ReadableStream<Uint8Array>;
+	return { output: ffmpeg.stdout, finished, stop };
+}
+
+export function streamTrackAsMp3(videoId: string, startSeconds: number): ReadableStream<Uint8Array> {
+	const transcode = startTranscode(videoId, startSeconds);
+	let listenerLeft = false;
+	transcode.output.on('close', () => {
+		listenerLeft = true;
+		transcode.stop();
+	});
+	transcode.finished.catch((error) => {
+		if (!listenerLeft) console.error(`Music stream failed for ${videoId}:`, error.message);
+	});
+	return Readable.toWeb(transcode.output) as ReadableStream<Uint8Array>;
 }

src/lib/server/musicCache.ts

@@ -0,0 +1,100 @@
+import { createReadStream, createWriteStream } from 'node:fs';
+import { mkdir, readdir, rename, rm, stat } from 'node:fs/promises';
+import path from 'node:path';
+import { Readable } from 'node:stream';
+import { pipeline } from 'node:stream/promises';
+import { playlistVideoIds, startTranscode } from './music';
+
+const cacheDirectory = path.resolve(process.env.MUSIC_CACHE_DIR ?? '.cache/music');
+const songExtension = '.mp3';
+const partialExtension = '.part';
+const rangePattern = /^bytes=(\d*)-(\d*)$/;
+
+const readySongs = new Set<string>();
+let refreshStarted = false;
+
+function songPath(videoId: string): string {
+	return path.join(cacheDirectory, `${videoId}${songExtension}`);
+}
+
+export function isSongCached(videoId: string): boolean {
+	return readySongs.has(videoId);
+}
+
+async function prepareCacheDirectory() {
+	await mkdir(cacheDirectory, { recursive: true });
+	const playlistIds = new Set(playlistVideoIds);
+
+	for (const fileName of await readdir(cacheDirectory)) {
+		const videoId = fileName.slice(0, -songExtension.length);
+		const isKeptSong = fileName.endsWith(songExtension) && playlistIds.has(videoId);
+		if (isKeptSong) readySongs.add(videoId);
+		else await rm(path.join(cacheDirectory, fileName), { force: true });
+	}
+}
+
+async function downloadSong(videoId: string) {
+	const partialPath = `${songPath(videoId)}${partialExtension}`;
+	const transcode = startTranscode(videoId, 0);
+
+	try {
+		await Promise.all([pipeline(transcode.output, createWriteStream(partialPath)), transcode.finished]);
+		const { size } = await stat(partialPath);
+		if (size === 0) throw new Error('ffmpeg produced an empty file');
+		await rename(partialPath, songPath(videoId));
+		readySongs.add(videoId);
+	} catch (error) {
+		transcode.stop();
+		await rm(partialPath, { force: true });
+		throw error;
+	}
+}
+
+export function refreshMusicCache() {
+	if (refreshStarted) return;
+	refreshStarted = true;
+
+	(async () => {
+		await prepareCacheDirectory();
+		for (const videoId of playlistVideoIds) {
+			try {
+				await downloadSong(videoId);
+				console.log(`Music cached: ${videoId}`);
+			} catch (error) {
+				console.error(`Music cache failed for ${videoId}:`, error instanceof Error ? error.message : error);
+			}
+		}
+	})().catch((error) => console.error('Music cache refresh failed:', error));
+}
+
+function parseRange(rangeHeader: string | null, fileSize: number): { start: number; end: number } | null {
+	const match = rangeHeader?.match(rangePattern);
+	if (!match) return { start: 0, end: fileSize - 1 };
+
+	const [, startText, endText] = match;
+	const start = startText ? Number(startText) : Math.max(0, fileSize - Number(endText));
+	const end = startText && endText ? Math.min(Number(endText), fileSize - 1) : fileSize - 1;
+	return start <= end && start < fileSize ? { start, end } : null;
+}
+
+export async function serveCachedSong(videoId: string, rangeHeader: string | null): Promise<Response> {
+	const filePath = songPath(videoId);
+	const { size } = await stat(filePath);
+	const range = parseRange(rangeHeader, size);
+
+	if (!range) {
+		return new Response(null, { status: 416, headers: { 'Content-Range': `bytes */${size}` } });
+	}
+
+	const fileStream = createReadStream(filePath, { start: range.start, end: range.end });
+	return new Response(Readable.toWeb(fileStream) as ReadableStream<Uint8Array>, {
+		status: rangeHeader ? 206 : 200,
+		headers: {
+			'Content-Type': 'audio/mpeg',
+			'Content-Length': `${range.end - range.start + 1}`,
+			'Content-Range': `bytes ${range.start}-${range.end}/${size}`,
+			'Accept-Ranges': 'bytes',
+			'Cache-Control': 'public, max-age=3600'
+		}
+	});
+}