fix(anime): strip disguised HLS segments and report real playback errors

Some hosts prepend a fake image header (a real 1x1 PNG) to every HLS
segment; ffmpeg probes the segment as a picture and mpv drops back to
idle with no window while the browser claims the episode is playing.

- Route bridge-served m3u8 streams through a local strip proxy that
  scans each segment for the first genuine MPEG-TS packet run and drops
  the junk in front of it; playlists get absolute origins rewritten so
  segment requests come back through the proxy. Non-TS bodies pass
  through untouched.
- Only report ok from playEpisode once mpv configures a video output;
  an end-file with reason error surfaces mpv's own message (e.g. "no
  audio or video data played") in the browser status bar instead of
  "Playing". New end-file event plumbed through the mpv IPC client.

src/anime-bridge/media-url.test.ts

@@ -1,8 +1,34 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
-import { parseAnimeStatus, resolveBridgeMediaUrl } from './media-url';
+import { parseAnimeStatus, resolveBridgeMediaUrl, routeHlsThroughProxy } from './media-url';
 
 const BRIDGE = 'http://127.0.0.1:56037';
+const PROXY = 'http://127.0.0.1:60001';
+
+test('a bridge m3u8 stream is routed through the strip proxy', () => {
+  assert.equal(
+    routeHlsThroughProxy(`${BRIDGE}/video/master.m3u8?q=1080`, BRIDGE, PROXY),
+    `${PROXY}/video/master.m3u8?q=1080`,
+  );
+});
+
+test('non-HLS bridge streams stay on the bridge', () => {
+  const direct = `${BRIDGE}/video/movie-token`;
+  assert.equal(routeHlsThroughProxy(direct, BRIDGE, PROXY), direct);
+});
+
+test('external m3u8 streams are not routed through the proxy', () => {
+  const remote = 'https://cdn.example.com/hls/master.m3u8';
+  assert.equal(routeHlsThroughProxy(remote, BRIDGE, PROXY), remote);
+});
+
+test('unparseable stream urls pass through routeHlsThroughProxy unchanged', () => {
+  assert.equal(routeHlsThroughProxy('not a url', BRIDGE, PROXY), 'not a url');
+  assert.equal(
+    routeHlsThroughProxy(`${BRIDGE}/video/a.m3u8`, 'garbage', PROXY),
+    `${BRIDGE}/video/a.m3u8`,
+  );
+});
 
 test('a loopback proxy url is rebased onto the live bridge port', () => {
   assert.equal(

src/anime-bridge/media-url.ts

@@ -47,6 +47,30 @@ export function resolveBridgeMediaUrl(bridgeBaseUrl: string, mediaUrl: string):
   return rebased.toString();
 }
 
+/**
+ * Send a bridge-served HLS stream through the local strip proxy instead. Only
+ * `.m3u8` URLs on the bridge origin qualify: direct files need no fixing, and
+ * an external URL would not resolve through a proxy that forwards to the
+ * bridge. Anything unparseable comes back unchanged.
+ */
+export function routeHlsThroughProxy(
+  streamUrl: string,
+  bridgeBaseUrl: string,
+  proxyOrigin: string,
+): string {
+  let stream: URL;
+  let bridge: URL;
+  try {
+    stream = new URL(streamUrl);
+    bridge = new URL(bridgeBaseUrl);
+  } catch {
+    return streamUrl;
+  }
+  if (stream.origin !== bridge.origin) return streamUrl;
+  if (!stream.pathname.endsWith('.m3u8')) return streamUrl;
+  return `${proxyOrigin}${stream.pathname}${stream.search}`;
+}
+
 /** Aniyomi's SAnime status constants. */
 export type AnimeStatus =
   | 'unknown'

src/anime-bridge/playback-outcome.test.ts

@@ -0,0 +1,93 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { watchPlaybackOutcome, type PlaybackEndFileEvent } from './playback-outcome';
+
+type Harness = {
+  emitEndFile: (event: PlaybackEndFileEvent) => void;
+  listenerCount: () => number;
+  setProperty: (name: string, value: unknown) => void;
+  failProperty: (name: string) => void;
+};
+
+function createHarness(overrides?: { timeoutMs?: number; probeIntervalMs?: number }) {
+  const listeners = new Set<(event: PlaybackEndFileEvent) => void>();
+  const properties = new Map<string, unknown>();
+  const failing = new Set<string>();
+
+  const watch = watchPlaybackOutcome({
+    onEndFile: (listener) => {
+      listeners.add(listener);
+      return () => listeners.delete(listener);
+    },
+    readProperty: async (name) => {
+      if (failing.has(name)) throw new Error(`Failed to read MPV property '${name}'`);
+      return properties.get(name);
+    },
+    wait: async () => {},
+    timeoutMs: overrides?.timeoutMs ?? 1000,
+    probeIntervalMs: overrides?.probeIntervalMs ?? 100,
+  });
+
+  const harness: Harness = {
+    emitEndFile: (event) => {
+      for (const listener of listeners) listener(event);
+    },
+    listenerCount: () => listeners.size,
+    setProperty: (name, value) => properties.set(name, value),
+    failProperty: (name) => failing.add(name),
+  };
+  return { watch, harness };
+}
+
+test('resolves ok once mpv configures a video output', async () => {
+  const { watch, harness } = createHarness();
+  harness.setProperty('vo-configured', true);
+  const outcome = await watch.wait();
+  assert.deepEqual(outcome, { ok: true });
+  watch.dispose();
+});
+
+test('resolves failure with the mpv error when the file ends in error', async () => {
+  const { watch, harness } = createHarness();
+  const pending = watch.wait();
+  harness.emitEndFile({ reason: 'error', fileError: 'no audio or video data played' });
+  const outcome = await pending;
+  assert.equal(outcome.ok, false);
+  assert.ok(!outcome.ok && outcome.error.includes('no audio or video data played'));
+  watch.dispose();
+});
+
+test('ignores the end-file fired for the file being replaced', async () => {
+  const { watch, harness } = createHarness();
+  const pending = watch.wait();
+  harness.emitEndFile({ reason: 'stop', fileError: null });
+  harness.setProperty('vo-configured', true);
+  const outcome = await pending;
+  assert.deepEqual(outcome, { ok: true });
+  watch.dispose();
+});
+
+test('times out with a failure when nothing ever starts', async () => {
+  const { watch } = createHarness({ timeoutMs: 300, probeIntervalMs: 100 });
+  const outcome = await watch.wait();
+  assert.equal(outcome.ok, false);
+  assert.ok(!outcome.ok && outcome.error.length > 0);
+  watch.dispose();
+});
+
+test('keeps polling through property read failures', async () => {
+  const { watch, harness } = createHarness();
+  harness.failProperty('vo-configured');
+  const pending = watch.wait();
+  harness.emitEndFile({ reason: 'error', fileError: null });
+  const outcome = await pending;
+  assert.equal(outcome.ok, false);
+  watch.dispose();
+});
+
+test('dispose removes the end-file subscription', () => {
+  const { watch, harness } = createHarness();
+  assert.equal(harness.listenerCount(), 1);
+  watch.dispose();
+  assert.equal(harness.listenerCount(), 0);
+});

src/anime-bridge/playback-outcome.ts

@@ -0,0 +1,80 @@
+/**
+ * Confirms that a `loadfile` handed to mpv actually turned into playback.
+ *
+ * Sending the command proves nothing: mpv accepts the file, fails to decode it
+ * (a dead host, a disguised stream the proxy could not fix), fires `end-file`
+ * with reason "error", and drops back to `--idle` — with no window, because
+ * idle mpv shows none. The UI would happily say "Playing" over a blank desktop.
+ *
+ * Success is mpv configuring a video output (`vo-configured`), which is
+ * literally "a window with frames in it". `file-loaded` is not enough — the
+ * broken stream in the wild reached it before dying. Failure is an `end-file`
+ * with reason "error"; the end-file of the file being *replaced* arrives with
+ * "stop"/"redirect" and is ignored.
+ */
+
+export interface PlaybackEndFileEvent {
+  reason: string;
+  fileError: string | null;
+}
+
+export type PlaybackOutcome = { ok: true } | { ok: false; error: string };
+
+export interface WatchPlaybackOutcomeDeps {
+  /** Subscribe to mpv end-file events; returns the unsubscribe. */
+  onEndFile: (listener: (event: PlaybackEndFileEvent) => void) => () => void;
+  /** One-shot mpv property read; may reject while the file is still loading. */
+  readProperty: (name: string) => Promise<unknown>;
+  wait: (ms: number) => Promise<void>;
+  timeoutMs?: number;
+  probeIntervalMs?: number;
+}
+
+export interface PlaybackOutcomeWatch {
+  wait: () => Promise<PlaybackOutcome>;
+  dispose: () => void;
+}
+
+export const DEFAULT_PLAYBACK_OUTCOME_TIMEOUT_MS = 20_000;
+const DEFAULT_PROBE_INTERVAL_MS = 500;
+
+/**
+ * Call *before* sending `loadfile` so the error subscription cannot lose a
+ * race against a fast failure; await `wait()` after the commands went out.
+ */
+export function watchPlaybackOutcome(deps: WatchPlaybackOutcomeDeps): PlaybackOutcomeWatch {
+  const timeoutMs = deps.timeoutMs ?? DEFAULT_PLAYBACK_OUTCOME_TIMEOUT_MS;
+  const probeIntervalMs = deps.probeIntervalMs ?? DEFAULT_PROBE_INTERVAL_MS;
+
+  let failure: PlaybackOutcome | null = null;
+  const unsubscribe = deps.onEndFile((event) => {
+    if (event.reason !== 'error') return;
+    failure = {
+      ok: false,
+      error: event.fileError
+        ? `mpv could not play this stream: ${event.fileError}`
+        : 'mpv could not play this stream.',
+    };
+  });
+
+  async function wait(): Promise<PlaybackOutcome> {
+    for (let elapsed = 0; elapsed < timeoutMs; elapsed += probeIntervalMs) {
+      if (failure) return failure;
+      try {
+        if ((await deps.readProperty('vo-configured')) === true) return { ok: true };
+      } catch {
+        // The property is unreadable while mpv is between files; keep polling.
+      }
+      if (failure) return failure;
+      await deps.wait(probeIntervalMs);
+    }
+    return (
+      failure ?? {
+        ok: false,
+        error: 'Playback did not start. mpv gave no error; try another server or quality.',
+      }
+    );
+  }
+
+  return { wait, dispose: unsubscribe };
+}

src/anime-bridge/stream-strip-proxy.test.ts

@@ -0,0 +1,165 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import http from 'node:http';
+import type { AddressInfo } from 'node:net';
+import {
+  findTsSyncOffset,
+  rewritePlaylistOrigins,
+  startStreamStripProxy,
+  TS_PACKET_LENGTH,
+} from './stream-strip-proxy';
+
+/** A valid MPEG-TS payload: 0x47 sync byte every 188 bytes. */
+function makeTsPackets(count: number): Buffer {
+  const data = Buffer.alloc(count * TS_PACKET_LENGTH, 0x11);
+  for (let i = 0; i < count; i++) data[i * TS_PACKET_LENGTH] = 0x47;
+  return data;
+}
+
+/** The disguise seen in the wild: a real PNG header glued before the TS data. */
+const PNG_HEADER = Buffer.from([
+  0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52,
+]);
+
+test('findTsSyncOffset returns 0 for clean TS data', () => {
+  assert.equal(findTsSyncOffset(makeTsPackets(6)), 0);
+});
+
+test('findTsSyncOffset finds TS data behind a PNG prefix', () => {
+  const disguised = Buffer.concat([PNG_HEADER, makeTsPackets(6)]);
+  assert.equal(findTsSyncOffset(disguised), PNG_HEADER.length);
+});
+
+test('findTsSyncOffset ignores a lone sync byte in the junk prefix', () => {
+  const junk = Buffer.alloc(64, 0x00);
+  junk[10] = 0x47; // decoy: no packet run follows it
+  const disguised = Buffer.concat([junk, makeTsPackets(6)]);
+  assert.equal(findTsSyncOffset(disguised), junk.length);
+});
+
+test('findTsSyncOffset returns null when no TS run exists', () => {
+  assert.equal(findTsSyncOffset(Buffer.alloc(4096, 0x42)), null);
+});
+
+test('findTsSyncOffset returns null when the run starts past the scan limit', () => {
+  const disguised = Buffer.concat([Buffer.alloc(600, 0x00), makeTsPackets(6)]);
+  assert.equal(findTsSyncOffset(disguised, 500), null);
+});
+
+test('rewritePlaylistOrigins swaps absolute upstream URLs and keeps relative lines', () => {
+  const body = [
+    '#EXTM3U',
+    '#EXTINF:6.006,',
+    '/video/aaa.ts',
+    '#EXTINF:4.463,',
+    'http://127.0.0.1:41569/video/bbb.ts',
+  ].join('\n');
+  const rewritten = rewritePlaylistOrigins(body, 'http://127.0.0.1:41569', 'http://127.0.0.1:9999');
+  assert.ok(rewritten.includes('http://127.0.0.1:9999/video/bbb.ts'));
+  assert.ok(rewritten.includes('\n/video/aaa.ts\n'));
+  assert.ok(!rewritten.includes('41569'));
+});
+
+/* ---------- proxy end-to-end ---------- */
+
+type Route = { status: number; contentType: string; body: Buffer };
+
+async function withProxy(
+  routes: Record<string, Route>,
+  run: (proxyOrigin: string, upstreamOrigin: string) => Promise<void>,
+): Promise<void> {
+  const upstream = http.createServer((req, res) => {
+    const route = routes[req.url ?? ''];
+    if (!route) {
+      res.writeHead(404).end('missing');
+      return;
+    }
+    res.writeHead(route.status, { 'content-type': route.contentType });
+    res.end(route.body);
+  });
+  await new Promise<void>((resolve) => upstream.listen(0, '127.0.0.1', resolve));
+  const upstreamOrigin = `http://127.0.0.1:${(upstream.address() as AddressInfo).port}`;
+
+  const proxy = await startStreamStripProxy({ upstreamOrigin: () => upstreamOrigin });
+  try {
+    await run(proxy.origin, upstreamOrigin);
+  } finally {
+    await proxy.close();
+    await new Promise<void>((resolve) => upstream.close(() => resolve()));
+  }
+}
+
+async function fetchBytes(url: string): Promise<{ status: number; body: Buffer }> {
+  const response = await fetch(url);
+  return { status: response.status, body: Buffer.from(await response.arrayBuffer()) };
+}
+
+test('proxy strips the PNG disguise off a segment', async () => {
+  const ts = makeTsPackets(8);
+  const disguised = Buffer.concat([PNG_HEADER, ts]);
+  await withProxy(
+    { '/video/seg.ts': { status: 200, contentType: 'video/mp2t', body: disguised } },
+    async (origin) => {
+      const { status, body } = await fetchBytes(`${origin}/video/seg.ts`);
+      assert.equal(status, 200);
+      assert.deepEqual(body, ts);
+    },
+  );
+});
+
+test('proxy passes a clean segment through unchanged', async () => {
+  const ts = makeTsPackets(8);
+  await withProxy(
+    { '/video/seg.ts': { status: 200, contentType: 'video/mp2t', body: ts } },
+    async (origin) => {
+      const { body } = await fetchBytes(`${origin}/video/seg.ts`);
+      assert.deepEqual(body, ts);
+    },
+  );
+});
+
+test('proxy leaves non-TS bodies alone', async () => {
+  const vtt = Buffer.from('WEBVTT\n\n00:00.000 --> 00:01.000\nhello\n');
+  await withProxy(
+    { '/video/sub.vtt': { status: 200, contentType: 'text/vtt', body: vtt } },
+    async (origin) => {
+      const { body } = await fetchBytes(`${origin}/video/sub.vtt`);
+      assert.deepEqual(body, vtt);
+    },
+  );
+});
+
+test('proxy rewrites absolute upstream playlist entries to its own origin', async () => {
+  const upstream = http.createServer((req, res) => {
+    const origin = `http://127.0.0.1:${(upstream.address() as AddressInfo).port}`;
+    if (req.url === '/video/list.m3u8') {
+      res.writeHead(200, { 'content-type': 'application/vnd.apple.mpegurl' });
+      res.end(`#EXTM3U\n#EXTINF:6,\n${origin}/video/abs.ts\n`);
+      return;
+    }
+    res.writeHead(404).end();
+  });
+  await new Promise<void>((resolve) => upstream.listen(0, '127.0.0.1', resolve));
+  const upstreamOrigin = `http://127.0.0.1:${(upstream.address() as AddressInfo).port}`;
+  const proxy = await startStreamStripProxy({ upstreamOrigin: () => upstreamOrigin });
+  try {
+    const { body } = await fetchBytes(`${proxy.origin}/video/list.m3u8`);
+    const text = body.toString('utf8');
+    assert.ok(text.includes(`${proxy.origin}/video/abs.ts`));
+    assert.ok(!text.includes(upstreamOrigin));
+  } finally {
+    await proxy.close();
+    await new Promise<void>((resolve) => upstream.close(() => resolve()));
+  }
+});
+
+test('proxy forwards error statuses without touching the body', async () => {
+  await withProxy(
+    { '/video/gone.ts': { status: 404, contentType: 'text/plain', body: Buffer.from('nope') } },
+    async (origin) => {
+      const { status, body } = await fetchBytes(`${origin}/video/gone.ts`);
+      assert.equal(status, 404);
+      assert.equal(body.toString('utf8'), 'nope');
+    },
+  );
+});

src/anime-bridge/stream-strip-proxy.ts

@@ -0,0 +1,232 @@
+import http from 'node:http';
+import type { AddressInfo } from 'node:net';
+
+/**
+ * Loopback proxy between mpv and the anime bridge that undoes segment
+ * disguises. Some hosts prepend a real image header (a 1x1 PNG in the wild) to
+ * every HLS segment so scrapers see "an image"; ffmpeg then probes the segment
+ * as a picture and playback dies with "no audio or video data played". Aniyomi
+ * strips this in its player; mpv needs the bytes fixed before it sees them.
+ *
+ * Only bridge-origin `.m3u8` streams are routed through here (see
+ * anime-browser-runtime). Playlist bodies get their absolute upstream origins
+ * rewritten so segment requests come back through the proxy; segment bodies are
+ * scanned for the first genuine MPEG-TS packet run and any junk before it is
+ * dropped. Anything that is not TS (fMP4, VTT, keys) passes through untouched.
+ */
+
+export const TS_PACKET_LENGTH = 188;
+const TS_SYNC_BYTE = 0x47;
+/**
+ * Sync bytes that must repeat at exact packet spacing before an offset counts
+ * as TS data. One or two matches happen by chance in binary data; five in a
+ * row at 188-byte strides do not.
+ */
+const SYNC_RUN = 5;
+/** A disguise prefix is small; give up scanning after this much. */
+export const DEFAULT_SCAN_LIMIT_BYTES = 1024 * 1024;
+/** Bytes needed to either find a run within the limit or rule one out. */
+const DECISION_BYTES = DEFAULT_SCAN_LIMIT_BYTES + (SYNC_RUN - 1) * TS_PACKET_LENGTH + 1;
+
+/**
+ * First offset at which a confirmed MPEG-TS packet run starts, or null when
+ * the data does not look like TS at all (within the scan limit).
+ */
+export function findTsSyncOffset(
+  data: Buffer,
+  scanLimit = DEFAULT_SCAN_LIMIT_BYTES,
+): number | null {
+  const lastConfirmable = data.length - (SYNC_RUN - 1) * TS_PACKET_LENGTH - 1;
+  const end = Math.min(lastConfirmable, scanLimit);
+  for (let offset = 0; offset <= end; offset++) {
+    if (data[offset] !== TS_SYNC_BYTE) continue;
+    let confirmed = true;
+    for (let packet = 1; packet < SYNC_RUN; packet++) {
+      if (data[offset + packet * TS_PACKET_LENGTH] !== TS_SYNC_BYTE) {
+        confirmed = false;
+        break;
+      }
+    }
+    if (confirmed) return offset;
+  }
+  return null;
+}
+
+/**
+ * Point absolute playlist entries at the proxy. Relative entries already
+ * resolve against whatever origin served the playlist, so they need no help.
+ */
+export function rewritePlaylistOrigins(
+  body: string,
+  upstreamOrigin: string,
+  proxyOrigin: string,
+): string {
+  return body.split(upstreamOrigin).join(proxyOrigin);
+}
+
+export interface StreamStripProxyOptions {
+  /** Read per request so a bridge restart on a new port keeps working. */
+  upstreamOrigin: () => string;
+  log?: (message: string) => void;
+}
+
+export interface StreamStripProxyHandle {
+  origin: string;
+  port: number;
+  close: () => Promise<void>;
+}
+
+/** Response headers that must not be forwarded verbatim. */
+const DROPPED_HEADERS = new Set([
+  'connection',
+  'keep-alive',
+  'transfer-encoding',
+  'content-length',
+]);
+
+function forwardableHeaders(headers: http.IncomingHttpHeaders): http.OutgoingHttpHeaders {
+  const result: http.OutgoingHttpHeaders = {};
+  for (const [name, value] of Object.entries(headers)) {
+    if (value === undefined || DROPPED_HEADERS.has(name.toLowerCase())) continue;
+    result[name] = value;
+  }
+  return result;
+}
+
+export function startStreamStripProxy(
+  options: StreamStripProxyOptions,
+): Promise<StreamStripProxyHandle> {
+  const log = options.log ?? (() => {});
+
+  const server = http.createServer((req, res) => {
+    if (req.method !== 'GET' && req.method !== 'HEAD') {
+      res.writeHead(405).end();
+      return;
+    }
+
+    let upstreamUrl: URL;
+    try {
+      upstreamUrl = new URL(req.url ?? '/', options.upstreamOrigin());
+    } catch {
+      res.writeHead(502).end();
+      return;
+    }
+
+    const requestHeaders = forwardableHeaders(req.headers);
+    delete requestHeaders.host;
+
+    const upstreamRequest = http.request(
+      upstreamUrl,
+      { method: req.method, headers: requestHeaders },
+      (upstream) => handleUpstreamResponse(req, res, upstream),
+    );
+    upstreamRequest.on('error', (error) => {
+      log(`[stream-proxy] upstream request failed: ${String(error)}`);
+      if (!res.headersSent) res.writeHead(502);
+      res.end();
+    });
+    upstreamRequest.end();
+  });
+
+  function handleUpstreamResponse(
+    req: http.IncomingMessage,
+    res: http.ServerResponse,
+    upstream: http.IncomingMessage,
+  ): void {
+    const status = upstream.statusCode ?? 502;
+    const pathname = (req.url ?? '').split('?', 1)[0] ?? '';
+    const contentType = String(upstream.headers['content-type'] ?? '');
+    const isPlaylist = pathname.endsWith('.m3u8') || contentType.includes('mpegurl');
+
+    upstream.on('error', () => res.destroy());
+
+    // Only a full 200 body is safe to modify; everything else (errors, range
+    // responses, HEAD) forwards untouched.
+    if (status !== 200 || req.method === 'HEAD') {
+      res.writeHead(status, forwardableHeaders(upstream.headers));
+      upstream.pipe(res);
+      return;
+    }
+
+    if (isPlaylist) {
+      const chunks: Buffer[] = [];
+      upstream.on('data', (chunk: Buffer) => chunks.push(chunk));
+      upstream.on('end', () => {
+        const body = rewritePlaylistOrigins(
+          Buffer.concat(chunks).toString('utf8'),
+          options.upstreamOrigin(),
+          origin,
+        );
+        res.writeHead(status, {
+          ...forwardableHeaders(upstream.headers),
+          'content-length': Buffer.byteLength(body, 'utf8'),
+        });
+        res.end(body);
+      });
+      return;
+    }
+
+    stripSegment(res, upstream);
+  }
+
+  /**
+   * Buffer just enough of the body to find (or rule out) a TS packet run,
+   * drop everything before it, then stream the rest through untouched.
+   */
+  function stripSegment(res: http.ServerResponse, upstream: http.IncomingMessage): void {
+    const chunks: Buffer[] = [];
+    let buffered = 0;
+
+    const respond = (data: Buffer, remainderFollows: boolean): void => {
+      const offset = findTsSyncOffset(data) ?? 0;
+      if (offset > 0) log(`[stream-proxy] stripped ${offset} disguise bytes off a segment`);
+      const body = offset > 0 ? data.subarray(offset) : data;
+
+      const headers = forwardableHeaders(upstream.headers);
+      const upstreamLength = Number(upstream.headers['content-length']);
+      if (remainderFollows) {
+        if (Number.isFinite(upstreamLength)) headers['content-length'] = upstreamLength - offset;
+      } else {
+        headers['content-length'] = body.length;
+      }
+
+      res.writeHead(upstream.statusCode ?? 200, headers);
+      res.write(body);
+    };
+
+    const onData = (chunk: Buffer): void => {
+      chunks.push(chunk);
+      buffered += chunk.length;
+      if (buffered < DECISION_BYTES) return;
+      upstream.off('data', onData);
+      upstream.off('end', onEnd);
+      respond(Buffer.concat(chunks), true);
+      upstream.pipe(res);
+    };
+    const onEnd = (): void => {
+      respond(Buffer.concat(chunks), false);
+      res.end();
+    };
+    upstream.on('data', onData);
+    upstream.on('end', onEnd);
+  }
+
+  let origin = '';
+
+  return new Promise((resolve, reject) => {
+    server.once('error', reject);
+    server.listen(0, '127.0.0.1', () => {
+      const { port } = server.address() as AddressInfo;
+      origin = `http://127.0.0.1:${port}`;
+      resolve({
+        origin,
+        port,
+        close: () =>
+          new Promise<void>((resolveClose) => {
+            server.closeAllConnections?.();
+            server.close(() => resolveClose());
+          }),
+      });
+    });
+  });
+}