feat: LAN Wi-Fi casting to Chromecast via local HTTP relay

Re-serve decrypted fMP4 video over HTTP on the LAN so a Chromecast can
play the stream. The relay runs in the Electron main process — same trust
boundary as downloads and MSE playback.

- cast-relay.js: HTTP server with BoxAccumulator (reassembles WebRTC
  chunks into moof+mdat pairs), ring buffer, backpressure, finish() for
  clean end-of-stream, fixed port range 19550-19553
- cast-chromecast.js: mDNS discovery (bonjour-service) + CASTV2 protocol
  (castv2-client), connect/reload/disconnect lifecycle
- Seek-aware: relay restarts on every seek, Chromecast reloads new URL;
  generation counter prevents stale async errors from killing active
  restarts; landingPlayheadRef suppresses programmatic seeking events
- Device picker in video top bar with scan, device selection, copy-URL
  fallback, and cast status indicator
- IPC bridge (main/preload/platform) for start/push/stop/finish/status/
  discover/chromecastConnect/chromecastReload/chromecastDisconnect
- Phase 3 design doc for DLNA/Smart TV in docs/cast-smart-tv.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 293 insertions, 0 deletions

diff --git a/packages/meshbay-client/src/cast-relay.js b/packages/meshbay-client/src/cast-relay.js
new file mode 100644
index 0000000..12939c4
--- /dev/null
+++ b/packages/meshbay-client/src/cast-relay.js
@@ -0,0 +1,293 @@
+/**
+ * Local HTTP relay for LAN casting.
+ *
+ * Re-serves decrypted fMP4 segments from the renderer over HTTP so that a
+ * Chromecast, Smart TV or any player on the same Wi-Fi can stream the video.
+ *
+ * The renderer sends raw byte chunks from the WebRTC DataChannel — these are
+ * arbitrary-sized slices of the fMP4 stream, NOT aligned to MP4 box boundaries.
+ * MSE handles this internally, but external players (VLC, Chromecast) need
+ * properly framed fMP4 fragments. The BoxAccumulator reassembles the byte
+ * stream and emits complete moof+mdat pairs.
+ *
+ * Mitigations:
+ *   · Bind to the LAN interface, never 0.0.0.0
+ *   · Fixed port range (19550-19553), opened only during active cast
+ *   · Unguessable token in the URL (32 hex chars)
+ *   · Cache-Control: no-store on every response
+ *   · Server destroyed when playback stops — zero residual surface
+ */
+
+'use strict';
+
+const crypto = require('node:crypto');
+const http = require('node:http');
+const os = require('node:os');
+
+const RING_CAP = 64;
+const BACKPRESSURE_HIGH = 8 * 1024 * 1024;
+const MOOF = 0x6d6f6f66;
+const PORT_BASE = 19550;
+const PORT_COUNT = 4;
+
+function lanAddress() {
+  const ifaces = os.networkInterfaces();
+  for (const name of Object.keys(ifaces)) {
+    for (const iface of ifaces[name]) {
+      if (!iface.internal && iface.family === 'IPv4') {
+        return iface.address;
+      }
+    }
+  }
+  return '127.0.0.1';
+}
+
+class BoxAccumulator {
+  constructor() {
+    this._buf = Buffer.alloc(0);
+    this._synced = false;
+  }
+
+  push(data) {
+    this._buf = Buffer.concat([this._buf, data]);
+    const fragments = [];
+
+    if (!this._synced) {
+      const idx = this._findMoof();
+      if (idx === -1) return fragments;
+      console.log(`[cast-relay] box sync: found first moof at byte offset ${idx}, discarded ${idx} bytes`);
+      this._buf = this._buf.subarray(idx);
+      this._synced = true;
+    }
+
+    while (this._buf.length >= 8) {
+      const size = this._buf.readUInt32BE(0);
+      const type = this._buf.readUInt32BE(4);
+
+      if (size < 8) {
+        console.log(`[cast-relay] box sync lost: invalid size ${size}, rescanning`);
+        this._synced = false;
+        const idx = this._findMoof();
+        if (idx === -1) return fragments;
+        this._buf = this._buf.subarray(idx);
+        this._synced = true;
+        continue;
+      }
+
+      if (type === MOOF) {
+        if (this._buf.length < size + 8) break;
+        const mdatSize = this._buf.readUInt32BE(size);
+        const pairSize = size + mdatSize;
+        if (this._buf.length < pairSize) break;
+
+        fragments.push(Buffer.from(this._buf.subarray(0, pairSize)));
+        this._buf = this._buf.subarray(pairSize);
+      } else {
+        if (this._buf.length < size) break;
+        this._buf = this._buf.subarray(size);
+      }
+    }
+
+    return fragments;
+  }
+
+  reset() {
+    this._buf = Buffer.alloc(0);
+    this._synced = false;
+  }
+
+  _findMoof() {
+    for (let i = 0; i <= this._buf.length - 8; i++) {
+      if (this._buf.readUInt32BE(i + 4) === MOOF) {
+        const size = this._buf.readUInt32BE(i);
+        if (size >= 8 && size < 1_000_000) {
+          return i;
+        }
+      }
+    }
+    return -1;
+  }
+}
+
+class CastRelay {
+  constructor() {
+    this._server = null;
+    this._port = null;
+    this._token = null;
+    this._lanIP = null;
+    this._initSegment = null;
+    this._ring = [];
+    this._clients = new Set();
+    this._accum = new BoxAccumulator();
+    this._fragCount = 0;
+    this._chunkCount = 0;
+    this._bytesSent = 0;
+  }
+
+  get active() { return this._server !== null; }
+
+  get url() {
+    if (!this._server) return null;
+    return `http://${this._lanIP}:${this._port}/stream.mp4?t=${this._token}`;
+  }
+
+  async start({ codec, initSegment }) {
+    if (this._server) await this.stop();
+
+    this._token = crypto.randomBytes(16).toString('hex');
+    this._lanIP = lanAddress();
+    this._initSegment = initSegment ? Buffer.from(initSegment) : null;
+    this._ring = [];
+    this._clients = new Set();
+    this._accum = new BoxAccumulator();
+    this._fragCount = 0;
+    this._chunkCount = 0;
+    this._bytesSent = 0;
+
+    const server = http.createServer((req, res) => this._handle(req, res));
+
+    let bound = false;
+    for (let i = 0; i < PORT_COUNT && !bound; i++) {
+      const port = PORT_BASE + i;
+      try {
+        await new Promise((resolve, reject) => {
+          const onError = (err) => {
+            server.removeListener('error', onError);
+            reject(err);
+          };
+          server.on('error', onError);
+          server.listen(port, this._lanIP, () => {
+            server.removeListener('error', onError);
+            this._port = port;
+            resolve();
+          });
+        });
+        bound = true;
+      } catch (err) {
+        if (err.code !== 'EADDRINUSE') throw err;
+        console.log(`[cast-relay] port ${port} busy, trying next`);
+      }
+    }
+    if (!bound) throw new Error('All cast relay ports are in use');
+
+    this._server = server;
+    console.log(`[cast-relay] started on ${this.url}`);
+    console.log(`[cast-relay] init segment: ${this._initSegment ? this._initSegment.length + ' bytes' : 'none'}`);
+    return { url: this.url, port: this._port, token: this._token };
+  }
+
+  pushSegment(data) {
+    const buf = Buffer.isBuffer(data) ? data : Buffer.from(data);
+    this._chunkCount++;
+
+    const fragments = this._accum.push(buf);
+
+    for (const frag of fragments) {
+      this._fragCount++;
+      if (this._fragCount <= 3 || this._fragCount % 50 === 0) {
+        console.log(`[cast-relay] fragment #${this._fragCount}: ${frag.length} bytes (from ${this._chunkCount} chunks), ${this._clients.size} client(s)`);
+      }
+
+      if (this._ring.length >= RING_CAP) {
+        this._ring.shift();
+      }
+      this._ring.push(frag);
+
+      for (const res of this._clients) {
+        if (res.writableLength > BACKPRESSURE_HIGH) {
+          console.log(`[cast-relay] backpressure: dropping fragment for slow client`);
+          continue;
+        }
+        res.write(frag);
+        this._bytesSent += frag.length;
+      }
+    }
+  }
+
+  finish() {
+    console.log('[cast-relay] finishing stream');
+    for (const res of this._clients) {
+      try { res.end(); } catch { /* already closed */ }
+    }
+  }
+
+  async stop() {
+    console.log(`[cast-relay] stopping — ${this._chunkCount} chunks, ${this._fragCount} fragments, ${(this._bytesSent / 1048576).toFixed(1)} MB sent`);
+    for (const res of this._clients) {
+      try { res.end(); } catch { /* already closed */ }
+    }
+    this._clients.clear();
+
+    if (this._server) {
+      const srv = this._server;
+      this._server = null;
+      await new Promise((resolve) => srv.close(resolve));
+    }
+
+    this._port = null;
+    this._token = null;
+    this._initSegment = null;
+    this._ring = [];
+    this._accum.reset();
+    this._fragCount = 0;
+    this._chunkCount = 0;
+    this._bytesSent = 0;
+  }
+
+  _handle(req, res) {
+    if (req.method !== 'GET') {
+      res.writeHead(405);
+      res.end();
+      return;
+    }
+
+    let url;
+    try {
+      url = new URL(req.url, `http://${req.headers.host}`);
+    } catch {
+      res.writeHead(400);
+      res.end();
+      return;
+    }
+
+    if (url.searchParams.get('t') !== this._token) {
+      res.writeHead(403);
+      res.end();
+      return;
+    }
+
+    if (url.pathname !== '/stream.mp4') {
+      res.writeHead(404);
+      res.end();
+      return;
+    }
+
+    let sent = 0;
+    res.writeHead(200, {
+      'Content-Type': 'video/mp4',
+      'Cache-Control': 'no-store',
+      'Accept-Ranges': 'none',
+      'Connection': 'keep-alive',
+    });
+
+    if (this._initSegment) {
+      res.write(this._initSegment);
+      sent += this._initSegment.length;
+    }
+
+    for (const frag of this._ring) {
+      res.write(frag);
+      sent += frag.length;
+    }
+
+    console.log(`[cast-relay] client connected from ${req.socket.remoteAddress} — sent init + ${this._ring.length} fragments (${(sent / 1024).toFixed(0)} KB)`);
+
+    this._clients.add(res);
+    req.on('close', () => {
+      this._clients.delete(res);
+      console.log(`[cast-relay] client disconnected, ${this._clients.size} remaining`);
+    });
+  }
+}
+
+module.exports = CastRelay;