ipx/web/sw/sw.ts

// iPX offline (#389): the page and what it last read from the API, kept so iPX opens and reads
// with no connection; pictures in items' text, through ipx's /api/art, to see them too; and the
// reads, flags and places in an episode made offline, kept and sent when the connection is back.
//
// Online, everything goes to the network first and only its answers are kept, so nothing seen
// online is ever an old copy, and a deploy's new page arrives as it would without this (#388).
// Offline, the kept copy answers. Cleared on sign-out and on a change of account (the page).
//
// Not part of app.js: a service worker is a script of its own, at the top of the site so it sees
// every request, and typed against the worker's globals, not the page's (tsconfig.sw.json). Its
// scope is the origin's; it never sees another site's.

// The worker's own global: the WebWorker library declares `self` for workers of every kind.
const sw = self as unknown as ServiceWorkerGlobalScope;
/// A write kept for later, as the outbox holds it.
type Saved = {url: string, body: string, type: string};

const PAGE = 'ipx-page', API = 'ipx-api', PICS = 'ipx-pics', OUTBOX = 'ipx-outbox', KEPT = 'ipx-episodes';
// shortcut: a byte budget over pictures counted when kept, oldest out first; nothing tracks
// which were looked at since. Raise it, or prune by use, if 200 MB proves too little or too much.
const PICS_BUDGET = 200 << 20;
// The writes made offline that are worth keeping for later: an item read or flagged, and a place
// in an episode. Anything else (adding a feed, settings) fails offline as it always has.
const QUEUED = /^\/api\/entries\/[^/]+\/[^/]+\/(flags|position)$/;

sw.addEventListener('install', () => { void sw.skipWaiting(); });
sw.addEventListener('activate', e => e.waitUntil(sw.clients.claim()));

sw.addEventListener('fetch', e => {
  const req = e.request, url = new URL(req.url);
  if (url.origin !== location.origin) {
    // A picture in an item's text, asked for from its publisher: the publisher while online, the
    // copy ipx kept for it while not.
    if (req.method === 'GET' && req.destination === 'image') e.respondWith(picture(req));
    return;
  }
  // An episode kept on this device (#412), which the page asks for as such.
  if (req.method === 'GET' && url.pathname.startsWith('/media/') && url.searchParams.has('offline')) {
    e.respondWith(keptEpisode(req, url.pathname));
    return;
  }
  // The live stream, audio and video pass by: a stream cannot be kept, a file is too big to
  // unless someone keeps it, above.
  if (url.pathname === '/api/events' || url.pathname.startsWith('/media/') || url.pathname === '/sw.js') return;
  if (req.method !== 'GET') {
    if (req.method === 'POST' && QUEUED.test(url.pathname)) e.respondWith(write(req));
    return;
  }
  if (req.mode === 'navigate') e.respondWith(page(req));
  // Named by a hash of their contents: what is kept is what the network would send.
  else if (url.searchParams.has('v') && !url.pathname.startsWith('/api/')) e.respondWith(hashed(req, e.clientId));
  else e.respondWith(networkFirst(req, url.pathname.startsWith('/api/') ? API : PAGE));
});

sw.addEventListener('message', e => {
  const m = e.data as {flush?: boolean, pictures?: string[]} | null;
  if (m?.flush) e.waitUntil(flush());
  if (Array.isArray(m?.pictures)) e.waitUntil(keepPictures(m.pictures));
});

/// The page itself, under one name whatever its query: a redirect to sign in, or to Cloudflare
/// Access, is not kept, and the browser follows it as it would.
async function page(req: Request): Promise<Response> {
  try {
    const r = await fetch(req);
    if (r.ok && r.type === 'basic' && new URL(req.url).pathname === '/') (await caches.open(PAGE)).put('/', r.clone());
    return r;
  } catch {
    return (await caches.match('/', { cacheName: PAGE })) || Response.error();
  }
}

async function hashed(req: Request, client: string): Promise<Response> {
  const kept = await caches.match(req, { cacheName: PAGE });
  if (kept) return kept;
  const r = await fetch(req);
  if (r.ok) {
    const url = new URL(req.url);
    // The server sends what it has now, whatever version the name asks for. A page kept from
    // before a deploy, shown on waking before the network was back, got the new script under its
    // old name, died at the first element it lacked (#449), and the new script was kept under the
    // old name. Such a page is stale: it is loaded again, from the network, and not given the file.
    if (await sha12(await r.clone().arrayBuffer()) !== url.searchParams.get('v')) {
      void reloadStale(client);
      return Response.error();
    }
    const c = await caches.open(PAGE), path = url.pathname;
    // The version before this one is no use once this one is here.
    for (const k of await c.keys()) if (new URL(k.url).pathname === path) await c.delete(k);
    await c.put(req, r.clone());
  }
  return r;
}

/// What web/build.mts names a file by: the first 12 hex digits of its SHA-256.
async function sha12(body: ArrayBuffer): Promise<string> {
  const d = new Uint8Array(await crypto.subtle.digest('SHA-256', body));
  return Array.from(d.slice(0, 6), b => b.toString(16).padStart(2, '0')).join('');
}

// Once in a while at most: if the page itself still cannot be had from the network, the kept one
// comes back, asks for its old files again, and would be reloaded round and round.
let reloadedAt = 0;
async function reloadStale(id: string) {
  if (Date.now() - reloadedAt < 30_000) return;
  const c = await sw.clients.get(id);
  if (!c || !('navigate' in c)) return;
  reloadedAt = Date.now();
  await (c as WindowClient).navigate(c.url);
}

async function networkFirst(req: Request, name: string): Promise<Response> {
  try {
    const r = await fetch(req);
    if (r.ok && r.type === 'basic') (await caches.open(name)).put(req, r.clone());
    // Back online: whatever waited to be sent goes now.
    if (name === API) void flush();
    return r;
  } catch {
    const kept = await caches.match(req, { cacheName: name });
    if (!kept) return Response.error();
    return name === API ? withQueuedReads(kept) : kept;
  }
}

/// A list kept before reads made offline still has those items unread: they are marked as the
/// outbox has them, so a page opened again offline shows what was read.
async function withQueuedReads(kept: Response): Promise<Response> {
  const reads = new Map<string, boolean>();
  for (const w of await queued()) {
    const m = new URL(w.url, location.origin).pathname.match(/^\/api\/entries\/([^/]+)\/([^/]+)\/flags$/);
    if (!m) continue;
    const read: unknown = (JSON.parse(w.body || '{}') as {read?: unknown}).read;
    if (typeof read === 'boolean') reads.set(decodeURIComponent(m[1]!) + '\n' + decodeURIComponent(m[2]!), read);
  }
  if (!reads.size || !(kept.headers.get('Content-Type') || '').includes('json')) return kept;
  // Only the fields read here: a page of items, or one item, as the API sends them.
  type Item = {feed_id: string, guid: string, read: boolean};
  const body = await kept.clone().json().catch((): null => null) as {entries?: Item[]} & Partial<Item> | null;
  const mark = (e: Partial<Item>) => { const r = reads.get(e.feed_id + '\n' + e.guid); if (r !== undefined) e.read = r; };
  if (Array.isArray(body?.entries)) body.entries.forEach(mark);
  else if (body?.guid) mark(body);
  else return kept;
  // Its own headers only: the kept one's length and encoding are not this body's.
  return new Response(JSON.stringify(body), { status: kept.status, headers: { 'Content-Type': 'application/json' } });
}

// ---- writes made offline -------------------------------------------------------------------

let seq = 0;
async function write(req: Request): Promise<Response> {
  const saved: Saved = { url: req.url, body: await req.clone().text(), type: req.headers.get('Content-Type') || 'application/json' };
  // One already waiting goes first: sent now, this one would land before it and be undone by it.
  if ((await queued()).length === 0) {
    try { return await fetch(req); } catch { /* offline: kept below */ }
  }
  await (await caches.open(OUTBOX)).put(`/__outbox/${Date.now()}-${seq++}`, new Response(JSON.stringify(saved)));
  void flush();
  // What the server answers for a write the page does not need back: it carries on as if sent.
  return new Response(null, { status: 204, headers: { 'X-Ipx-Queued': '1' } });
}

async function queued(): Promise<(Saved & {key: Request})[]> {
  const c = await caches.open(OUTBOX);
  const all = await Promise.all((await c.keys()).map(async k => {
    const r = await c.match(k);
    return r ? { key: k, ...(await r.json() as Saved) } : null;
  }));
  return all.filter(w => w !== null);
}

let flushing: Promise<void> | null = null;
/// Sends the writes in the order they were made; the first that cannot reach the server stops
/// it for the next try. One the server answered at all is done, whatever it said: a refusal now
/// would be a refusal again.
function flush(): Promise<void> {
  flushing ||= (async () => {
    const c = await caches.open(OUTBOX);
    for (const w of await queued()) {
      try {
        await fetch(w.url, { method: 'POST', body: w.body, headers: { 'Content-Type': w.type }, credentials: 'same-origin' });
      } catch { break; }
      await c.delete(w.key);
    }
  })().finally(() => { flushing = null; });
  return flushing;
}

// ---- episodes kept on the device (#412) ------------------------------------------------------

/// The kept copy, or the byte range of it a player asks for: an audio element asks for ranges and
/// Safari plays nothing it cannot have them for. Not kept (cleared, or another device's page), the
/// server's own, as without `?offline`.
async function keptEpisode(req: Request, path: string): Promise<Response> {
  const kept = await caches.match(path, { cacheName: KEPT });
  const range = req.headers.get('Range');
  if (!kept) return fetch(path, { headers: range ? { Range: range } : {}, credentials: 'same-origin' });
  const blob = await kept.blob();
  return ranged(blob, range, kept.headers.get('Content-Type') || 'audio/mpeg');
}

/// `blob` answered as a server answers a Range request: 206 with the part asked for, 416 for a
/// part past its end, 200 with all of it when no range is asked for or the one asked is not
/// understood.
function ranged(blob: Blob, range: string | null, type: string): Response {
  const size = blob.size;
  const m = range?.match(/^bytes=(\d*)-(\d*)$/);
  const head = { 'Content-Type': type, 'Accept-Ranges': 'bytes' };
  if (!m || (m[1] === '' && m[2] === '')) return new Response(blob, { headers: { ...head, 'Content-Length': String(size) } });
  // `bytes=-N` is the last N bytes.
  const start = m[1] === '' ? Math.max(0, size - Number(m[2])) : Number(m[1]);
  const end = m[1] === '' || m[2] === '' ? size - 1 : Math.min(Number(m[2]), size - 1);
  if (start >= size || start > end) {
    return new Response(null, { status: 416, headers: { ...head, 'Content-Range': `bytes */${size}` } });
  }
  return new Response(blob.slice(start, end + 1), { status: 206, headers: {
    ...head, 'Content-Length': String(end - start + 1), 'Content-Range': `bytes ${start}-${end}/${size}`,
  } });
}

// ---- pictures ----------------------------------------------------------------------------------

const artFor = (src: string) => `/api/art?u=${encodeURIComponent(src)}`;

async function picture(req: Request): Promise<Response> {
  try {
    return await fetch(req);
  } catch {
    return (await caches.match(artFor(req.url), { cacheName: PICS })) || Response.error();
  }
}

/// Fetches through ipx, three at a time, the pictures not kept yet, then trims to the budget,
/// oldest kept first out.
async function keepPictures(srcs: string[], budget = PICS_BUDGET) {
  const c = await caches.open(PICS);
  const todo: string[] = [];
  // Kept under the address the browser will ask for, which it normalises (a space is %20); asked
  // of ipx as the text names it, which is what ipx knows it by.
  const key = (s: string) => { try { return artFor(new URL(s).href); } catch { return null; } };
  for (const s of srcs) { const k = key(s); if (k && !(await c.match(k))) todo.push(s); }
  const one = async () => {
    for (let s; (s = todo.shift()) !== undefined;) {
      const k = key(s);
      if (!k) continue;
      try {
        const r = await fetch(artFor(s));
        if (!r.ok) continue;
        const blob = await r.blob();
        await c.put(k, new Response(blob, { headers: { 'Content-Type': r.headers.get('Content-Type') || '', 'X-Size': String(blob.size) } }));
      } catch { return; }   // offline: the rest wait for the next list
    }
  };
  await Promise.all([one(), one(), one()]);
  const keys = await c.keys();
  const sizes = await Promise.all(keys.map(async k => Number((await c.match(k))?.headers.get('X-Size')) || 0));
  let total = sizes.reduce((a, b) => a + b, 0);
  for (let i = 0; i < keys.length && total > budget; i++) { await c.delete(keys[i]!); total -= sizes[i]!; }
}