Signage/src/browser/storage.ts

/**
 * A display's own local storage: **IndexedDB for structured state, Cache Storage for image bytes**
 * (ADR 0006).
 *
 * Ordinary HTTP cache is deliberately not used as the asset manager. It offers no eviction control,
 * no way to prune a superseded rendition on purpose, and no way to answer the one question a display
 * has to answer honestly — *what can I actually render if the network is gone?*
 *
 * Everything here is behind a small interface with an in-memory implementation, because a display's
 * offline behaviour is the part that must be tested and IndexedDB is the part a test runner cannot
 * reliably host. Both implementations satisfy the same contract; the browser one is thin.
 */

/** One key-value store of structured, JSON-serializable state. */
export interface SignageLocalStore {
  read<Value>(key: string): Promise<Value | undefined>;
  write(key: string, value: unknown): Promise<void>;
  delete(key: string): Promise<void>;
}

/**
 * Image bytes a display can still paint with no network.
 *
 * Everything is addressed by a **stable content key**, never by the delivery URL. A delivery URL is
 * signed and short-lived: it changes on every refresh of the same bytes, so a cache keyed on one
 * would store the same image over and over and find none of it after a reconnect. The key is derived
 * from identity that does not change — the provider, the item, and the item's revision — so the
 * bytes a display cached an hour ago are the bytes it can still paint now.
 */
export interface SignageMediaCache {
  /** Fetches from `url` and stores under `key`, returning a local URL to render from. */
  store(key: string, url: string): Promise<string | undefined>;
  /** A local URL for previously stored bytes, or `undefined` if they were never stored. */
  find(key: string): Promise<string | undefined>;
  /** Removes every key not in the keep set. Called only after a new snapshot is usable, so a prune
   * can never leave a screen with less than it had. */
  prune(keep: readonly string[]): Promise<void>;
  count(): Promise<number>;
  /** Gives back every local URL this cache handed out. Called when a display stops; a URL handed
   * out before this is no longer valid afterwards. */
  release(): void;
}

/** The stable key a native image's bytes are cached under. Deliberately built from identity, never
 * from a URL. */
export function signageMediaKey(input: {
  readonly providerId: string;
  readonly itemId: string;
  readonly itemRevision: string;
  readonly rendition: string;
}): string {
  return `signage-media/${encodeURIComponent(input.providerId)}/${encodeURIComponent(input.itemId)}/${encodeURIComponent(input.itemRevision)}/${input.rendition}`;
}

export const SIGNAGE_LOCAL_DATABASE = "ekklesia-signage";
export const SIGNAGE_LOCAL_STORE = "state";
export const SIGNAGE_MEDIA_CACHE = "ekklesia-signage-media-v1";

export function createMemoryStore(): SignageLocalStore {
  const values = new Map<string, unknown>();
  return {
    delete(key: string): Promise<void> {
      values.delete(key);
      return Promise.resolve();
    },
    read<Value>(key: string): Promise<Value | undefined> {
      return Promise.resolve(values.get(key) as Value | undefined);
    },
    write(key: string, value: unknown): Promise<void> {
      // Structured state only. Round-tripping through JSON here is what keeps a `Date`, a function,
      // or a cycle from being written now and failing to come back later.
      values.set(key, JSON.parse(JSON.stringify(value)) as unknown);
      return Promise.resolve();
    },
  };
}

export function createMemoryMediaCache(): SignageMediaCache {
  const stored = new Map<string, string>();
  return {
    count: () => Promise.resolve(stored.size),
    find: (key: string) => Promise.resolve(stored.get(key)),
    prune(keep: readonly string[]): Promise<void> {
      const retained = new Set(keep);
      for (const key of [...stored.keys()]) {
        if (!retained.has(key)) {
          stored.delete(key);
        }
      }
      return Promise.resolve();
    },
    release(): void {
      // Holds plain URLs, nothing the browser has to be given back.
    },
    store(key: string, url: string): Promise<string | undefined> {
      stored.set(key, url);
      return Promise.resolve(url);
    },
  };
}

/** The IndexedDB-backed store a real display uses. Falls back to memory where IndexedDB is absent —
 * a private window, a policy that blocks storage — because a display that cannot persist must still
 * render, online. */
export function createBrowserStore(scope: typeof globalThis = globalThis): SignageLocalStore {
  const maybeIndexedDb = (scope as { indexedDB?: IDBFactory }).indexedDB;
  if (!maybeIndexedDb) {
    return createMemoryStore();
  }
  const indexedDb: IDBFactory = maybeIndexedDb;

  function open(): Promise<IDBDatabase> {
    return new Promise((resolve, reject) => {
      const request = indexedDb.open(SIGNAGE_LOCAL_DATABASE, 1);
      request.onupgradeneeded = () => {
        const database = request.result;
        if (!database.objectStoreNames.contains(SIGNAGE_LOCAL_STORE)) {
          database.createObjectStore(SIGNAGE_LOCAL_STORE);
        }
      };
      request.onsuccess = () => resolve(request.result);
      request.onerror = () => reject(new Error("Signage could not open its local store."));
    });
  }

  async function transact<Result>(
    mode: IDBTransactionMode,
    run: (store: IDBObjectStore) => IDBRequest,
  ): Promise<Result | undefined> {
    let database: IDBDatabase;
    try {
      database = await open();
    } catch {
      return undefined;
    }
    return new Promise<Result | undefined>((resolve) => {
      try {
        const transaction = database.transaction(SIGNAGE_LOCAL_STORE, mode);
        const request = run(transaction.objectStore(SIGNAGE_LOCAL_STORE));
        request.onsuccess = () => resolve(request.result as Result | undefined);
        request.onerror = () => resolve(undefined);
        transaction.oncomplete = () => database.close();
      } catch {
        resolve(undefined);
      }
    });
  }

  return {
    async delete(key: string): Promise<void> {
      await transact("readwrite", (store) => store.delete(key));
    },
    read<Value>(key: string): Promise<Value | undefined> {
      return transact<Value>("readonly", (store) => store.get(key));
    },
    async write(key: string, value: unknown): Promise<void> {
      await transact("readwrite", (store) => store.put(value, key));
    },
  };
}

/** The Cache Storage-backed media cache a real display uses. Absent Cache Storage — an insecure
 * origin in development, a policy that refuses — a display works exactly as it did before: online. */
export function createBrowserMediaCache(scope: typeof globalThis = globalThis): SignageMediaCache {
  const maybeCaches = (scope as { caches?: CacheStorage }).caches;
  const fetchFn = (scope as { fetch?: typeof fetch }).fetch;
  if (!maybeCaches || !fetchFn) {
    return createMemoryMediaCache();
  }
  const caches: CacheStorage = maybeCaches;

  async function open(): Promise<Cache | undefined> {
    try {
      return await caches.open(SIGNAGE_MEDIA_CACHE);
    } catch {
      return undefined;
    }
  }

  /** Cache Storage keys on a request URL, so a stable key becomes a stable absolute URL on this
   * origin. It is never fetched; it exists only to be a key. */
  function cacheRequest(key: string): string {
    const origin =
      (scope as { location?: { origin?: string } }).location?.origin ?? "https://signage.invalid";
    return `${origin}/${key}`;
  }

  /**
   * The one object URL each cached slide is rendered from, by cache entry.
   *
   * An object URL pins its blob in the browser until it is revoked. Every state refresh looks every
   * cached slide up again, so minting a URL per lookup leaked a blob per slide per refresh, and
   * within minutes the browser refused the display's own requests for want of resources. So each
   * entry gets exactly one URL, revoked when the entry is pruned or replaced, and all of them when
   * the display stops.
   */
  const objectUrls = new Map<string, string>();

  function forget(request: string): void {
    const url = objectUrls.get(request);
    if (url !== undefined) {
      objectUrls.delete(request);
      URL.revokeObjectURL(url);
    }
  }

  async function localUrl(cache: Cache, request: string): Promise<string | undefined> {
    const existing = objectUrls.get(request);
    if (existing !== undefined) {
      return existing;
    }
    const response = await cache.match(request);
    if (!response) {
      return undefined;
    }
    try {
      const blob = await response.blob();
      // Two lookups of one slide can be in flight at once; the first to finish wins, and the other
      // gives its URL straight back rather than leaving it pinned.
      const raced = objectUrls.get(request);
      if (raced !== undefined) {
        return raced;
      }
      const url = URL.createObjectURL(blob);
      objectUrls.set(request, url);
      return url;
    } catch {
      return undefined;
    }
  }

  return {
    async count(): Promise<number> {
      const cache = await open();
      if (!cache) {
        return 0;
      }
      try {
        return (await cache.keys()).length;
      } catch {
        return 0;
      }
    },

    async find(key: string): Promise<string | undefined> {
      const cache = await open();
      return cache ? localUrl(cache, cacheRequest(key)) : undefined;
    },

    async prune(keep: readonly string[]): Promise<void> {
      const cache = await open();
      if (!cache) {
        return;
      }
      const retained = new Set(keep.map((key) => cacheRequest(key)));
      try {
        for (const request of await cache.keys()) {
          if (!retained.has(request.url)) {
            await cache.delete(request);
            forget(request.url);
          }
        }
      } catch {
        // A prune that fails leaves a screen with more than it needs, which is the safe direction.
      }
    },

    release(): void {
      for (const request of [...objectUrls.keys()]) {
        forget(request);
      }
    },

    async store(key: string, url: string): Promise<string | undefined> {
      const cache = await open();
      if (!cache) {
        return undefined;
      }
      try {
        const response = await fetchFn(url);
        // Only a complete, successful response is stored. A `404` or a redirect replayed later as a
        // slide would be worse than having nothing cached.
        if (!response.ok || response.status !== 200) {
          return undefined;
        }
        // Stored under the *stable* key, so the next refresh's freshly signed URL finds these
        // bytes instead of downloading them again.
        await cache.put(cacheRequest(key), response.clone());
        // New bytes under this key: the URL for the old ones is given back before one is made.
        forget(cacheRequest(key));
        return localUrl(cache, cacheRequest(key));
      } catch {
        return undefined;
      }
    },
  };
}