Function fetchAndCacheOnce

packages/service-worker/worker/src/assets.ts:300–364

* Fetch the given resource from the network, and cache it if able.

(req: Request, used: boolean = true)

   * Fetch the given resource from the network, and cache it if able.
   */
  protected async fetchAndCacheOnce(req: Request, used: boolean = true): Promise<Response> {
    // The `inFlightRequests` map holds information about which caching operations are currently
    // underway for known resources. If this request appears there, another "thread" is already
    // in the process of caching it, and this work should not be duplicated.
    if (this.inFlightRequests.has(req.url)) {
      // There is a caching operation already in progress for this request. Wait for it to
      // complete, and hopefully it will have yielded a useful response.
      return this.inFlightRequests.get(req.url)!;
    }

    // No other caching operation is being attempted for this resource, so it will be owned here.
    // Go to the network and get the correct version.
    const fetchOp = this.fetchFromNetwork(req);

    // Save this operation in `inFlightRequests` so any other "thread" attempting to cache it
    // will block on this chain instead of duplicating effort.
    this.inFlightRequests.set(req.url, fetchOp);

    // Make sure this attempt is cleaned up properly on failure.
    try {
      // Wait for a response. If this fails, the request will remain in `inFlightRequests`
      // indefinitely.
      const res = await fetchOp;

      // It's very important that only successful responses are cached. Unsuccessful responses
      // should never be cached as this can completely break applications.
      if (!res.ok) {
        throw new Error(
          `Response not Ok (fetchAndCacheOnce): request for ${req.url} returned response ${res.status} ${res.statusText}`,
        );
      }

      try {
        // This response is safe to cache (as long as it's cloned). Wait until the cache operation
        // is complete.
        const cache = await this.cache;
        await cache.put(req, res.clone());

        // If the request is not hashed, update its metadata, especially the timestamp. This is
        // needed for future determination of whether this cached response is stale or not.
        if (!this.hashes.has(this.adapter.normalizeUrl(req.url))) {
          // Metadata is tracked for requests that are unhashed.
          const meta: UrlMetadata = {ts: this.adapter.time, used};
          const metaTable = await this.metadata;
          await metaTable.write(req.url, meta);
        }

        return res;
      } catch (err) {
        // Among other cases, this can happen when the user clears all data through the DevTools,
        // but the SW is still running and serving another tab. In that case, trying to write to the
        // caches throws an `Entry was not found` error.
        // If this happens the SW can no longer work correctly. This situation is unrecoverable.
        throw new SwCriticalError(
          `Failed to update the caches for request to '${
            req.url
          }' (fetchAndCacheOnce): ${errorToString(err)}`,
        );