Handling concurrent access to resource

concurrency.handler.ts

type PendingRequests = { [url: string]: Function[] }

// Initialize an object to track pending resource requests
let pendingRequests: PendingRequests = {};

/**
 * Check if there is a pending request for a given URL.
 * @param {string} url - The URL to check for pending requests.
 * @returns {boolean} - True if there are pending requests, false otherwise.
 */
export function hasPendingRequest(url: string) {
  return pendingRequests.hasOwnProperty(url);
}

/**
 * Lock a resource to prevent concurrent requests for the same URL.
 * @param {string} url - The URL of the resource to lock.
 */
export function lockResource(url: string) {
  if (!pendingRequests[url]) {
    // Create a queue for pending requests for this URL if it doesn't exist
    pendingRequests[url] = [];
  }
}

/**
 * Wait for a resource request to complete. This function returns a promise that
 * resolves when the request finishes.
 * @param {string} url - The URL of the resource being requested.
 * @returns {Promise<Response>} - A promise that resolves when the request completes.
 */
export function waitForResourceCompletion(url: string): Promise<Response> {
  // We wait until the ongoing request finishes by queuing the promise until completion
  const waitForRequestPromise = new Promise<Response>((resolve) => {
    // Add the resolve from promise to the queue for this URL
    pendingRequests[url].push(resolve);
  });
  return waitForRequestPromise;
}

/**
 * Notify that a resource request has completed and resolve any pending requests
 * for the same URL.
 * @param {string} url - The URL of the completed resource request.
 * @param {Response} response - The response of the completed request.
 */
export function notifyResourceCompletion(url: string, response: Response) {
  // Notify other requests in the queue and remove the URL entry
  if (pendingRequests[url]?.length > 0) {
    pendingRequests[url].forEach((resolve) => {
      // Resolve each pending request with a clone of the response
      resolve(response.clone());
    });
  }
  // Remove the URL entry as all requests are now resolved
  delete pendingRequests[url];
}

/**
 * Clear the queue of pending resource requests.
 */
export function clearPendingRequestQueue() {
  // Reset the pendingRequests object to clear all pending requests
  pendingRequests = {};
}