/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

const Services = require("Services");

loader.lazyRequireGetter(this, "asyncStorage", "devtools/shared/async-storage");

/**
 * Downloads and caches a JSON file from an URL given by a pref.
 *
 * @param {String} prefName
 *        The preference for the target URL
 *
 * @return {Promise}
 *         - Resolved with the JSON object in case of successful request
 *           or cache hit
 *         - Rejected with an error message in case of failure
 */
exports.getJSON = function(prefName) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest();

    // We used to store cached data in preferences, but now we use asyncStorage
    // Migration step: if it still exists, move this now useless preference in its
    // new location and clear it
    if (Services.prefs.prefHasUserValue(prefName + "_cache")) {
      const json = Services.prefs.getCharPref(prefName + "_cache");
      asyncStorage.setItem(prefName + "_cache", json).catch(function(e) {
        // Could not move the cache, let's log the error but continue
        console.error(e);
      });
      Services.prefs.clearUserPref(prefName + "_cache");
    }

    function readFromStorage(networkError) {
      asyncStorage
        .getItem(prefName + "_cache")
        .then(function(json) {
          if (!json) {
            return reject("Empty cache for " + prefName);
          }
          return resolve(json);
        })
        .catch(function(e) {
          reject(
            "JSON not available, CDN error: " +
              networkError +
              ", storage error: " +
              e
          );
        });
    }

    xhr.onload = () => {
      try {
        const json = JSON.parse(xhr.responseText);
        asyncStorage.setItem(prefName + "_cache", json).catch(function(e) {
          // Could not update cache, let's log the error but continue
          console.error(e);
        });
        resolve(json);
      } catch (e) {
        readFromStorage(e);
      }
    };

    xhr.onerror = e => {
      readFromStorage(e);
    };

    xhr.open("get", Services.prefs.getCharPref(prefName));
    xhr.send();
  });
};