Bug 1694515 - Part 3: Add BackgroundTasksUtils method for reading Telemetry client ID. r=bytesized,chutten

This will be used from the `backgroundupdate` task to fish the default
profile's Telemetry client ID in order to correlate the task's Glean
ping with regular main pings.

Differential Revision: https://phabricator.services.mozilla.com/D107712

toolkit/components/backgroundtasks/BackgroundTasksUtils.jsm

@@ -154,4 +154,37 @@ var BackgroundTasksUtils = {
     log.debug(`readPreferences: parsed prefs from buffer`, prefs);
     return prefs;
   },
+
+  /**
+   * Reads the Telemetry Client ID out of a profile.
+   *
+   * If no `lock` is given, the default profile is locked and the preferences
+   * read from it.  If `lock` is given, read from the given lock's directory.
+   *
+   * @param {nsIProfileLock} [lock] optional lock to use
+   * @returns {string}
+   */
+  async readTelemetryClientID(lock = null) {
+    if (!lock) {
+      return this.withProfileLock(profileLock =>
+        this.readTelemetryClientID(profileLock)
+      );
+    }
+
+    this._throwIfNotLocked(lock);
+
+    let stateFile = lock.directory.clone();
+    stateFile.append("datareporting");
+    stateFile.append("state.json");
+
+    log.info(
+      `readPreferences: will read Telemetry client ID from ${stateFile.path}`
+    );
+
+    // This JSON is always UTF-8.
+    let data = await IOUtils.readUTF8(stateFile.path);
+    let state = JSON.parse(data);
+
+    return state.clientID;
+  },
 };

toolkit/components/backgroundtasks/tests/xpcshell/test_backgroundtasksutils.js

@@ -97,3 +97,41 @@ add_task(async function test_readPreferences() {
   };
   Assert.deepEqual(prefs, expected, "Filtered prefs read are correct");
 });
+
+add_task(async function test_readTelemetryClientID() {
+  let profileService = Cc["@mozilla.org/toolkit/profile-service;1"].getService(
+    Ci.nsIToolkitProfileService
+  );
+
+  let profilePath = do_get_profile();
+  profilePath.append(`test_readTelemetryClientID`);
+  let profile = profileService.createUniqueProfile(
+    profilePath,
+    "test_readTelemetryClientID"
+  );
+
+  // Before we write any state, we fail to read.
+  await Assert.rejects(
+    BackgroundTasksUtils.withProfileLock(
+      lock => BackgroundTasksUtils.readTelemetryClientID(lock),
+      profile
+    ),
+    /NotFoundError/
+  );
+
+  let expected = {
+    clientID: "9cc75672-6830-4cb6-a7fd-089d6c7ce34c",
+    ecosystemClientID: "752f9d53-73fc-4006-a93c-d3e2e427f238",
+  };
+  let prefsFile = profile.rootDir.clone();
+  prefsFile.append("datareporting");
+  prefsFile.append("state.json");
+  await IOUtils.writeUTF8(prefsFile.path, JSON.stringify(expected));
+
+  // Now we can read the state.
+  let state = await BackgroundTasksUtils.withProfileLock(
+    lock => BackgroundTasksUtils.readTelemetryClientID(lock),
+    profile
+  );
+  Assert.deepEqual(state, expected.clientID, "State read is correct");
+});