fix(GUI): provide a friendly message when no polkit agent is available (#1221)

`sudo-prompt`, the module we use to provide elevation for GNU/Linux,
relies on polkit to show the elevation dialog. Polkit implements a
"backend" to control privileges, but relies on different packages to
provide a GUI frontend.

If no GUI frontend can be found on the system, then `sudo-prompt` will
fail with a `No polkit authentication agent found.` In this case, we
shouldn't present a scary stack trace to the user, and we should also
not send this error to TrackJS.

As a solution, we intercept such error in
`lib/child-writer/writer-proxy.js`, and convert it into a human readable
user error using `errors.createUserError()`, which is passed to the
Etcher GUI using the "robot" mechanism.

Fixes: https://github.com/resin-io/etcher/issues/1179
Change-Type: patch
Changelog-Entry: Provide a user friendly error message when no polkit authentication agent is available on the system.
Signed-off-by: Juan Cruz Viotti <jviotti@openmailbox.org>

lib/child-writer/index.js

@@ -113,25 +113,43 @@ exports.write = (image, drive, options) => {
     emitter.emit('error', error);
   };
 
-  ipc.server.on('error', emitError);
-  ipc.server.on('message', (data) => {
-    let message = null;
-
+  /**
+   * @summary Bridge robot message to the child writer caller
+   * @function
+   * @private
+   *
+   * @param {String} message - robot message
+   *
+   * @example
+   * bridgeRobotMessage(robot.buildMessage('foo', {
+   *   bar: 'baz'
+   * }));
+   */
+  const bridgeRobotMessage = (message) => {
     try {
-      message = robot.parseMessage(data);
+      const parsedMessage = robot.parseMessage(message);
+
+      // These are lighweight accessor methods for
+      // the properties of the parsed message
+      const messageCommand = robot.getCommand(parsedMessage);
+      const messageData = robot.getData(parsedMessage);
+
+      // The error object is decomposed by the CLI for serialisation
+      // purposes. We compose it back to an `Error` here in order
+      // to provide better encapsulation.
+      if (messageCommand === 'error') {
+        emitError(robot.recomposeErrorMessage(parsedMessage));
+      } else {
+        emitter.emit(messageCommand, messageData);
+      }
+
     } catch (error) {
-      return emitError(error);
+      emitError(error);
     }
+  };
 
-    // The error object is decomposed by the CLI for serialisation
-    // purposes. We compose it back to an `Error` here in order
-    // to provide better encapsulation.
-    if (robot.getCommand(message) === 'error') {
-      return emitError(robot.recomposeErrorMessage(message));
-    }
-
-    return emitter.emit(robot.getCommand(message), robot.getData(message));
-  });
+  ipc.server.on('error', emitError);
+  ipc.server.on('message', bridgeRobotMessage);
 
   ipc.server.on('start', () => {
     const child = childProcess.fork(CONSTANTS.WRITER_PROXY_SCRIPT, argv, {
@@ -144,7 +162,7 @@ exports.write = (image, drive, options) => {
     });
 
     child.stderr.on('data', (data) => {
-      emitError(new Error(data.toString()));
+      bridgeRobotMessage(data.toString());
 
       // This function causes the `close` event to be emitted
       child.kill();

lib/child-writer/writer-proxy.js

@@ -28,6 +28,7 @@ const sudoPrompt = Bluebird.promisifyAll(require('sudo-prompt'));
 const utils = require('./utils');
 const EXIT_CODES = require('../shared/exit-codes');
 const errors = require('../shared/errors');
+const robot = require('../shared/robot');
 const packageJSON = require('../../package.json');
 
 // This script is in charge of spawning the writer process and
@@ -153,10 +154,23 @@ return isElevated().then((elevated) => {
       if (!_.isEmpty(stderr)) {
         throw errors.createError(stderr);
       }
+
+    // We're hardcoding internal error messages declared by `sudo-prompt`.
+    // There doesn't seem to be a better way to handle these errors, so
+    // for now, we should make sure we double check if the error messages
+    // have changed every time we upgrade `sudo-prompt`.
+
     }).catch({
       message: 'User did not grant permission.'
     }, () => {
       process.exit(EXIT_CODES.CANCELLED);
+    }).catch({
+      message: 'No polkit authentication agent found.'
+    }, () => {
+      throw errors.createUserError(
+        'No polkit authentication agent found',
+        'Please install a polkit authentication agent for your desktop environment of choice to continue'
+      );
     });
   }
 
@@ -226,6 +240,6 @@ return isElevated().then((elevated) => {
     process.exit(exitCode);
   });
 }).catch((error) => {
-  console.error(error);
+  robot.printError(error);
   process.exit(EXIT_CODES.GENERAL_ERROR);
 });