The remote agent has unit- and functional tests located under remote/test/{unit,browser}.

You may run all the tests under a particular subfolder like this:

% ./mach test remote

Unit tests

Because tests are run in parallel and xpcshell itself is quite chatty, it can sometimes be useful to run the tests in sequence:

% ./mach xcpshell-test --sequential remote/test/unit/test_Assert.js

The unit tests will appear as part of the X (for xpcshell) jobs on Treeherder.

Browser chrome tests

We also have a set of functional browser chrome tests located under remote/test/browser:

% ./mach mochitest remote/test/browser/browser_cdp.js

The functional tests will appear under the M (for mochitest) category in the remote jobs on Treeherder.

As the functional tests will sporadically pop up new Firefox application windows, a helpful tip is to run them in headless mode:

% ./mach mochitest --headless remote/test/browser

The --headless flag is equivalent to setting the MOZ_HEADLESS environment variable. You can additionally use MOZ_HEADLESS_WIDTH and MOZ_HEADLESS_HEIGHT to control the dimensions of the virtual display.

The add_task() function used for writing asynchronous tests is replaced to provide some additional test setup and teardown useful for writing tests against the remote agent and the targets.

Before the task is run, the nsIRemoteAgent listener is started and a CDP client is connected. You will use this CDP client for interacting with the agent just as any other CDP client would.

Also target discovery is getting enabled, which means that targetCreated, targetDestroyed, and targetInfoChanged events will be received by the client.

The task function you provide in your test will be called with the three arguments client, CDP, and tab:

  • client is the connection to the nsIRemoteAgent listener, and it provides the a client CDP API

  • CDP is the CDP client class

  • tab is a fresh tab opened for each new test, and is automatically removed after the test has run

This is what it looks like all put together:

add_task(async function testName({client, CDP, tab}) {
  // test tab is implicitly created for us
  info("Current URL: " + tab.linkedBrowser.currentURI.spec);

  // manually connect to a specific target
  const { mainProcessTarget } = RemoteAgent.targets;
  const target = mainProcessTarget.wsDebuggerURL;
  const client = await CDP({ target });

  // retrieve the Browser domain, and call getVersion() on it
  const { Browser } = client;
  const version = await Browser.getVersion();

       await client.close();

  // tab is implicitly removed

You can control the tab creation behaviour with the createTab option to add_task(taskFunction, options):

add_task(async function testName({client}) {
  // tab is not implicitly created
}, { createTab: false });

If you want to write an asynchronous test without this implicit setup you may instead use add_plain_task(), which works exactly like the original add_task().

Puppeteer tests

In addition to our own Firefox-specific tests, we run the upstream Puppeteer test suite against our implementation to track progress towards achieving full Puppeteer support in Firefox.

These tests are vendored under remote/test/puppeteer/ and are run locally like this:

% ./mach test remote/test/puppeteer/test

On try they appear under the remote(pup) symbol, but because they’re a Tier-3 class test job they’re not automatically scheduled. To schedule the tests, look for source-test-remote-puppeteer in ./mach try fuzzy.

The tests are written in the behaviour-driven testing framework Mocha, which doesn’t support selecting tests by file path like other harnesses. It does however come with a myriad of flags to narrow down the selection a bit: some of the most useful ones include --grep, --ignore, and --file.

Perhaps the most useful trick is to rename the describe() or it() functions to fdescribe() and fit(), respectively, in order to run a specific subset of tests. This does however force you to edit the test files manually.

A real-world example:

diff --git a/remote/test/puppeteer/test/frame.spec.js b/remote/test/puppeteer/test/frame.spec.js
index 58e57934a7b8..0531d49d7a12 100644
--- a/remote/test/puppeteer/test/frame.spec.js
+++ b/remote/test/puppeteer/test/frame.spec.js
@@ -48,7 +48,7 @@ module.exports.addTests = function({testRunner, expect}) {

-  describe('Frame.evaluateHandle', function() {
+  fdescribe('Frame.evaluateHandle', function() {
     it('should work', async({page, server}) => {
       await page.goto(server.EMPTY_PAGE);
       const mainFrame = page.mainFrame();
@@ -58,7 +58,7 @@ module.exports.addTests = function({testRunner, expect}) {

   describe('Frame.evaluate', function() {
-    it('should throw for detached frames', async({page, server}) => {
+    fit('should throw for detached frames', async({page, server}) => {
       const frame1 = await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       await utils.detachFrame(page, 'frame1');
       let error = null;