Electrolysis/e10s test tips

There isn't a standard formula to use to fix broken tests for e10s. Instead, one has to read the test and look for potential problem areas. Sometimes, it's pretty obvious (there are a lot of browser-chrome tests that use contentWindow or contentDocument). Other times it's much more subtle.

Here are a few tips to look for.

About CPOWs

CPOWs are allowed in tests but tend to be a little flaky (search for "dead CPOW" in bugzilla) so it's probably worth spending a little extra time to use message passing and setting things up "correctly".

Mochitest (plain)

• Setting prefs
  • Prefs should only be set using SpecialPowers (pushPrefEnv).
  • Getting prefs is fine.

• Setting permissions
  • SpecialPowers.addPermission() is racy, so use SpecialPowers.pushPermissions() instead.

• Observers:
  • Only receive observer notifications for the content process (possibly OK).
  • SpecialPowers.addObserver does the Right Thing (TM).

• Services
  • Some services are only available in the parent process (form history, window watcher, etc).
  • SpecialPowers.loadChromeScript allows you to have a "worker" in the parent process with which to communicate.
  • SpecialPowers.importInMainProcess("resource://gre/modules/SomeService.jsm") will call Cu.import in the main process with the given URI, which is useful when your test needs to start some main process service.

• SpecialPowers.wrap
  • Sometimes OK (getting window utils, controller, etc.)
  • Sometimes not (getting the chrome window).
  • Fixing might require something like loadChromeScript.

• SpecialPowers.Cc
  • Big red flag. Especially for windowwatcher!

Browser-chrome

• Use BrowserTestUtils!
  • Lots of examples in the tree.
  • This + ContentTask.spawn make for nice control flow.
  • Can also use browser.messageManager.loadFrameScript("data:,(" + fn.toString() + ")()")

• browser.contentWindow/contentDocument
  • Tests that use this are asking to fail!
  • Replace uses with ContentTask.spawn to retrieve needed information.