1756910 - Write test to ensure scripts aren't loaded in content process before content-process-ready-for-script

Status: NEW

(Core :: XPCOM, task)

Description

[Michael Comella (:mcomella) [NI reported issues only: ex-Mozilla]]

In bug 1632682, we implemented a new observer topic, content-process-ready-for-script, and moved all scripts in the content process to start at or after that topic to ensure they are cached and have optimal performance. To prevent regressions, we should write a test to ensure no scripts are started before content-process-ready-for-script in the content process.

Comment 1

[Michael Comella (:mcomella) [NI reported issues only: ex-Mozilla]]

For the impact a regression could cause, see https://bugzilla.mozilla.org/show_bug.cgi?id=1632682#c5 and https://bugzilla.mozilla.org/show_bug.cgi?id=1632682#c8.

Here are some suggestions from :mccr8 in the dependent bug's review on what this test could look like:

@mccr8 Do you know if this is feasible to write a test for? If so, can you point me in the right direction?

It would probably be possible to write a test that checks which scripts can be loaded at various phases in the content process. browser/base/content/test/performance/browser_startup.js does that for the main process, and the browser_startup_content.js test in the same directory checks which scripts we load in content processes, so presumably you could modify the latter to check scripts a bit. I think it is a bit tricky because there's no guarantee about the order that observers are called in, so they have to check what scripts are loaded in the next phase. For app-startup in the main process, it looks like the test is using "profile-do-change", but I'm not sure that exists in the content process. I'm not sure if it is worth the hassle.