po-resource.html has never been run in our automation tests because PerformanceObserver is not yet enabled by default. It will be enabled by bug 1271487. A try[1] run with enabling PerformanceObserver made me realize that there is a risk of intermittent failures in po-resource.html [1] https://treeherder.mozilla.org/logviewer.html#?job_id=21421337&repo=try#L15163 There is an obvious anti-pattern[2] to cause intermittent failures. We should fix it. https://dxr.mozilla.org/mozilla-central/rev/4d63dde701b47b8661ab7990f197b6b60e543839/testing/web-platform/tests/performance-timeline/po-resource.html#21

Review commit: https://reviewboard.mozilla.org/r/56084/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/56084/

Hi, Joel. Could you please take a look this? I am not sure you are a proper person for reviewing this web platform test, but I know you have tremendous amount of knowledge of automation test field. A try run with this patch: https://treeherder.mozilla.org/#/jobs?repo=try&revision=657984a2903a Thank you,

Comment on attachment 8757674 [details] MozReview Request: Bug 1276490 - Do not use Math.random() to create unique values to avoid intermittent failrues. r?jgraham https://reviewboard.mozilla.org/r/56084/#review52734 I see no problem with this- although img2 was theoretically 10x the value of img1 and now they will roughly be the same magnitude. I am going to ask jgraham to review this- I do not know if this needs to be done upstream as well.

I guess this is OK, but I think it's unnecessary; in practice we run tests in a clean profile, so the only chance of a collision is from the two random numbers being identical in a single run. The probability of that is sufficiently small that it doesn't matter. But if you want to land this, please fix the typo in the commit message.

Comment on attachment 8757674 [details] MozReview Request: Bug 1276490 - Do not use Math.random() to create unique values to avoid intermittent failrues. r?jgraham https://reviewboard.mozilla.org/r/56084/#review52986 Typo in the commit message.

Thank you, jgraham. Right, the collision must be rare, but it actually happened the first try. I guess it's because I like oranges. ;-)

Comment on attachment 8757674 [details] MozReview Request: Bug 1276490 - Do not use Math.random() to create unique values to avoid intermittent failrues. r?jgraham Review request updated; see interdiff: https://reviewboard.mozilla.org/r/56084/diff/1-2/