Attempting to draw a tooltip for a flat marker track (where all samples have the same value) causes a "Error: No samples for marker CPU" about profiler

[fqueze]

Can be reproduced with the profiles at https://treeherder.mozilla.org/jobs?repo=autoland&revision=f768135b85f29ef20a22825a054d1a6938475ba5&searchStr=a51&selectedTaskRun=XDBFxGowQwKYx0YAJk_1YQ.0

Somehow the resource monitoring fails for these jobs / on that hardware (Samsung A51 in bitbar). The test log says for example:
"Total resource usage - Wall time: 1843s; CPU: Can't collect data; Read bytes: 0; Write bytes: 0; Read time: 0; Write time: 0"

[julienw]

As another example see this profile from #4845 https://share.firefox.dev/3tcvuuo

[julienw]

I think the problem isn't about "having the same value" but the fact the marker schema for the "CPU" marker has an empty "graphs" array (empty as in "array with no items"). So I believe the error is on the code that generates this profile. Although it woulkd be good to not crash at it, or at least crash with a better error.

[julienw]

I made a PR to avoid the crash.

I think this is failing here:
https://searchfox.org/mozilla-central/rev/fa86401b80f19afb6ed9bfca127ecc5e3a6f0cdc/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py#1126-1135

From the comment it looks like you planned to add just the right properties to graphs but you forgot, what do you think @fqueze ?

[julienw]

Actually this happens a bit later in https://searchfox.org/mozilla-central/rev/fa86401b80f19afb6ed9bfca127ecc5e3a6f0cdc/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py#1156-1157
So maybe having an empty graphs array is the expected outcome, and my PR is just the right fix.

[fqueze]

Actually this happens a bit later in https://searchfox.org/mozilla-central/rev/fa86401b80f19afb6ed9bfca127ecc5e3a6f0cdc/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py#1156-1157 So maybe having an empty graphs array is the expected outcome, and my PR is just the right fix.

We could make the code at
https://searchfox.org/mozilla-central/rev/fa86401b80f19afb6ed9bfca127ecc5e3a6f0cdc/testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py#1148-1157 check len(cpuGraphs) and delete the "graphs" property entirely when it's empty, but your fix looks good enough to me in terms of error handling.
In any case this is an unexpected edge case where resourcemonitor fails to access the data from psutil, so the 'right' fix would be to figure out why it's not working (I'm guessing a python package is missing on some of our builders in CI).

Thanks for looking into it!