1402609 - Empty fields on about:support probably because of invalid header in experiments.json file

Status: RESOLVED FIXED

(Toolkit :: Telemetry, defect, P1)

Description

[Marco Castelluccio [:marco]]

When I open about:support, the structure of the page is there, but all the fields are empty.

There's the following exception in the browser console:
> Error: uncaught exception: Invalid header (no magic number) - Data: C\Users\Marco\AppData\Roaming\Mozilla\Profiles\fv9zh0no.default-1387804988712\experiments.json

Comment 1

[Mike Taylor [:miketaylr]]

Does this still happen for you Marco? It WFM on Mac and Win10.

Comment 2

[Marco Castelluccio [:marco]]

Yes, I can still reproduce (not in a new profile, only in my used profile).

Comment 3

[Mike Taylor [:miketaylr]]

Interesting, thanks. Georg, have you ever seen any error like in Comment #0?

Comment 4

[Georg Fritzsche [:gfritzsche]]

I can reproduce an equivalent error using: echo "foo" > $PROFDIR/experiments.json
I think this is unlikely to be a regression. That file could be in a bad state for multiple reasons (disk/fs issues, crash while writing it, ...).

From looking at the code that fills out about:support:
https://dxr.mozilla.org/mozilla-central/rev/19b32a138d08f73961df878a29de6f0aad441683/toolkit/modules/Troubleshoot.jsm#143
... turns out, if any of the providers there doesn't invoke the "done" callback, about:support won't render anything.
If `Experiments.getExperiments()` fails/rejects, about:support will never render:
https://dxr.mozilla.org/mozilla-central/rev/19b32a138d08f73961df878a29de6f0aad441683/toolkit/modules/Troubleshoot.jsm#320

Moving on to Experiments.jsm... when loading of experiments.json fails, we only recover gracefully if it was because the file didn't exist:
https://dxr.mozilla.org/mozilla-central/rev/19b32a138d08f73961df878a29de6f0aad441683/browser/experiments/Experiments.jsm#1043
The proper solution there seems to be to always recover gracefully as we can't fix broken experiments.json files.

Comment 5

[Georg Fritzsche [:gfritzsche]]

Attachment: Part 1 - Fix missing rejection handling in Troubleshoot.jsm

Felipe, can you review this part? I see you in the log as a past reviewer.

Comment 6

[Georg Fritzsche [:gfritzsche]]

Attachment: Part 2 - Make Experiments.jsm recover from unloadable manifest cache

This fixes the Experiments.jsm part, to make it recover gracefully when we are unable to load the cache file.

Comment 7

[Georg Fritzsche [:gfritzsche]]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=808f03a1aea24cb35d742351bd46ebc521c5541a

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/3ea78aa38c05
Part 1: Fix missing rejection handling in Troubleshoot.jsm. r=felipe
https://hg.mozilla.org/integration/mozilla-inbound/rev/934f0c1076f5
Part 2: Make Experiments.jsm recover from unloadable manifest cache. r=Dexter

Comment 9

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

https://hg.mozilla.org/mozilla-central/rev/3ea78aa38c05
https://hg.mozilla.org/mozilla-central/rev/934f0c1076f5