The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision 866a2ed32b60.

I briefly looked at this and it seems benign, we are double-writing true to sWebExtensionsRemoteWasEverDisabled. We can probably make that variable a relaxed atomic bool to avoid the race.

General information about TSan reports

Why fix races?

Data races are undefined behavior and can cause crashes as well as correctness issues. Compiler optimizations can cause racy code to have unpredictable and hard-to-reproduce behavior.

Rating

If you think this race can cause crashes or correctness issues, it would be great to rate the bug appropriately as P1/P2 and/or indicating this in the bug. This makes it a lot easier for us to assess the actual impact that these reports make and if they are helpful to you.

False Positives / Benign Races

Typically, races reported by TSan are not false positives [1], but it is possible that the race is benign. Even in this case it would be nice to come up with a fix if it is easily doable and does not regress performance. Every race that we cannot fix will have to remain on the suppression list and slows down the overall TSan performance. Also note that seemingly benign races can possibly be harmful (also depending on the compiler, optimizations and the architecture) [2][3].

[1] One major exception is the involvement of uninstrumented code from third-party libraries.
[2] http://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong
[3] How to miscompile programs with "benign" data races: https://www.usenix.org/legacy/events/hotpar11/tech/final_files/Boehm.pdf

Suppressing unfixable races

If the bug cannot be fixed, then a runtime suppression needs to be added in mozglue/build/TsanOptions.cpp. The suppressions match on the full stack, so it should be picked such that it is unique to this particular race. The bug number of this bug should also be included so we have some documentation on why this suppression was added.

Yes; I realized when I was writing the code that sWebExtensionsRemoteWasEverDisabled could race between write and check; or write and write but it's benign. Didn't realize TSAN would catch it. I will look into that Relaxed thing to (a) fix the TSAN issue and (b) provide a sample commit you could link people to in the future.

This boolean may be written from multiple threads at the same time.
That causes a data race that is detected by TSAN. In this case it is
benign because we're setting it to the same value (true); but by
switching it to Atomic Relaxed bool, we will prevent the race and
the TSAN report.

There's a r+ patch which didn't land and no activity in this bug for 2 weeks.
:tjr, could you have a look please?
For more information, please visit auto_nag documentation.

Tom, there is an r+ patch that apparently never landed. Can we land it, is something blocking us or has this bug become invalid in the meantime?

While we can land it, the better fix is to refactor this code due to other changes that happened with this pref. So land if you wish, but I want to eventually refactor it. Leaving ni.

(In reply to Tom Ritter [:tjr] (OOTO until 6/1?) from comment #6)

While we can land it, the better fix is to refactor this code due to other changes that happened with this pref. So land if you wish, but I want to eventually refactor it. Leaving ni.

I haven't really looked at the patch, just trying to find the right severity - if you think S3 is wrong, please update - thanks!

Bug 1502525 removed the ability for the Web Extension process to be toggled mid-session.
We don't need this weird edge-case feature anymore.

https://hg.mozilla.org/mozilla-central/rev/718f52b9618c

https://hg.mozilla.org/mozilla-central/rev/b56287027158