Thanks for the insight, much appreciated. I'll let Andrew answer regarding the indexedDB behavior with file descriptors, I don't know about it.
I'd have no problem temporarily testing a patch in Nightly, but the crash spike is only visible in Beta, Nightly has a few reports, but not many, that means either some timing difference is covering the crash, something changed in indexedDB, or just that the population is too small to hit this case often enough.
My fear is that we couldn't observe a meaningful shift in the number of crashes with the Nightly population, to be able to extrapolate reasoning out of it.
Testing your patch in Beta is a question for release drivers, I can't make such a decision by myself. The patch looks unharmful and from a technical point of view it should have no effect on any of our measurements.
Julien, what do you think, would release drivers allow this small diagnostic patch (see the link in comment 7) in Beta for a week?