Subject: bug#46722: test-map-into fails

From: Stefan Monnier <[email-masked]>
Date: Tue, 23 Feb 2021 12:30:12 -0500

>> Something between 8d5dfaf and efe42c2 (inclusive) causes test-map-into
>> to reliably fail on hydra.nixos.org. We can't be more specific since
>> intermediate revisions don't build. So far I could not reproduce the issue.
>>
>> Ref eg https://hydra.nixos.org/build/137474120
>>
>> (cl-no-applicable-method map-into ((a . 1) (b . 2)) (hash-table :test equal))
>
> FWIW, I've been working on map.el and map-tests.el since yesterday, and
> I occasionally run into this heisenerror locally maybe once in every 50
> runs of 'make test/map-tests RET'.

This kind of code is supposed to be fully deterministic, so it's quite
worrisome.  It sounds/smells like memory corruption or some other bug in
the memory management.

If you could come up with a reasonably simple code (in the sense that it
doesn't take too long to run) to reproduce this problem with a high
probability, maybe we can start adding some tracing to try and figure
out what's going on.

IOW, try and write a loop that runs the problematic code a couple
hundred times, so that one of those should fail.  Then try and cut down
the code as much as possible, verifying along the way that the problem
didn't disappear.

        Stefan