opt out of accursedUnutterablePerformIO

[Ben Franksen]

Am 04.03.2018 um 22:48 schrieb Ben Gamari:
>> Ben Franksen <ben.franksen at online.de> writes:
>> Anyway. Would you perchance have any idea what could possibly make a
>> program work fine with the first version of linesPS and crash with the
>> second one? I find this pretty scary and would like to understand it.
>>
> Indeed that is quite scary. The cause is not at all obvious. Do you have
> an isolated reproducer that you could share?

Sharing is no problem, this happened with darcs, which notably has its
own share of dark and dangerous corners. I have considered trying to
cook this down to some small example but my hopes of succeeding with
this are almost nil. As I said, I have no evidence whatsoever that the
problem has even anything to do with that-what-must-not-be-named, it's
been purely a guess.

FWIW, I re-checked that the behavior is reproducible. I have uploaded a
clone of darcs-screened with my patch applied on top to
https://hub.darcs.net/bf/darcs-screened
To reproduce the crash, cabal configure with --enable-tests, then do
./dist...whatever...darcs-test --shell=no
Unpull the last patch and everything works.

Cheers
Ben