Re: [PATCH] Makefile: suppress annotated leaks with certain ASan options

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> I do think there's some complexity here, though.
>
> One problem UNLEAK() is that compile-time switch, but whether ASan does
> leak detection is a run-time choice. So you are stuck with either:
>
>   - you always turn on UNLEAK() for ASan builds, in which case test runs
>     using the default ASAN_OPTIONS we set do the extra work even though
>     they are not doing any leak detection. I doubt it's very measurable,
>     though (it's just shoving a few bytes onto a linked list),
>     especially compared to the overall slowness of ASan.
>
>   - you predicate the build-time choice on ASAN_OPTIONS. But this means
>     that:
>
>       make SANITIZE=address
>       cd t
>       ASAN_OPTIONS=detect_leaks=1 ./t0000-*.sh
>
>     will confusingly fail to use UNLEAK().
>
> Your patch does the second one, but I think the first may be the
> least-bad option.

Thanks, I totally missed the fact that ASAN_OPTIONS was a runtime
thing.  If we were to pursue this topic of enabling UNLEAK() outside
LSan, I agree the first would be necessary.

> But for leak-checking, we've already seen real cases where using LSan
> with higher optimization levels can lead to false positives (because the
> optimizer drops a value that is still in scope but not used in a code
> path that leads to exit()).
> ...
> So it may be that we really do want to keep leak-checking to "-O0
> -fsanitize=leak", and reserve "-O2 -fsanitize=address" for finding
> address bugs.

Yup, we have been burned a few times with this, IIRC.