Subject: Re: [PATCH v3 6/6] Makefile: build "$(FUZZ_OBJS)" in CI, not under "all"

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Sun, 28 Feb 2021 21:13:54 +0100

On Tue, Feb 23 2021, Jeff King wrote:

> On Tue, Feb 23, 2021 at 12:41:32PM +0100, Ævar Arnfjörð Bjarmason wrote:
>
>> Adding $(FUZZ_OBJS) as a dependency on "all" was intentionally done in
>> 5e472150800 (fuzz: add basic fuzz testing target., 2018-10-12).
>> 
>> Rather than needlessly build these objects which aren't required for
>> the build every time we make "all", let's instead move them to be
>> built by the CI jobs.
>> 
>> The goal is to make sure that we don't inadvertently break these, we
>> can accomplish that goal by building them in CI, rather than slowing
>> down every build of git for everyone everywhere.
>
> The current state is that regular devs are responsible for avoiding
> compile breakages in the fuzz objects, even if they don't care
> themselves. Your earlier patches turned this into: regular devs are not
> on the hook for breaking fuzz objects; they are the responsibility of
> fuzz people. I'm OK with either of those, but this approach seems to me
> like the worst of both worlds. ;)
>
> If you do a refactor, you are still on the hook for breaking the fuzz
> objects because CI will fail (and you have to investigate it, and fix it
> for CI to remain a useful tool). But instead of finding out about the
> problem quickly as you're working, instead you push up what you think is
> a finished result, and then from minutes to hours later you get a
> notification telling you that oops, you missed a spot. I find that the
> shorter the error-fix-compile cycle is, the less time I waste waiting or
> context-switching.
>
> If we had a ton of fuzz object files that took forever to build, the
> savings on each build might be worth it. But AFAICT (from timing "make
> clean; make -j1" before and after), we are saving less than 1% of the
> build time (which is way less than the run-to-run noise).
>
> It doesn't seem like the right tradeoff to me. (Likewise, if other
> CI-only checks we have, like coccinelle, could be run at a similar cost,
> I'd recommend sticking them into the default developer build).

It's mainly psychological and doesn't contribute much to overall build
time as a percentage, but I find it grating that the last thing I see
before I switch away from that terminal when firing off a build on a
slower GCC farm box I can only use -j1 on, is these fuzz objects taking
2-3 seconds to build, knowing I'm wasting time on something I'll never
need.

I think when we build something we should narrowly be compiling only the
things we need, not running some sort of pseudo-CI on every developer's
computer. We can have CI or other targets for that.

Besides, if we were going for some sane cost-benefit here we'd have
targets to try compiling with NO_CURL=1 or some other conditional setups
that are actually common in the wild.

> One thing we _could_ do is stop building fuzz objects as part of "all",
> but include them for DEVELOPER=1 builds (which includes CI). That keeps
> them from hurting normal users (who don't actually need them), but
> prevents bitrot. It doesn't address your original motivation though (you
> as a developer would probably still be building them).

Please no. A very good thing about how DEVELOPER=1 works is that we're
not doing anything extra except advisory compilation flags. It's turned
on for "production" builds in a lot of settings because of that.

It would also be very annoying to e.g. have some failure on Solaris or
whatever, debug it with DEVELOPER=1, and then get some completely
unrelated failure in the developer-only code, e.g. because we'd decided
to compile all of fuzz/NO_OPENSSL/NO_CURL etc. and had some bug there.

Yes that bug would be worthwhile to fix, but not right there and
then. So having it under some "make all-combinations" flag or whatever
would be better.