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

On Tue, Feb 23 2021, Jeff King wrote:

On Tue, Feb 23, 2021 at 12:41:32PM +0100, Ævar Arnfjörð Bjarmason wrote:

quoted

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.