Re: [RFC PATCH] t/Makefile: use dependency graph for "check-chainlint"

From: Eric Sunshine <sunshine@sunshineco.com>
Date: Dec 14, 2021, 07:44 UTC
Message-ID: <CAPig+cSFtpt6ExbVDbcx3tZodrKFuM-r2GMW4TQ2tJmLvHBFtQ@mail.gmail.com>
In-Reply-To: <RFC-patch-1.1-bb3f1577829-20211213T095456Z-avarab@gmail.com>

On Mon, Dec 13, 2021 at 5:09 AM Ævar Arnfjörð Bjarmason <avarab@gmail.com> wrote:

Show 10 quoted lines

> On Mon, Dec 13 2021, Eric Sunshine wrote:
> > Rather than running `chainlint` and `diff` once per self-test -- which
> > may become expensive as more tests are added -- instead run `chainlint`
> > a single time over all tests bodies collectively and compare the result
> > to the collective "expected" output.
>
> I think that "optimizing" things like this is an anti-pattern. I.e. we
> have N chainlint test files, and N potential outputs from that (ok or
> not, and with/without error). If one of the chainlint tests changes
> we'd like to re-run it, if not we can re-use an earlier run.

As mentioned in a reply elsewhere, the commit message sells this as an optimization, but that's not the real reason for the change, which is that the rewritten `check-chainlint` target for the upcoming new chainlint really wants to have a composite file of the "test" input and a composite of the "expect" output. I didn't know how to sell that change in this preparatory patch series, so I weakly framed it as an optimization. The reason for making this a preparatory step is that it makes for a less noisy patch later on when the new chainlint is actually plugged into the `check-chainlint` target. At least, it was less noisy originally... in the final implementation, I think it ends up being noisy anyhow. So, maybe it makes sense to drop this patch altogether(?).

Show 17 quoted lines

> This is something make's dependency logic is perfectly suited for, and
> will be faster than any optimization of turning a for-loop into a
> "sed" command we run every time, since we'll only need to "stat" a few
> things to see that there's nothing to do.
>
> +BUILT_CHAINLINTTESTS = $(patsubst %,.build/%.actual,$(CHAINLINTTESTS))
> +
> +.build/chainlint:
> +       mkdir -p $@
> +
> +$(BUILT_CHAINLINTTESTS): | .build/chainlint
> +$(BUILT_CHAINLINTTESTS): .build/%.actual: %
> +       $(CHAINLINT) <$< | \
> +       sed -e '/^# LINT: /d' >$@ && \
> +       diff -u $(basename $<).expect $@
> +
> +check-chainlint: $(BUILT_CHAINLINTTESTS)

This sort of optimization makes sense (I think) even with the new chainlint preferring to see composite "test" and "expect" files rather than the individual files. The individual files would be prerequisites of the composite files, thus the composites would only be regenerated if the individual files change. And the composite files would be prerequisites of the `check-chainlint` target, so it would only run if the composite files change (or if chainlint itself changes).

In fact, with the new chainlint checking all tests in all scripts at once, this technique should apply nicely to it, as well, since the names of test scripts (t????-*.sh) are fed to it as command-line arguments. Thus, the t????-*.sh files could be prerequisites of the chainlint rule which would use $? to check only test scripts which have changed since the previous run.

Having said all that, I don't think think the changes in this series or the upcoming new chainlint series make the situation any worse (Makefile-wise) than its current state, and the sort of optimizations discussed here can easily be made after those series land. (And, as my Git time is rather limited these days, I'd really like to focus on getting the core components landed.)