Re: [PATCH v2] describe: enable sparse index for describe

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

"Raghul Nanth A via GitGitGadget" <gitgitgadget@gmail.com> writes:

> From: Raghul Nanth A <nanth.raghul@gmail.com>
>
> Add usage and performance tests for describe
>
> Performance metrics
> ...

The description is a bit skimpy.  At least it should explain why
blindly flipping the "requires-full-index" bit off is all that is
necessary. I think in the review discussion on v1, Derrick gave some
explanation you can regurgitate and reuse.

> Signed-off-by: Raghul Nanth A <nanth.raghul@gmail.com>
> ...

> diff --git a/t/t6121-describe-sparse.sh b/t/t6121-describe-sparse.sh
> new file mode 100755
> index 00000000000..ce53603c387
> --- /dev/null
> +++ b/t/t6121-describe-sparse.sh
> @@ -0,0 +1,67 @@
> +#!/bin/sh
> +
> +test_description='git describe in sparse checked out trees'
> +
> +GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
> +export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
> +
> +. ./test-lib.sh
> +
> +check_describe () {
> +	indir= &&
> +...
> +	'
> +}

Having this almost identical helper copied from a near-by test
script means maintenance nightmare.  People will forget to side port
to this copy any fixes they make to the other one.

I was hoping there would be a cleaner approach to reuse t6120, by
doing something similar to either how t8001-annotate shares blame
tests, or how t5559 takes advantage of t5551.  One way might be ...

 * prepare a prerequisite like so near the beginning of t6120

	test_lazy_prereq WITH_SPARSE_INDEX '
		test "$TEST_NAME" = t6121-describe
	'

 * add tests to be run with sparse-index enabled, but guarded with
   some variable, e.g.

	test_expect_success TESTING_SPARSE_INDEX 'a new test' '
		... do sparse-index testing specific code ...
	'

   Such "sparse-index testing specific" code may include turning the
   working tree the previous test already prepared into sparse
   (i.e. additional "setup"), or running commands under
   "ensure_not_expanded" (i.e. new tests).

 * create t6121 that works similar to how t5559 takes advantage of
   t5551, something like

	#/bin/sh
	. ./t6120-describe.sh

Ideally we should be able to do this without adding a new t6121, by
adding new tests that are specific to sparse-index at the end of
t6120, and avoid duplicated code.

Another possibility that may take the least amount of effort (but
may give us a lot less satisfactory outcome) may be to add a
lib-describe.sh library that is sourced from t6120 and t6121, and
move check_describe there.  If check_describe has to behave slightly
differently, have a new conditional in the implementation so that
the caller can make a choice.