Subject: Re: [PATCH v2 2/2] blame: enable and test the sparse index

From: Taylor Blau <[email-masked]>
Date: Mon, 25 Oct 2021 16:53:19 -0400

On Fri, Oct 15, 2021 at 09:20:35PM +0000, Lessley Dennington via GitGitGadget wrote:
> From: Lessley Dennington <[email-masked]>
>
> Enable the sparse index for the 'git blame' command. The index was already
> not expanded with this command, so the most interesting thing to do is to
> add tests that verify that 'git blame' behaves correctly when the sparse
> index is enabled and that its performance improves. More specifically, these
> cases are:
>
> 1. The index is not expanded for 'blame' when given paths in the sparse
> checkout cone at multiple levels.
>
> 2. Performance measurably improves for 'blame' with sparse index when given
> paths in the sparse checkout cone at multiple levels.
>
> The `p2000` tests demonstrate a ~60% execution time reduction when running
> 'blame' for a file two levels deep and and a ~30% execution time reduction
> for a file three levels deep.

Eek. What's eating up the other 30% when we have to open up another
layer of trees?

>
> Test                                         before  after
> ----------------------------------------------------------------
> 2000.62: git blame f2/f4/a (full-v3)         0.31    0.32 +3.2%
> 2000.63: git blame f2/f4/a (full-v4)         0.29    0.31 +6.9%
> 2000.64: git blame f2/f4/a (sparse-v3)       0.55    0.23 -58.2%
> 2000.65: git blame f2/f4/a (sparse-v4)       0.57    0.23 -59.6%
> 2000.66: git blame f2/f4/f3/a (full-v3)      0.77    0.85 +10.4%
> 2000.67: git blame f2/f4/f3/a (full-v4)      0.78    0.81 +3.8%
> 2000.68: git blame f2/f4/f3/a (sparse-v3)    1.07    0.72 -32.7%
> 2000.99: git blame f2/f4/f3/a (sparse-v4)    1.05    0.73 -30.5%
>
> We do not include paths outside the sparse checkout cone because blame
> currently does not support blaming files outside of the sparse definition.
> Attempting to do so fails with the following error:
>
> fatal: no such path '<path outside sparse definition>' in HEAD.

Small nit; this error message should be indented with a couple of space
characters to indicate that it's the output of running Git instead of
part of your patch message. Not worth a reroll on its own, but something
to keep in mind for your many future patches :).

>
> Signed-off-by: Lessley Dennington <[email-masked]>
> ---
>  builtin/blame.c                          |  3 +++
>  t/perf/p2000-sparse-operations.sh        |  2 ++
>  t/t1092-sparse-checkout-compatibility.sh | 24 +++++++++++++++++-------
>  3 files changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/builtin/blame.c b/builtin/blame.c
> index 641523ff9af..af3d81e2bd4 100644
> --- a/builtin/blame.c
> +++ b/builtin/blame.c
> @@ -902,6 +902,9 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
>  	long anchor;
>  	const int hexsz = the_hash_algo->hexsz;
>
> +	prepare_repo_settings(the_repository);
> +	the_repository->settings.command_requires_full_index = 0;
> +

By now we're quite used to seeing this ;). Makes sense to me.

>  	setup_default_color_by_age();
>  	git_config(git_blame_config, &output_option);
>  	repo_init_revisions(the_repository, &revs, NULL);
> diff --git a/t/perf/p2000-sparse-operations.sh b/t/perf/p2000-sparse-operations.sh
> index bff93f16e93..9ac76a049b8 100755
> --- a/t/perf/p2000-sparse-operations.sh
> +++ b/t/perf/p2000-sparse-operations.sh
> @@ -115,5 +115,7 @@ test_perf_on_all git reset --hard
>  test_perf_on_all git reset -- does-not-exist
>  test_perf_on_all git diff
>  test_perf_on_all git diff --staged
> +test_perf_on_all git blame $SPARSE_CONE/a
> +test_perf_on_all git blame $SPARSE_CONE/f3/a

Good.

>  test_done
> diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
> index e5d15be9d45..960ccf2d150 100755
> --- a/t/t1092-sparse-checkout-compatibility.sh
> +++ b/t/t1092-sparse-checkout-compatibility.sh
> @@ -488,15 +488,16 @@ test_expect_success 'blame with pathspec inside sparse definition' '
>  	test_all_match git blame deep/deeper1/deepest/a
>  '
>
> -# TODO: blame currently does not support blaming files outside of the
> -# sparse definition. It complains that the file doesn't exist locally.
> -test_expect_failure 'blame with pathspec outside sparse definition' '
> +# Blame does not support blaming files outside of the sparse
> +# definition, so we verify this scenario.
> +test_expect_success 'blame with pathspec outside sparse definition' '
>  	init_repos &&
>
> -	test_all_match git blame folder1/a &&
> -	test_all_match git blame folder2/a &&
> -	test_all_match git blame deep/deeper2/a &&
> -	test_all_match git blame deep/deeper2/deepest/a
> +	test_sparse_match git sparse-checkout set &&
> +	test_sparse_match test_must_fail git blame folder1/a &&
> +	test_sparse_match test_must_fail git blame folder2/a &&
> +	test_sparse_match test_must_fail git blame deep/deeper2/a &&
> +	test_sparse_match test_must_fail git blame deep/deeper2/deepest/a
>  '

test_must_fail used to allow for segfaults, but doesn't these days. So
this is a good test of "it should fail in sparse checkouts but not
crash", although I think it would be good to ensure that it's failing in
the way you expect (i.e., by checking that stderr contains "no such path
<xyz> in HEAD").
>
>  test_expect_success 'checkout and reset (mixed)' '
> @@ -874,6 +875,15 @@ test_expect_success 'sparse-index is not expanded: merge conflict in cone' '
>  	)
>  '
>
> +test_expect_success 'sparse index is not expanded: blame' '
> +	init_repos &&
> +
> +	ensure_not_expanded blame a &&
> +	ensure_not_expanded blame deep/a &&
> +	ensure_not_expanded blame deep/deeper1/a &&
> +	ensure_not_expanded blame deep/deeper1/deepest/a
> +'

Makes sense. Probably just one of these is necessary, but I haven't
looked into init_repos (or the "setup" test) enough to know for sure.
Either way, not worth changing.

Thanks,
Taylor