Re: [PATCH 8/8] sparse-checkout: integrate with sparse index

> From: Derrick Stolee <dstolee@microsoft.com>
> 
> When modifying the sparse-checkout definition, the sparse-checkout
> builtin calls update_sparsity() to modify the SKIP_WORKTREE bits of all
> cache entries in the index. Before, we needed the index to be fully
> expanded in order to ensure we had the full list of files necessary that
> match the new patterns.
> 
> Insert a call to reset_sparse_directories() that expands sparse
> directories that are within the new pattern list, but only far enough
> that every necessary file path now exists as a cache entry. The
> remaining logic within update_sparsity() will modify the SKIP_WORKTREE
> bits appropriately.
> 
> This allows us to disable command_requires_full_index within the
> sparse-checkout builtin. Add tests that demonstrate that we are not
> expanding to a full index unnecessarily.
> 
> We can see the improved performance in the p2000 test script:
> 
> Test                           HEAD~1            HEAD
> ------------------------------------------------------------------------
> 2000.24: git ... (sparse-v3)   2.14(1.55+0.58)   1.57(1.03+0.53) -26.6%
> 2000.25: git ... (sparse-v4)   2.20(1.62+0.57)   1.58(0.98+0.59) -28.2%
> 
> These reductions of 26-28% are small compared to most examples, but the
> time is dominated by writing a new copy of the base repository to the
> worktree and then deleting it again. The fact that the previous index
> expansion was such a large portion of the time is telling how important
> it is to complete this sparse index integration.
> 
> Signed-off-by: Derrick Stolee <derrickstolee@github.com>
> ---
>  builtin/sparse-checkout.c                |  3 +++
>  t/t1092-sparse-checkout-compatibility.sh | 25 ++++++++++++++++++++++++
>  unpack-trees.c                           |  4 ++++
>  3 files changed, 32 insertions(+)
> 
> diff --git a/builtin/sparse-checkout.c b/builtin/sparse-checkout.c
> index cbff6ad00b0..0157b292b36 100644
> --- a/builtin/sparse-checkout.c
> +++ b/builtin/sparse-checkout.c
> @@ -937,6 +937,9 @@ int cmd_sparse_checkout(int argc, const char **argv, const char *prefix)
>  
>  	git_config(git_default_config, NULL);
>  
> +	prepare_repo_settings(the_repository);
> +	the_repository->settings.command_requires_full_index = 0;
> +
>  	if (argc > 0) {
>  		if (!strcmp(argv[0], "list"))
>  			return sparse_checkout_list(argc, argv);
> diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
> index 93bcfd20bbc..614357fc48c 100755
> --- a/t/t1092-sparse-checkout-compatibility.sh
> +++ b/t/t1092-sparse-checkout-compatibility.sh
> @@ -1552,6 +1552,31 @@ test_expect_success 'ls-files' '
>  	ensure_not_expanded ls-files --sparse
>  '
>  
> +test_expect_success 'sparse index is not expanded: sparse-checkout' '
> +	init_repos &&
> +
> +	ensure_not_expanded sparse-checkout set deep/deeper2 &&
> +	ensure_not_expanded sparse-checkout set deep/deeper1 &&
> +	ensure_not_expanded sparse-checkout set deep &&
> +	ensure_not_expanded sparse-checkout add folder1 &&
> +	ensure_not_expanded sparse-checkout set deep/deeper1 &&
> +	ensure_not_expanded sparse-checkout set folder2 &&
> +
> +	# Demonstrate that the checks that "folder1/a" is a file
> +	# do not cause a sparse-index expansion (since it is in the
> +	# sparse-checkout cone).
> +	echo >>sparse-index/folder2/a &&
> +	git -C sparse-index add folder2/a &&
> +
> +	ensure_not_expanded sparse-checkout add folder1 &&
> +
> +	# Skip checks here, since deep/deeper1 is inside a sparse directory
> +	# that must be expanded to check whether `deep/deeper1` is a file
> +	# or not.
> +	ensure_not_expanded sparse-checkout set --skip-checks deep/deeper1 &&
> +	ensure_not_expanded sparse-checkout set
> +'
> +

>  # NEEDSWORK: a sparse-checkout behaves differently from a full checkout
>  # in this scenario, but it shouldn't.
>  test_expect_success 'reset mixed and checkout orphan' '
> diff --git a/unpack-trees.c b/unpack-trees.c
> index 7f528d35cc2..9745e0dfc34 100644
> --- a/unpack-trees.c
> +++ b/unpack-trees.c
> @@ -18,6 +18,7 @@
>  #include "promisor-remote.h"
>  #include "entry.h"
>  #include "parallel-checkout.h"
> +#include "sparse-index.h"
>  
>  /*
>   * Error messages expected by scripts out of plumbing commands such as
> @@ -2018,6 +2019,9 @@ enum update_sparsity_result update_sparsity(struct unpack_trees_options *o)
>  			goto skip_sparse_checkout;
>  	}
>  
> +	/* Expand sparse directories as needed */
> +	expand_to_pattern_list(o->src_index, o->pl);
> +
>  	/* Set NEW_SKIP_WORKTREE on existing entries. */
>  	mark_all_ce_unused(o->src_index);
>  	mark_new_skip_worktree(o->pl, o->src_index, 0,