[PATCH v1] worktree: integrate with sparse-index

[Shuqi Liang <cheskaqiqi@gmail.com>]

The index is read in 'worktree.c' at two points:

1.The 'validate_no_submodules' function, which checks if there are any
submodules present in the worktree.

2.The 'check_clean_worktree' function, which verifies if a worktree is
'clean', i.e., there are no untracked or modified but uncommitted files.
This is done by running the 'git status' command, and an error message
is thrown if the worktree is not clean. Given that 'git status' is
already sparse-aware, the function is also sparse-aware.

Hence we can just set the requires-full-index to false for
"git worktree".

Add tests that verify that 'git worktree' behaves correctly when the
sparse index is enabled and test to ensure the index is not expanded.

The `p2000` tests demonstrate a ~20% execution time reduction for
'git worktree' using a sparse index:

(Note:the p2000 test results did't reflect the huge speedup because of
the index reading time is minuscule comparing to the filesystem
operations.)

Test                                       before  after
-----------------------------------------------------------------------
2000.102: git worktree add....(full-v3)    3.15    2.82  -10.5%
2000.103: git worktree add....(full-v4)    3.14    2.84  -9.6%
2000.104: git worktree add....(sparse-v3)  2.59    2.14  -16.4%
2000.105: git worktree add....(sparse-v4)  2.10    1.57  -25.2%

Helped-by: Victoria Dye <vdye@github.com>
Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
---
 builtin/worktree.c                       |  4 ++++
 t/perf/p2000-sparse-operations.sh        |  1 +
 t/t1092-sparse-checkout-compatibility.sh | 23 +++++++++++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/builtin/worktree.c b/builtin/worktree.c
index f3180463be..db14bff1a3 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -1200,5 +1200,9 @@ int cmd_worktree(int ac, const char **av, const char *prefix)
 		prefix = "";
 
 	ac = parse_options(ac, av, prefix, options, git_worktree_usage, 0);
+
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	return fn(ac, av, prefix);
 }
diff --git a/t/perf/p2000-sparse-operations.sh b/t/perf/p2000-sparse-operations.sh
index 901cc493ef..1422136c73 100755
--- a/t/perf/p2000-sparse-operations.sh
+++ b/t/perf/p2000-sparse-operations.sh
@@ -131,5 +131,6 @@ test_perf_on_all git describe --dirty
 test_perf_on_all 'echo >>new && git describe --dirty'
 test_perf_on_all git diff-files
 test_perf_on_all git diff-files -- $SPARSE_CONE/a
+test_perf_on_all "git worktree add ../temp && git worktree remove ../temp"
 
 test_done
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index a63d0cc222..6ed691d338 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -2180,4 +2180,27 @@ test_expect_success 'sparse index is not expanded: diff-files' '
 	ensure_not_expanded diff-files -- "deep/*"
 '
 
+test_expect_success 'worktree' '
+	init_repos &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>"$1"
+	EOF
+
+	test_all_match git worktree add .worktrees/hotfix &&
+	test_sparse_match ls .worktrees/hotfix &&
+	test_all_match git worktree remove .worktrees/hotfix &&
+
+	test_all_match git worktree add .worktrees/hotfix &&
+	run_on_all ../edit-contents .worktrees/hotfix/deep/a &&
+	test_all_match test_must_fail git worktree remove .worktrees/hotfix
+'
+
+test_expect_success 'worktree is not expanded' '
+	init_repos &&
+
+	test_all_match git worktree add .worktrees/hotfix &&
+	ensure_not_expanded worktree remove .worktrees/hotfix
+'
+
 test_done
-- 
2.39.0