[PATCH v2] write-tree: integrate with sparse index

[Shuqi Liang <cheskaqiqi@gmail.com>]

Update 'git write-tree' to allow using the sparse-index in memory
without expanding to a full one.

The recursive algorithm for update_one() was already updated in 2de37c5
(cache-tree: integrate with sparse directory entries, 2021-03-03) to
handle sparse directory entries in the index. Hence we can just set the
requires-full-index to false for "write-tree".

The `p2000` tests demonstrate a ~96% execution time reduction for 'git
write-tree' using a sparse index:

Test                                           before  after
-----------------------------------------------------------------
2000.78: git write-tree (full-v3)              0.34    0.33 -2.9%
2000.79: git write-tree (full-v4)              0.32    0.30 -6.3%
2000.80: git write-tree (sparse-v3)            0.47    0.02 -95.8%
2000.81: git write-tree (sparse-v4)            0.45    0.02 -95.6%

Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
---

* change the position of "settings.command_requires_full_index = 0"

Range-diff against v1:
1:  d8a9ccd0b3 ! 1:  8873c79759 write-tree: integrate with sparse index
    @@ Commit message
     
      ## builtin/write-tree.c ##
     @@ builtin/write-tree.c: int cmd_write_tree(int argc, const char **argv, const char *cmd_prefix)
    - 	};
    - 
    - 	git_config(git_default_config, NULL);
    -+	
    -+	prepare_repo_settings(the_repository);
    -+	the_repository->settings.command_requires_full_index = 0;
    -+
      	argc = parse_options(argc, argv, cmd_prefix, write_tree_options,
      			     write_tree_usage, 0);
      
    ++	prepare_repo_settings(the_repository);
    ++	the_repository->settings.command_requires_full_index = 0;
    ++	
    + 	ret = write_cache_as_tree(&oid, flags, tree_prefix);
    + 	switch (ret) {
    + 	case 0:
     
      ## t/perf/p2000-sparse-operations.sh ##
     @@ t/perf/p2000-sparse-operations.sh: test_perf_on_all git checkout-index -f --all


 builtin/write-tree.c                     |  3 +++
 t/perf/p2000-sparse-operations.sh        |  1 +
 t/t1092-sparse-checkout-compatibility.sh | 28 ++++++++++++++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/builtin/write-tree.c b/builtin/write-tree.c
index 45d61707e7..4492da0912 100644
--- a/builtin/write-tree.c
+++ b/builtin/write-tree.c
@@ -38,6 +38,9 @@ int cmd_write_tree(int argc, const char **argv, const char *cmd_prefix)
 	argc = parse_options(argc, argv, cmd_prefix, write_tree_options,
 			     write_tree_usage, 0);
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+	
 	ret = write_cache_as_tree(&oid, flags, tree_prefix);
 	switch (ret) {
 	case 0:
diff --git a/t/perf/p2000-sparse-operations.sh b/t/perf/p2000-sparse-operations.sh
index 3242cfe91a..9924adfc26 100755
--- a/t/perf/p2000-sparse-operations.sh
+++ b/t/perf/p2000-sparse-operations.sh
@@ -125,5 +125,6 @@ test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove $SPARSE_CONE/a
 test_perf_on_all "git rm -f $SPARSE_CONE/a && git checkout HEAD -- $SPARSE_CONE/a"
 test_perf_on_all git grep --cached --sparse bogus -- "f2/f1/f1/*"
+test_perf_on_all git write-tree 
 
 test_done
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 801919009e..3b8191b390 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -2055,4 +2055,32 @@ test_expect_success 'grep sparse directory within submodules' '
 	test_cmp actual expect
 '
 
+test_expect_success 'write-tree on all' '
+	init_repos &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>"$1"
+	EOF
+
+	run_on_all ../edit-contents deep/a &&
+	run_on_all git update-index deep/a &&
+	test_all_match git write-tree &&
+
+	run_on_all mkdir -p folder1 &&
+	run_on_all cp a folder1/a &&
+	run_on_all ../edit-contents folder1/a &&
+	run_on_all git update-index folder1/a &&
+	test_all_match git write-tree
+'
+
+test_expect_success 'sparse-index is not expanded: write-tree' '
+	init_repos &&
+
+	ensure_not_expanded write-tree &&
+
+	echo "test1" >>sparse-index/a &&
+	git -C sparse-index update-index a &&
+	ensure_not_expanded write-tree 
+'
+
 test_done
-- 
2.39.0