[PATCH] ls-files: conditionally leave index sparse

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <stolee@gmail.com>

When running 'git ls-files' with a pathspec, the index entries get
filtered according to that pathspec before iterating over them in
show_files().  In 78087097b8 (ls-files: add --sparse option,
2021-12-22), this iteration was prefixed with a check for the '--sparse'
option which allows the command to output directory entries; this
created a pre-loop call to ensure_full_index().

However, when a user runs 'git ls-files' where the pathspec matches
directories that are recursively matched in the sparse-checkout, there
are not any sparse directories that match the pathspec so they would not
be written to the output. The expansion in this case is just a
performance drop for no behavior difference.

Replace this global check to expand the index with a check inside the
loop for a matched sparse directory. If we see one, then expand the
index and continue from the current location. This is safe since the
previous entries in the index did not have any sparse directories and
thus would remain stable in this expansion.

A test in t1092 confirms that this changes the behavior.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
    ls-files: conditionally leave index sparse
    
    Here's a small sparse index performance update based on a user report.
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1955%2Fderrickstolee%2Fls-files-sparse-index-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1955/derrickstolee/ls-files-sparse-index-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1955

 builtin/ls-files.c                       | 13 ++++++++++---
 t/t1092-sparse-checkout-compatibility.sh | 13 +++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/builtin/ls-files.c b/builtin/ls-files.c
index c06a6f33e41..b148607f7a1 100644
--- a/builtin/ls-files.c
+++ b/builtin/ls-files.c
@@ -414,14 +414,21 @@ static void show_files(struct repository *repo, struct dir_struct *dir)
 	if (!(show_cached || show_stage || show_deleted || show_modified))
 		return;
 
-	if (!show_sparse_dirs)
-		ensure_full_index(repo->index);
-
 	for (i = 0; i < repo->index->cache_nr; i++) {
 		const struct cache_entry *ce = repo->index->cache[i];
 		struct stat st;
 		int stat_err;
 
+		if (S_ISSPARSEDIR(ce->ce_mode) && !show_sparse_dirs) {
+			/*
+			 * This is the first time we've hit a sparse dir,
+			 * so expansion will leave the first 'i' entries
+			 * alone.
+			 */
+			ensure_full_index(repo->index);
+			ce = repo->index->cache[i];
+		}
+
 		construct_fullname(&fullname, repo, ce);
 
 		if ((dir->flags & DIR_SHOW_IGNORED) &&
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index d8101139b40..b0f691c151a 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1506,6 +1506,8 @@ test_expect_success 'sparse-index is not expanded' '
 	ensure_not_expanded reset --hard &&
 	ensure_not_expanded restore -s rename-out-to-out -- deep/deeper1 &&
 
+	ensure_not_expanded ls-files deep/deeper1 &&
+
 	echo >>sparse-index/README.md &&
 	ensure_not_expanded add -A &&
 	echo >>sparse-index/extra.txt &&
@@ -1607,6 +1609,17 @@ test_expect_success 'describe tested on all' '
 	test_all_match git describe --dirty
 '
 
+test_expect_success 'ls-files filtering and expansion' '
+	init_repos &&
+
+	# This filtering will hit a sparse directory midway
+	# through the iteration.
+	test_all_match git ls-files deep &&
+
+	# This pathspec will filter the index to only a sparse
+	# directory.
+	test_all_match git ls-files folder1
+'
 
 test_expect_success 'sparse-index is not expanded: describe' '
 	init_repos &&

base-commit: 724518f3884d8707c5f51428ba98c115818229b8
-- 
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