ls-files: add --sparse option

Existing callers to 'git ls-files' are expecting file names, not
directories. It is best to expand a sparse index to show all of the
contained files in this case.

However, expert users may want to inspect the contents of the index
itself including which directories are sparse. Add a --sparse option to
allow users to request this information.

During testing, I noticed that options such as --modified did not affect
the output when the files in question were outside the sparse-checkout
definition. Tests are added to document this preexisting behavior and
how it remains unchanged with the sparse index and the --sparse option.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-files.txt

@@ -187,6 +187,11 @@ Both the <eolinfo> in the index ("i/<eolinfo>")
 and in the working tree ("w/<eolinfo>") are shown for regular files,
 followed by the  ("attr/<eolattr>").
 
+--sparse::
+	If the index is sparse, show the sparse directories without expanding
+	to the contained files. Sparse directories will be shown with a
+	trailing slash, such as "x/" for a sparse directory "x".
+
 \--::
 	Do not interpret any more arguments as options.
 

builtin/ls-files.c

@@ -37,6 +37,7 @@ static int debug_mode;
 static int show_eol;
 static int recurse_submodules;
 static int skipping_duplicates;
+static int show_sparse_dirs;
 
 static const char *prefix;
 static int max_prefix_len;
@@ -315,8 +316,10 @@ static void show_files(struct repository *repo, struct dir_struct *dir)
 
 	if (!(show_cached || show_stage || show_deleted || show_modified))
 		return;
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(repo->index);
+
+	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;
@@ -670,6 +673,8 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 		OPT_BOOL(0, "debug", &debug_mode, N_("show debugging data")),
 		OPT_BOOL(0, "deduplicate", &skipping_duplicates,
 			 N_("suppress duplicate entries")),
+		OPT_BOOL(0, "sparse", &show_sparse_dirs,
+			 N_("show sparse directories in the presence of a sparse index")),
 		OPT_END()
 	};
 	int ret = 0;
@@ -677,6 +682,9 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage_with_options(ls_files_usage, builtin_ls_files_options);
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	prefix = cmd_prefix;
 	if (prefix)
 		prefix_len = strlen(prefix);

t/t1092-sparse-checkout-compatibility.sh

@@ -816,6 +816,12 @@ test_expect_success 'sparse-index is expanded and converted back' '
 	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
 		git -C sparse-index reset -- folder1/a &&
 	test_region index convert_to_sparse trace2.txt &&
+	test_region index ensure_full_index trace2.txt &&
+
+	# ls-files expands on read, but does not write.
+	rm trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+		git -C sparse-index ls-files &&
 	test_region index ensure_full_index trace2.txt
 '
 
@@ -871,6 +877,7 @@ test_expect_success 'sparse-index is not expanded' '
 	init_repos &&
 
 	ensure_not_expanded status &&
+	ensure_not_expanded ls-files --sparse &&
 	ensure_not_expanded commit --allow-empty -m empty &&
 	echo >>sparse-index/a &&
 	ensure_not_expanded commit -a -m a &&
@@ -1019,6 +1026,90 @@ test_expect_success 'sparse index is not expanded: fetch/pull' '
 	ensure_not_expanded pull full base
 '
 
+test_expect_success 'ls-files' '
+	init_repos &&
+
+	# Use a smaller sparse-checkout for reduced output
+	test_sparse_match git sparse-checkout set &&
+
+	# Behavior agrees by default. Sparse index is expanded.
+	test_all_match git ls-files &&
+
+	# With --sparse, the sparse index data changes behavior.
+	git -C sparse-index ls-files --sparse >actual &&
+
+	cat >expect <<-\EOF &&
+	a
+	deep/
+	e
+	folder1-
+	folder1.x
+	folder1/
+	folder10
+	folder2/
+	g
+	x/
+	z
+	EOF
+
+	test_cmp expect actual &&
+
+	# With --sparse and no sparse index, nothing changes.
+	git -C sparse-checkout ls-files >dense &&
+	git -C sparse-checkout ls-files --sparse >sparse &&
+	test_cmp dense sparse &&
+
+	# Set up a strange condition of having a file edit
+	# outside of the sparse-checkout cone. This is just
+	# to verify that sparse-checkout and sparse-index
+	# behave the same in this case.
+	write_script edit-content <<-\EOF &&
+	mkdir folder1 &&
+	echo content >>folder1/a
+	EOF
+	run_on_sparse ../edit-content &&
+
+	# ls-files does not currently notice modified files whose
+	# cache entries are marked SKIP_WORKTREE. This may change
+	# in the future, but here we test that sparse index does
+	# not accidentally create a change of behavior.
+	test_sparse_match git ls-files --modified &&
+	test_must_be_empty sparse-checkout-out &&
+	test_must_be_empty sparse-index-out &&
+
+	git -C sparse-index ls-files --sparse --modified >sparse-index-out &&
+	test_must_be_empty sparse-index-out &&
+
+	# Add folder1 to the sparse-checkout cone and
+	# check that ls-files shows the expanded files.
+	test_sparse_match git sparse-checkout add folder1 &&
+	test_sparse_match git ls-files --modified &&
+
+	test_all_match git ls-files &&
+	git -C sparse-index ls-files --sparse >actual &&
+
+	cat >expect <<-\EOF &&
+	a
+	deep/
+	e
+	folder1-
+	folder1.x
+	folder1/0/0/0
+	folder1/0/1
+	folder1/a
+	folder10
+	folder2/
+	g
+	x/
+	z
+	EOF
+
+	test_cmp expect actual &&
+
+	# Double-check index expansion is avoided
+	ensure_not_expanded ls-files --sparse
+'
+
 # 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' '