mv: integrate with sparse-index

Teach 'git mv' to work with a sparse index without expanding it for
in-cone file moves.

Set command_requires_full_index to 0 so that the sparse index is
preserved when reading. For --sparse operations that reference
out-of-cone paths, expand the full index immediately since those
operations need access to individual file entries within sparse
directory entries.

For in-cone moves, the entries are already individual in the sparse
index, so no expansion is needed. This provides a performance benefit
for the common case of renaming files within the working directory.

Update empty_dir_has_sparse_contents() to use index_name_pos_sparse()
(which avoids triggering expansion) and to recognize sparse directory
entries directly via S_ISSPARSEDIR, in addition to the existing logic
that finds individual skip-worktree entries beneath a directory prefix.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: derrickstolee

builtin/mv.c

@@ -24,9 +24,11 @@
 #include "read-cache-ll.h"
 
 #include "setup.h"
+#include "sparse-index.h"
 #include "strvec.h"
 #include "submodule.h"
 #include "entry.h"
+#include "repository.h"
 
 static const char * const builtin_mv_usage[] = {
 	N_("git mv [-v] [-f] [-n] [-k] <source> <destination>"),
@@ -149,20 +151,26 @@ static int empty_dir_has_sparse_contents(const char *name)
 	char *with_slash = add_slash(name);
 	int length = strlen(with_slash);
 
-	int pos = index_name_pos(the_repository->index, with_slash, length);
+	int pos = index_name_pos_sparse(the_repository->index, with_slash, length);
 	const struct cache_entry *ce;
 
-	if (pos < 0) {
-		pos = -pos - 1;
-		if (pos >= the_repository->index->cache_nr)
-			goto free_return;
+	if (pos >= 0) {
+		/* Exact match: sparse directory entry in a sparse index */
 		ce = the_repository->index->cache[pos];
-		if (strncmp(with_slash, ce->name, length))
-			goto free_return;
-		if (ce_skip_worktree(ce))
+		if (S_ISSPARSEDIR(ce->ce_mode))
 			ret = 1;
+		goto free_return;
 	}
 
+	pos = -pos - 1;
+	if (pos >= the_repository->index->cache_nr)
+		goto free_return;
+	ce = the_repository->index->cache[pos];
+	if (strncmp(with_slash, ce->name, length))
+		goto free_return;
+	if (ce_skip_worktree(ce))
+		ret = 1;
+
 free_return:
 	free(with_slash);
 	return ret;
@@ -241,6 +249,8 @@ int cmd_mv(int argc,
 	struct repo_config_values *cfg = repo_config_values(the_repository);
 
 	repo_config(the_repository, git_default_config, NULL);
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
 
 	argc = parse_options(argc, argv, prefix, builtin_mv_options,
 			     builtin_mv_usage, 0);
@@ -251,6 +261,16 @@ int cmd_mv(int argc,
 	if (repo_read_index(the_repository) < 0)
 		die(_("index file corrupt"));
 
+	/*
+	 * When --sparse is used, we may need to operate on entries outside
+	 * the sparse-checkout cone which requires a full index. Expand now
+	 * so that the rest of the command works on individual file entries.
+	 * Without --sparse, only in-cone moves are allowed, and those
+	 * entries are already expanded in the sparse index.
+	 */
+	if (ignore_sparse)
+		ensure_full_index(the_repository->index);
+
 	internal_prefix_pathspec(&sources, prefix, argv, argc, 0);
 	CALLOC_ARRAY(modes, argc);
 

t/t1092-sparse-checkout-compatibility.sh

@@ -2876,6 +2876,15 @@ test_expect_success 'mv: in-cone to existing file collision' '
 	test_all_match git diff --cached --name-status
 '
 
+test_expect_success 'sparse-index is not expanded: mv in-cone' '
+	init_repos &&
+
+	# Moving a file within the sparse-checkout cone does not
+	# require expanding the index since those entries are
+	# already individual in the sparse index.
+	ensure_not_expanded mv deep/a deep/moved-a
+'
+
 test_expect_success 'sparse-index is expanded: mv with out-of-cone source' '
 	init_repos &&