[PATCH 2/2] restore: avoid sparse index expansion

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

From: Derrick Stolee <stolee@gmail.com>

Teach update_some() to handle sparse directory entries at the tree
level rather than expanding the entire sparse index. When iterating a
source tree during checkout/restore operations:

 - If a directory matches a sparse directory entry with the same OID,
   skip it entirely (no change needed).

 - If the OID differs and we are in non-overlay mode (e.g., restore
   --staged), update the sparse directory entry's OID in place. This
   is semantically correct because non-overlay mode removes paths not
   in the source tree anyway.

 - In overlay mode (e.g., checkout <tree> -- .), fall through to
   recursive descent so individual file entries are preserved
   correctly.

Also switch from index_name_pos() to index_name_pos_sparse() for
individual file lookups to avoid triggering ensure_full_index() when
the file is already individually tracked in the index.

Update the test expectation in t1092 to assert that 'restore --staged'
no longer expands the sparse index.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
 builtin/checkout.c                       | 57 +++++++++++++++++++++---
 t/t1092-sparse-checkout-compatibility.sh |  8 ++--
 2 files changed, 55 insertions(+), 10 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 1345e8574a..67f03dea10 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -31,6 +31,7 @@
 #include "revision.h"
 #include "sequencer.h"
 #include "setup.h"
+#include "sparse-index.h"
 #include "strvec.h"
 #include "submodule.h"
 #include "symlinks.h"
@@ -142,14 +143,56 @@ static int post_checkout_hook(struct commit *old_commit, struct commit *new_comm
 }
 
 static int update_some(const struct object_id *oid, struct strbuf *base,
-		       const char *pathname, unsigned mode, void *context UNUSED)
+		       const char *pathname, unsigned mode, void *context)
 {
 	int len;
 	struct cache_entry *ce;
 	int pos;
+	int overlay_mode = context ? *((int *)context) : 1;
 
-	if (S_ISDIR(mode))
+	if (S_ISDIR(mode)) {
+		/*
+		 * If this directory exists as a sparse directory entry in
+		 * the index, we can handle it at the tree level without
+		 * descending into individual files.
+		 */
+		if (the_repository->index->sparse_index) {
+			struct strbuf dirpath = STRBUF_INIT;
+
+			strbuf_addbuf(&dirpath, base);
+			strbuf_addstr(&dirpath, pathname);
+			strbuf_addch(&dirpath, '/');
+
+			pos = index_name_pos_sparse(the_repository->index,
+						    dirpath.buf, dirpath.len);
+			if (pos >= 0) {
+				struct cache_entry *old =
+					the_repository->index->cache[pos];
+				if (S_ISSPARSEDIR(old->ce_mode)) {
+					if (oideq(oid, &old->oid)) {
+						strbuf_release(&dirpath);
+						return 0;
+					}
+					if (!overlay_mode) {
+						/*
+						 * In non-overlay mode (e.g.,
+						 * restore --staged), we can
+						 * replace the sparse dir OID
+						 * directly since files not in
+						 * the source tree should be
+						 * removed anyway.
+						 */
+						oidcpy(&old->oid, oid);
+						old->ce_flags |= CE_UPDATE;
+						strbuf_release(&dirpath);
+						return 0;
+					}
+				}
+			}
+			strbuf_release(&dirpath);
+		}
 		return READ_TREE_RECURSIVE;
+	}
 
 	len = base->len + strlen(pathname);
 	ce = make_empty_cache_entry(the_repository->index, len);
@@ -165,7 +208,7 @@ static int update_some(const struct object_id *oid, struct strbuf *base,
 	 * entry in place. Whether it is UPTODATE or not, checkout_entry will
 	 * do the right thing.
 	 */
-	pos = index_name_pos(the_repository->index, ce->name, ce->ce_namelen);
+	pos = index_name_pos_sparse(the_repository->index, ce->name, ce->ce_namelen);
 	if (pos >= 0) {
 		struct cache_entry *old = the_repository->index->cache[pos];
 		if (ce->ce_mode == old->ce_mode &&
@@ -182,10 +225,11 @@ static int update_some(const struct object_id *oid, struct strbuf *base,
 	return 0;
 }
 
-static int read_tree_some(struct tree *tree, const struct pathspec *pathspec)
+static int read_tree_some(struct tree *tree, const struct pathspec *pathspec,
+			  int overlay_mode)
 {
 	read_tree(the_repository, tree,
-		  pathspec, update_some, NULL);
+		  pathspec, update_some, &overlay_mode);
 
 	/* update the index with the given tree's info
 	 * for all args, expanding wildcards, and exit
@@ -580,7 +624,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 		return error(_("index file corrupt"));
 
 	if (opts->source_tree)
-		read_tree_some(opts->source_tree, &opts->pathspec);
+		read_tree_some(opts->source_tree, &opts->pathspec,
+			       opts->overlay_mode);
 	if (opts->merge)
 		unmerge_index(the_repository->index, &opts->pathspec, CE_MATCHED);
 
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index d69434e7ab..8186da5c88 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -2608,19 +2608,19 @@ test_expect_success 'restore --staged with wildcards' '
 	test_all_match git diff --cached
 '
 
-test_expect_success 'sparse-index is expanded: restore --staged' '
+test_expect_success 'sparse-index is not expanded: restore --staged' '
 	init_repos &&
 
 	git -C sparse-index checkout -b restore-staged-exp base &&
 	git -C sparse-index reset --soft update-folder1 &&
-	ensure_expanded restore --staged .
+	ensure_not_expanded restore --staged .
 '
 
-test_expect_success 'sparse-index is expanded: restore --source --staged' '
+test_expect_success 'sparse-index is not expanded: restore --source --staged' '
 	init_repos &&
 
 	git -C sparse-index checkout -b restore-source-staged base &&
-	ensure_expanded restore --source update-folder1 --staged .
+	ensure_not_expanded restore --source update-folder1 --staged .
 '
 
 test_done
-- 
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