[PATCH] stash: reuse prepared state when creating stash

[jasmeet.bhatia.us@gmail.com]

From: "Jasmeet (Jazz) Bhatia" <jasmeet.bhatia.us@gmail.com>

do_push_stash() already preloads and refreshes the index and calls
check_changes() before invoking do_create_stash(). However, Git then
repeats this same index preparation and change detection, including
rebuilding the list of untracked files when requested, which is
duplicate work.

Instead, avoid this duplicate work by passing the already prepared
untracked file list to do_create_stash(). When the list is provided,
reuse it and skip the redundant index preload, refresh, and
check_changes() call.

This maintains the existing behavior of "git stash create", which
invokes do_create_stash() without prepared state and thus continues
to perform its own index preparation and change detection.

Testing this using the p3904 stash performance tests shows a decent
performance boost.

    stash -p, no fsmonitor
    0.12(0.11+0.17) -> 0.10(0.10+0.18)  -16.7%

    stash -p, builtin fsmonitor
    0.11(0.10+0.02) -> 0.09(0.08+0.01)  -18.2%

Testing this on the Linux kernel on my system, "git stash push"
improved from 254 ms to about 239 ms, which is about 6%.

Signed-off-by: Jasmeet (Jazz) Bhatia <jasmeet.bhatia.us@gmail.com>
---
 builtin/stash.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/builtin/stash.c b/builtin/stash.c
index e1a3a76bd0..58eb782597 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -1459,7 +1459,7 @@ static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_b
 			   int include_untracked, int patch_mode,
 			   struct interactive_options *interactive_opts,
 			   int only_staged, struct stash_info *info, struct strbuf *patch,
-			   int quiet)
+			   int quiet, struct strbuf *prepared_untracked_files)
 {
 	int ret = 0;
 	int flags = 0;
@@ -1473,14 +1473,17 @@ static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_b
 	struct strbuf msg = STRBUF_INIT;
 	struct strbuf commit_tree_label = STRBUF_INIT;
 	struct strbuf untracked_files = STRBUF_INIT;
+	struct strbuf *untracked_files_p = prepared_untracked_files ? prepared_untracked_files : &untracked_files;
 
 	prepare_fallback_ident("git stash", "git@stash");
 
-	repo_read_index_preload(the_repository, NULL, 0);
-	if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET, 0, 0,
-					 NULL, NULL, NULL) < 0) {
-		ret = error(_("could not write index"));
-		goto done;
+	if (!prepared_untracked_files) {
+		repo_read_index_preload(the_repository, NULL, 0);
+		if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET, 0, 0,
+						 NULL, NULL, NULL) < 0) {
+			ret = error(_("could not write index"));
+			goto done;
+		}
 	}
 
 	if (repo_get_oid(the_repository, "HEAD", &info->b_commit)) {
@@ -1493,7 +1496,8 @@ static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_b
 		head_commit = lookup_commit(the_repository, &info->b_commit);
 	}
 
-	if (!check_changes(ps, include_untracked, &untracked_files)) {
+	if (!prepared_untracked_files &&
+	    !check_changes(ps, include_untracked, untracked_files_p)) {
 		ret = 1;
 		goto done;
 	}
@@ -1529,7 +1533,7 @@ static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_b
 	parents = NULL;
 
 	if (include_untracked) {
-		if (save_untracked_files(info, &msg, &untracked_files)) {
+		if (save_untracked_files(info, &msg, untracked_files_p)) {
 			if (!quiet)
 				fprintf_ln(stderr, _("Cannot save "
 						     "the untracked files"));
@@ -1615,7 +1619,7 @@ static int create_stash(int argc, const char **argv, const char *prefix UNUSED,
 		return 0;
 
 	ret = do_create_stash(&ps, &stash_msg_buf, 0, 0, NULL, 0, &info,
-			      NULL, 0);
+			      NULL, 0, NULL);
 	if (!ret)
 		printf_ln("%s", oid_to_hex(&info.w_commit));
 
@@ -1698,7 +1702,8 @@ static int do_push_stash(const struct pathspec *ps, const char *stash_msg, int q
 	if (stash_msg)
 		strbuf_addstr(&stash_msg_buf, stash_msg);
 	if (do_create_stash(ps, &stash_msg_buf, include_untracked, patch_mode,
-			    interactive_opts, only_staged, &info, &patch, quiet)) {
+			    interactive_opts, only_staged, &info, &patch, quiet,
+			    &untracked_files)) {
 		ret = -1;
 		goto done;
 	}
-- 
2.56.0