stash: reuse cached index entries in --patch temporary index

`git stash -p` prepares the interactive selection by creating a
temporary index at HEAD, switching `GIT_INDEX_FILE` to it, and then
running the `add -p` machinery.

That temporary index was created by running `git read-tree HEAD`.  The
resulting index had no useful cached stat data or fsmonitor-valid bits
from the real index.  When `run_add_p()` refreshed that temporary index
before showing the first prompt, it could end up lstat(2)-ing every
tracked file, even in a repository where `git diff` and `git restore -p`
can use fsmonitor to avoid that work.

Create the temporary index in-process instead.  Use `unpack_trees()` to
reset the real index contents to HEAD while writing the result to the
temporary index path.  For paths whose index entries already match HEAD,
`oneway_merge()` reuses the existing cache entries, preserving their
cached stat data and `CE_FSMONITOR_VALID` state.

This makes the refresh performed by `run_add_p()` behave like the one
used by `git restore -p`: unchanged paths can be skipped via fsmonitor
instead of being scanned again.

In a 206k file repository with `core.fsmonitor` enabled and a one-line
change in one file, time to first prompt dropped from 34.774 seconds to
0.659 seconds. The new perf test file demonstrates similar improvements,
with maen times for without- and with-fsmonitor cases dropping from 6.90
and 6.83 seconds to 0.55 and 0.28 seconds, respectively.

Signed-off-by: Adam Johnson <me@adamj.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -372,6 +372,56 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
 	return 0;
 }
 
+static int create_index_from_tree(const struct object_id *tree_id,
+				  const char *index_path)
+{
+	int nr_trees = 1;
+	int ret = 0;
+	struct unpack_trees_options opts;
+	struct tree_desc t[MAX_UNPACK_TREES];
+	struct tree *tree;
+	struct index_state dst_istate = INDEX_STATE_INIT(the_repository);
+	struct lock_file lock_file = LOCK_INIT;
+
+	repo_read_index_preload(the_repository, NULL, 0);
+	refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL);
+
+	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
+
+	memset(&opts, 0, sizeof(opts));
+
+	tree = repo_parse_tree_indirect(the_repository, tree_id);
+	if (!tree || repo_parse_tree(the_repository, tree)) {
+		ret = -1;
+		goto done;
+	}
+
+	init_tree_desc(t, &tree->object.oid, tree->buffer, tree->size);
+
+	opts.head_idx = 1;
+	opts.src_index = the_repository->index;
+	opts.dst_index = &dst_istate;
+	opts.merge = 1;
+	opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
+	opts.fn = oneway_merge;
+
+	if (unpack_trees(nr_trees, t, &opts)) {
+		ret = -1;
+		goto done;
+	}
+
+	if (write_locked_index(&dst_istate, &lock_file, COMMIT_LOCK)) {
+		ret = error(_("unable to write new index file"));
+		goto done;
+	}
+
+done:
+	release_index(&dst_istate);
+	if (ret)
+		rollback_lock_file(&lock_file);
+	return ret;
+}
+
 static int diff_tree_binary(struct strbuf *out, struct object_id *w_commit)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
@@ -1309,18 +1359,26 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 		       struct interactive_options *interactive_opts)
 {
 	int ret = 0;
-	struct child_process cp_read_tree = CHILD_PROCESS_INIT;
 	struct child_process cp_diff_tree = CHILD_PROCESS_INIT;
+	struct commit *head_commit;
+	const struct object_id *head_tree;
 	struct index_state istate = INDEX_STATE_INIT(the_repository);
 	char *old_index_env = NULL, *old_repo_index_file;
 
 	remove_path(stash_index_path.buf);
 
-	cp_read_tree.git_cmd = 1;
-	strvec_pushl(&cp_read_tree.args, "read-tree", "HEAD", NULL);
-	strvec_pushf(&cp_read_tree.env, "GIT_INDEX_FILE=%s",
-		     stash_index_path.buf);
-	if (run_command(&cp_read_tree)) {
+	head_commit = lookup_commit(the_repository, &info->b_commit);
+	if (!head_commit || repo_parse_commit(the_repository, head_commit)) {
+		ret = -1;
+		goto done;
+	}
+	head_tree = get_commit_tree_oid(head_commit);
+	if (!head_tree) {
+		ret = -1;
+		goto done;
+	}
+
+	if (create_index_from_tree(head_tree, stash_index_path.buf)) {
 		ret = -1;
 		goto done;
 	}

t/perf/p3904-stash-patch.sh

@@ -0,0 +1,43 @@
+#!/bin/sh
+
+test_description="Performance tests for git stash -p"
+
+. ./perf-lib.sh
+
+test_perf_fresh_repo
+
+test_expect_success "setup" '
+	mkdir files &&
+	test_seq 1 100000 | while read i; do
+		echo "content $i" >files/$i.txt || return 1
+	done &&
+	git add files/ &&
+	git commit -q -m "add tracked files" &&
+	echo modified >files/1.txt
+'
+
+test_perf "stash -p, no fsmonitor" \
+	--setup 'echo modified >files/1.txt' '
+	printf "q\n" | git stash -p >/dev/null 2>&1 || true
+'
+
+if test_have_prereq FSMONITOR_DAEMON
+then
+	test_expect_success "enable builtin fsmonitor" '
+		git config core.fsmonitor true &&
+		git fsmonitor--daemon start &&
+		git update-index --fsmonitor &&
+		git status >/dev/null 2>&1
+	'
+
+	test_perf "stash -p, builtin fsmonitor" \
+		--setup 'echo modified >files/1.txt && git status >/dev/null 2>&1' '
+		printf "q\n" | git stash -p >/dev/null 2>&1 || true
+	'
+
+	test_expect_success "stop builtin fsmonitor" '
+		git fsmonitor--daemon stop
+	'
+fi
+
+test_done