[PATCH v2] stash: reuse cached index entries in --patch temporary index

From: Adam Johnson <me@adamj.eu>

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

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2306%2Fadamchainz%2Faj%2Foptimize-stash-patch-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2306/adamchainz/aj/optimize-stash-patch-v2
Pull-Request: https://github.com/git/git/pull/2306

Range-diff vs v1:

 1:  b228160cc4 ! 1:  8785572c4d stash: reuse cached index entries in --patch temporary index
     @@ Commit message
      
          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.
     +    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>
      
     @@ builtin/stash.c: static int reset_tree(struct object_id *i_tree, int update, int
      +	struct lock_file lock_file = LOCK_INIT;
      +
      +	repo_read_index_preload(the_repository, NULL, 0);
     -+	if (refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL))
     -+		return -1;
     ++	refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL);
      +
      +	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
      +
     @@ builtin/stash.c: static int stash_patch(struct stash_info *info, const struct pa
       		goto done;
       	}
      
     - ## t/t3904-stash-patch.sh ##
     -@@ t/t3904-stash-patch.sh: test_expect_success 'none of this moved HEAD' '
     - 	verify_saved_head
     - '
     - 
     -+test_expect_success 'stash -p with unmodified tracked files present' '
     -+	git reset --hard &&
     -+	echo line1 >alpha &&
     -+	echo line1 >beta &&
     -+	git add alpha beta &&
     -+	git commit -m "add alpha and beta" &&
     -+	echo line2 >>alpha &&
     -+	echo y | git stash -p &&
     -+	echo line1 >expect &&
     -+	test_cmp expect alpha &&
     -+	test_cmp expect beta &&
     -+	git stash pop &&
     -+	printf "line1\nline2\n" >expect &&
     -+	test_cmp expect alpha &&
     -+	echo line1 >expect &&
     -+	test_cmp expect beta
     + ## t/perf/p3904-stash-patch.sh (new) ##
     +@@
     ++#!/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_expect_success 'stash -p with split hunk' '
     - 	git reset --hard &&
     - 	cat >test <<-\EOF &&
     ++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

 builtin/stash.c             | 70 +++++++++++++++++++++++++++++++++----
 t/perf/p3904-stash-patch.sh | 43 +++++++++++++++++++++++
 2 files changed, 107 insertions(+), 6 deletions(-)
 create mode 100755 t/perf/p3904-stash-patch.sh