Re: [PATCH v2 13/13] merge-ort, diffcore-rename: employ cached renames when possible

[Derrick Stolee <stolee@gmail.com>]

On 5/3/21 10:12 PM, Elijah Newren via GitGitGadget wrote:
> From: Elijah Newren <newren@gmail.com>
...
> diff --git a/merge-ort.c b/merge-ort.c
> index 2fc98b803d1c..17dc3deb3c73 100644
> --- a/merge-ort.c
> +++ b/merge-ort.c
> @@ -753,15 +753,48 @@ static void add_pair(struct merge_options *opt,
>  	struct rename_info *renames = &opt->priv->renames;
>  	int names_idx = is_add ? side : 0;
>  
> -	if (!is_add) {
> +	if (is_add) {
> +		if (strset_contains(&renames->cached_target_names[side],
> +				    pathname))
> +			return;
> +	} else {
>  		unsigned content_relevant = (match_mask == 0);
>  		unsigned location_relevant = (dir_rename_mask == 0x07);
>  
> +		/*
> +		 * If pathname is found in cached_irrelevant[side] due to
> +		 * previous pick but for this commit content is relevant,
> +		 * then we need to remove it from cached_irrelevant.
> +		 */
> +		if (content_relevant)
> +			/* strset_remove is no-op if strset doesn't have key */
> +			strset_remove(&renames->cached_irrelevant[side],
> +				      pathname);

I see, content can become relevant again.

...

> diff --git a/t/t6429-merge-sequence-rename-caching.sh b/t/t6429-merge-sequence-rename-caching.sh
> index f47d8924ee73..035edc40b1eb 100755
> --- a/t/t6429-merge-sequence-rename-caching.sh
> +++ b/t/t6429-merge-sequence-rename-caching.sh
> @@ -101,10 +101,10 @@ test_expect_success 'caching renames does not preclude finding new ones' '
>  # dramatic change in size of the file, but remembering the rename and
>  # reusing it is reasonable too.
>  #
> -# Rename detection (diffcore_rename_extended()) will run twice here; it is
> -# not needed on the topic side of history for either of the two commits
> -# being merged, but it is needed on the upstream side of history for each
> -# commit being picked.
> +# We do test here that we expect rename detection to only be run once total
> +# (the topic side of history doesn't need renames, and with caching we
> +# should be able to only run rename detection on the upstream side one
> +# time.)
>  test_expect_success 'cherry-pick both a commit and its immediate revert' '
>  	test_create_repo pick-commit-and-its-immediate-revert &&
>  	(
> @@ -140,11 +140,11 @@ test_expect_success 'cherry-pick both a commit and its immediate revert' '
>  		GIT_TRACE2_PERF="$(pwd)/trace.output" &&
>  		export GIT_TRACE2_PERF &&
>  
> -		test_might_fail test-tool fast-rebase --onto HEAD upstream~1 topic &&
> +		test-tool fast-rebase --onto HEAD upstream~1 topic &&

Here is a change of behavior, but it appears to be a good one!

>  		#git cherry-pick upstream~1..topic &&
>  
>  		grep region_enter.*diffcore_rename trace.output >calls &&
> -		test_line_count = 2 calls
> +		test_line_count = 1 calls
>  	)
...
> @@ -450,7 +459,7 @@ test_expect_success 'dir rename unneeded, then add new file to old dir' '
>  		#git cherry-pick upstream..topic &&
>  
>  		grep region_enter.*diffcore_rename trace.output >calls &&
> -		test_line_count = 3 calls &&
> +		test_line_count = 2 calls &&
>  
>  		git ls-files >tracked &&
>  		test_line_count = 5 tracked &&
> @@ -516,7 +525,7 @@ test_expect_success 'dir rename unneeded, then rename existing file into old dir
>  		#git cherry-pick upstream..topic &&
>  
>  		grep region_enter.*diffcore_rename trace.output >calls &&
> -		test_line_count = 4 calls &&
> +		test_line_count = 3 calls &&

I appreciate that this use of tracing demonstrates a change of
internal behavior.

>  		test_path_is_missing somefile &&
>  		test_path_is_missing olddir/newfile &&
> @@ -648,9 +657,8 @@ test_expect_success 'caching renames only on upstream side, part 1' '
>  # for the wrong side of history.
>  #
>  #
> -# This testcase should only need three calls to diffcore_rename_extended(),
> -# because there are no renames on the topic side of history for picking
> -# Topic_2.
> +# This testcase should only need two calls to diffcore_rename_extended(),
> +# both for the first merge, one for each side of history.
>  #
>  test_expect_success 'caching renames only on upstream side, part 2' '
>  	test_setup_topic_rename cache-renames-only-upstream-rename-file &&
> @@ -677,7 +685,7 @@ test_expect_success 'caching renames only on upstream side, part 2' '
>  		#git cherry-pick upstream..topic &&
>  
>  		grep region_enter.*diffcore_rename trace.output >calls &&
> -		test_line_count = 3 calls &&
> +		test_line_count = 2 calls &&

Same here.

As I was reading, I was also thinking that it would be good to
have some kind of tracing, if only a summary of how often we
relied upon the cached renames. That would present a mechanism
for the test cases to verify that the rename cache is behaving
as expected, but also provides a way to diagnose any issues that
might arise in the future by asking a user to reproduce a
problematic rebase/merge with a GIT_TRACE2* target. Such a
change would fit as a follow-up, and does not need to insert
into an already heavy patch.

I have now read all of the patches in this series to the level
I can. It's all very deep stuff, so the more we can rely on the
tests to show correctness, the better.

I appreciate the extra tests that you added, which increases my
confidence in the series.

Thanks,
-Stolee