Re: [PATCH] rebase -i: fix rewording with --committer-date-is-author-date

Hi Peff

On 02/11/2021 22:32, Jeff King wrote:

On Tue, Nov 02, 2021 at 08:10:44PM +0000, Phillip Wood via GitGitGadget wrote:

quoted

From: Phillip Wood <redacted>

baf8ec8d3a (rebase -r: don't write .git/MERGE_MSG when
fast-forwarding, 2021-08-20) stopped reading the author script in
run_git_commit() when rewording a commit. This is normally safe
because "git commit --amend" preserves the authorship. However if the
user passes "--committer-date-is-author-date" then we need to read the
author date from the author script when rewording. Fix this regression
by tightening the check for when it is safe to skip reading the author
script.

That description makes sense, and the patch matches. Not being that
familiar with this area, my biggest question would be: are there are
other cases that would need the same treatment? And is there a way we
can make it easier to avoid forgetting such a case in the future?

I don't think there are any other cases (but then I thought that when I 
wrote the buggy patch...). The only time we change the authorship is if 
the user passes --committer-date-is-author-date or --reset-author-date. 
I agree it would be good to have a way to avoid this problem in the 
future but I haven't come up with an easy way to do that. One 
possibility would be to go back to always reading the author script. 
That would mean revisiting the changes to do_merge() in baf8ec8d3a so 
that it always writes the author script and .git/MERGE_MSG but removes 
them when fast-forwarding (the problem that baf8ec8d3a tried to solve 
was a left over .git/MERGE_MSG when do_merge() fast-forwarded) I don't 
want to do that in the rc window though.

quoted

diff --git a/sequencer.c b/sequencer.c
index cd2aabf1f76..ea96837cde3 100644
--- a/sequencer.c
+++ b/sequencer.c

@@ -997,7 +997,9 @@ static int run_git_commit(const char *defmsg,
  
  	cmd.git_cmd = 1;
  
-	if (is_rebase_i(opts) && !(!defmsg && (flags & AMEND_MSG)) &&
+	if (is_rebase_i(opts) &&
+	    ((opts->committer_date_is_author_date && !opts->ignore_date) ||
+	     !(!defmsg && (flags & AMEND_MSG))) &&
  	    read_env_script(&cmd.env_array)) {
  		const char *gpg_opt = gpg_sign_opt_quoted(opts);

This conditional is getting pretty complicated. I wonder if a helper
like:

   if (is_rebase_i(opts) && !needs_env_script(...))

might help, but I guess it needs a funky array of inputs (defmsg, flags,
and opts). So maybe it is just making things worse.

As you say it needs a lot of inputs so I'm not sure how much a function 
would help. I did consider changing it to

         if (is_rebase_i(opts) &&
	    ((opts->committer_date_is_author_date && !opts->ignore_date) ||
              defmsg || !(flags & AMEND_MSG)))

but as we're in the rc phase I decided to leave the existing condition 
alone.

quoted

+test_expect_success '--committer-date-is-author-date works when rewording' '
+	GIT_AUTHOR_DATE="@1234 +0300" git commit --amend --reset-author &&
+	(
+		set_fake_editor &&
+		FAKE_COMMIT_MESSAGE=edited \
+			FAKE_LINES="reword 1" \
+			git rebase -i --committer-date-is-author-date HEAD^
+	) &&
+	test_write_lines edited "" >expect &&
+	git log --format="%B" -1 >actual &&
+	test_cmp expect actual &&
+	test_ctime_is_atime -1
+'

This test make sense (I had to look up what "-1" means for
test_ctime_is_atime; it's passed to git-log to decide which commits to
look at).

Yeah I had to check what the -1 was for when writing the test, maybe we 
should change the helper to add the '-' for us once 2.34.0 is out.

quoted

+test_expect_success 'reset-author-date with --committer-date-is-author-date works when rewording' '
+	GIT_AUTHOR_DATE="@1234 +0300" git commit --amend --reset-author &&
+	(
+		set_fake_editor &&
+		FAKE_COMMIT_MESSAGE=edited \
+			FAKE_LINES="reword 1" \
+			git rebase -i --committer-date-is-author-date \
+				--reset-author-date HEAD^
+	) &&
+	test_write_lines edited "" >expect &&
+	git log --format="%B" -1 >actual &&
+	test_cmp expect actual &&
+	test_atime_is_ignored -1
+'

And this one I guess is covering the --ignore-date cut-out in the code?

Yes

I think it would pass even without it, as that is just noting a case
where we _don't_ need to call read_env_script().

That's right.

But I don't know if
there is any user-visible effect of accidentally calling it when we
don't need to (my impression is that it's just a performance thing).

There should not be any user-visible effects from reading the author 
script when --ignore-date is given but we don't need to read it in that 
case so I opted not to.

Thanks for your comments, are you happy for this to go in as is or 
should I look at simplifying the conditional?

Phillip

-Peff