RFC patchcontrib: rerere-train overwrites existing resolutions

From 41116889f7eb2ddd590d165d9ca89646f7b15aaf Mon Sep 17 00:00:00 2001
From: Raman Gupta <rocketraman@gmail.com>
Date: Tue, 25 Jul 2017 10:28:35 -0400
Subject: [PATCH] contrib: rerere-train overwrites existing resolutions

When running rerere-train, the user is explicitly asking the training to
occur based on the current merge commit. However, if the current cache
has a resolution for the same conflict (even if out of date or wrong),
rerere-train does not currently update the rr-cache. Now, forget
existing resolutions before training so that training is always
reflective of the trained data.
---
 contrib/rerere-train.sh | 1 +
 1 file changed, 1 insertion(+)

Raman Gupta <rocketraman@gmail.com> writes:

These do not belong to the body of the message.

Imagine that your title line appears in "git shortlog --no-merges" output together with 600 other commits, and you see that list 3 months later. Can you tell what this change was about?

To me, it sounds like a user is complaining that it overwrites and loses a precious one, implying that the change is to fix it by being more careful, i.e. checking if there is an existing one and avoid overwriting it.

As our convention is to write the log message as if you are giving an order to the codebase "to behave like so",

	contrib/rerere-train: overwrite existing resolutions

would convey what you are doing better, I would think.

As to the change itself, I do anticipate that a user who is used to rerere-train will complain that the updated one overwrites existing resolutions, losing data, as the existing one didn't, and they are used to the way to correct an incorrect resolution that is recorded earlier, which is:

 * perform the botched merge again, which will materialize the
   botched merge result in the working tree;

 * do "git rerere forget" for the path;

 * correct the path with the right merge result; and then finally

 * do "git rerere".

If this new behaviour is triggered only when a command line option is given (e.g. "--overwrite" which is better than "--force"), I would imagine that it might give us an easier way to achieve the same thing without learning about "rerere forget". So I do not fundamentally oppose to having such an option, but changing the default behaviour is not going to fly.

Actually, the user is asking to record what is not recorded yet, which is the originally intended use case. You may fetch my 'pu' branch into a repository you already have a copy of git.git, and want to grab conflict resolutions I did between master..pu that you still do not know about. The tool is conservative and avoids clobbering your resolution if you already have resolved some of the merges yourself.

In any case, please make it a habit to sign-off your work when posting here.

Provide the user an option to overwrite existing resolutions using an `--overwrite` flag. This might be used, for example, if the user knows that they already have an entry in their rerere cache for a conflict, but wish to drop it and retrain based on the merge commit(s) passed to the rerere-train script.

Signed-off-by: Raman Gupta <rocketraman@gmail.com>
---
 contrib/rerere-train.sh | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

Raman Gupta <rocketraman@gmail.com> writes:

It is very good that you initialize overwrite explicitly here, to prevent it from seeping through from the caller's environment.

I haven't tried this patch, but would this work well with options meant for the 'git rev-list --parents "$@"' that grabs the list of merge commits to learn from? e.g.

	$ contrib/rerere-train.sh -n 4 --merges master
	$ contrib/rerere-train.sh --overwrite -n 4 --merges master
	$ contrib/rerere-train.sh -n 4 --overwrite --merges master

I do not think it is necessary to make the last one work; as long as the first two work as expected, we are good even if the last one dies with a sensible message e.g. "options X, Y and Z must be given before other options" (currently "X, Y and Z" consists only of "--overwrite", but I think you get what I mean).

	if test "$overwrite" = 1

cf. Documentation/CodingGuidelines.

On 26/07/17 02:05 PM, Junio C Hamano wrote:

You're right -- I didn't try all the cases. I wasn't able to figure out how to get `rev-parse --parseopt` to deal with this situation, so I did it manually. I'm not super-happy with the result, but it does work. Look for PATCH v3.

Regards, Raman

Raman Gupta <rocketraman@gmail.com> writes:

Yes, I think you could squash the two case arms in the later loop into one i.e.

	-h|--help|-o|--overwrite)
		die "please don't." ;;

but still the repetition does look ugly.

As a contrib/ material, I do not care too deeply about it, though.

Will queue.

On 26/07/17 04:41 PM, Junio C Hamano wrote:

I considered that but decided the non-collapsed version better supports this list growing in the future.

Agreed.

Thanks.

Regards, Raman

Provide the user an option to overwrite existing resolutions using an `--overwrite` flag. This might be used, for example, if the user knows that they already have an entry in their rerere cache for a conflict, but wish to drop it and retrain based on the merge commit(s) passed to the rerere-train script.

Signed-off-by: Raman Gupta <rocketraman@gmail.com>
---
 contrib/rerere-train.sh | 54 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 52 insertions(+), 2 deletions(-)