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Re: Merge commit diff results are confusing and inconsistent




Hi Robert,

On 07/05/2019 15:10, Robert Dailey wrote:
The majority use case I'm interested in is seeing net-positive changes
that happen in merge commits. Normally I take for granted that merge
commits have nothing meaningful in them (meaningful here defined as
something unexpected for a merge commit). But what if someone makes a
poor decision and does some crazy refactoring in 1 file and amends it
into a merge commit? Let's also say that these changes are done to a
file that wasn't modified in any parent (say a unrelated.txt next to
your color.txt). Since neither parent cares about that file for the
purposes of the merge, I am trying to make sense of a revision
specification that can be used to see what they did to that file.

I see that you are specifically interested in seeing 'net-positive' changes.

Part of the problem is that for a merge commit there are multiple choices as to the implied initial central merge, where A and B are combined to create X [which I just called the central merge], to which further changes are made to create the final merge commit C. (Note: X is never committed, and is somewhat 'mythical')

These cases where there needs to be 'further changes', either to resolve conflicts because we never got a cleanly merged X, or the user added changes, we an "Evil Commit/Merge". Definitions vary slightly between different protagonists in the VCS world as to the best evil merge resolution starategies.

For your 'net-positive' changes, what is needed is to effectively generate that mythical clean initial merge X where either we delete from both sides, or we have a simple addition only from one side (addition/deletion normally being of whole lines). It is only that way that allows the changes from X to C to be addition only.

Unfortunately there is currently no diff representation that does that, as there is no method of indicating that middle X state. In the worst case there are always pathological cases.

A similar problem exists for the “reuse recorded resolution” (rerere / redo) storage of conflict resolutions. At present there isn't a way of exchanging such resolutions in a mechanism similar to a diff. In fact I was only just asking about that [1]  within the last two days! There is some discussion about the rerere database in [2], should you want a look.
--
Philip


[1] rerere - https://public-inbox.org/git/b8e56556-6c83-9e37-38e9-ac67f51b5cd2@xxxxxxx/ [2] https://github.com/git/git/blob/master/Documentation/technical/rerere.txt