No one understands diff3 "Temporary merge branch" conflict markers

From: Edward Anderson <nilbus@nilbus.com>
Date: Jul 7, 2015, 14:55 UTC
Message-ID: <CAOMsSXQVJsd0h1fnNMEJ5+cKpxbeF9mHraXva-wr6Y2zBCADbg@mail.gmail.com>

I have the diff3 conflictstyle enabled and want to be able to understand how to understand its output when there are criss-cross merges requiring temporary merge branches. Eg:

    <<<<<<< HEAD
      print(A);
    ||||||| merged common ancestors
    <<<<<<< Temporary merge branch 1
      print(B);
    =======
      print(C);
    >>>>>>> feature

I have combed Google but have been able to find no good explanation. People are hitting these and asking for help, but the suggestions are weak and fail to explain anything.

Background ----------

Our workflow often causes criss-cross merges. In this more extreme (but real) case, I have 16 ambiguously "best" merge bases:

    $ git merge-base --all HEAD feature | wc -l
    16

The 'resolve' strategy isn't an option for this many merge-bases:

    $ git merge -s resolve feature
    fatal: I cannot read more than 8 trees
    Merge with strategy resolve failed.

That brings me back to the recursive strategy.

https://lwn.net/Articles/210045/ "Branching and merging with git" describes what a criss-cross merge is and how the recursive strategy deals with them by merging the multiple merge-bases to form a single temporary merge base.

Show 21 quoted lines

> The classic confusing case is called a "criss-cross merge", and
> looks like this:
>
>          o--b-o-o--B
>         /    \ /
>  o--o--o      X
>         \    / \
>          o--a-o-o--A
>
> There are two common ancestors of A and B, marked a and b in the
> graph above.  And they're not the same.  You could use either one
> and get reasonable results, but how to choose?
>
> The details are too advanced for this discussion, but the default
> "recursive" merge strategy that git uses solves the answer by
> merging a and b into a temporary commit and using *that* as the
> merge base.
>
> Of course, a and b could have the same problem, so merging them
> could require another merge of still-older commits.  This is why the
> algorithm is called "recursive."

With the diff3 conflictstyle showing the merged common ancestors and the recursive merge strategy, the merged common ancestors may have a conflict

Conflict #1 -----------

     1 <<<<<<< HEAD
     2     unless admin
     3       fail Unauthorized.new("Admin only")
     4     end
     5 ||||||| merged common ancestors
     6 <<<<<<< Temporary merge branch 1
     7     unless admin
     8       fail Unauthorized.new("Admin only")
     9     end
    10 ||||||| merged common ancestors
    11     unless admin
    12       fail Unauthorized.new
    13     end
    14 =======
    15     fail Unauthorized.new unless admin
    16 >>>>>>> Temporary merge branch 2
    17 =======
    18     unless admin
    19         fail Unauthorized.new("Admin only")
    20       fail Unauthorized.new
    21     end
    22 >>>>>>> feature

It seems lines 6-16 are a conflict that occurred when merging the merge-bases. That conflict could be resolved by merging the change in Temporary merge branch 1 (add "Admin only") with Temporary merge branch 2 (convert multi-line unless to single-line) as this:

           fail Unauthorized.new("Admin only") unless admin

Thus, you might thing that you could refactor the above conflict to be this:

     1 <<<<<<< HEAD
     2     unless admin
     3       fail Unauthorized.new("Admin only")
     4     end
     5 ||||||| merged common ancestors
     6     fail Unauthorized.new("Admin only") unless admin
     7 =======
     8     unless admin
     9         fail Unauthorized.new("Admin only")
    10       fail Unauthorized.new
    11     end
    12 >>>>>>> feature

However this would be resolved by merging HEAD's apparent change (break single-line unless onto multiple lines) and feature's change, which appears to be a botched up conflict resolution.

Obviously I'm reading this wrong. What is the correct way?

Conflict #2 ------------

     1 <<<<<<< HEAD
     2     unless feature.enabled_for_user?(UserIdLookup.new(params).user_id)
     3       fail Unauthorized.new("Requires setting #{label}.")
     4 ||||||| merged common ancestors
     5 <<<<<<< Temporary merge branch 1
     6     unless feature.enabled_for_user?(params[:user_id])
     7       fail Unauthorized.new("Requires setting #{label}.")
     8 =======
     9     unless feature.enabled_for_user?(params[:user_id])
    10       fail Unauthorized.new("Requires setting #{label}.")
    11 >>>>>>> feature

This is the full conflict, and it doesn't seem to balance. In this case, there's only one Temporary merge branch. Can the line marker on line 5 just be ignored? If so, then this is easy to resolve, but I am left puzzled as to why the sections in lines 6-7 and 9-10 are identical. Is the Temporary merge branch 1 marker here simply because other conflicts had multiple Temporary merge branches that had to be labeled? Or is the 0 lines between lines 4 and 5 a significant deletion?

---

The git community needs some light and understanding shed on this topic, on how these conflicts are to be read and understood. I'm looking for something that can be applied methodically. If better understanding the recursive merge strategy is necessary, that is fine. However suggestions like "just resolve it to how you think it should be" or "talk to the authors" will not be appreciated, as they do not lend understanding regarding these conflict markers nor where the code they delineate comes from.

Does anyone out there understand this stuff?

Thanks, Edward