Re: RFC: Merge-related plans

[Stefan Beller <sbeller@google.com>]

On Mon, May 28, 2018 at 1:48 PM, Elijah Newren <newren@gmail.com> wrote:
> Hi everyone,
>
> I have some merge-related plans (and work in progress) that I'd like
> to get some feedback on in order to find what order would be best to
> address things in, if there are special steps I should take while
> approaching some of the bigger items, and even if folks disagree with
> any of the plans.
>
>
> Currently, I would like to:
>
> A) Fix cases where directory rename detection does not work with
>    rebase/am due to how they call merge-recursive.
>
>    Notes: Could just wait for D & E to land before fixing.
>    Alternatively, email RFC to list now explaining issues and how the
>    fix has performance implications; poll for opinions on whether to
>    fix before or after D.
>
> B) Implement a remerge-diff ability (compare a merge commit to what an
>    "automatic merge" would look like)[1].
>
>    Notes: Possibly for cherry-picks and reverts too.  Depends on C &
>    E.
>
> C) Modify/extend how path-based and mode-based conflicts are
>    communicated to the user.
>
>    Notes: Particularly important as a mechanism for handling
>    challenges[2] with implementing the remerge-diff ability.  Need to
>    send RFC to list with ideas to get feedback.
>
> D) Improve merge performance.
>
>    Notes: Includes 4-5 specific optimizations[5], some of which I
>    expect to be somewhat invasive and thus may make more sense to just
>    make part of the new merge strategy implemented in E.  Biggest
>    optimization depends on F.
>
> E) Write a new merge strategy meant to replace merge-recursive.
>
>    Notes: Suggested by Junio[3][4].  Depends on F & G.
>
> F) Make file collision conflict types more consistent.
>
>    Notes: Specifically, make rename/rename(2to1) & rename/add
>    conflicts behave more like add/add[6][7].  Depends on part of G.
>    Would prefer H to be accepted first.
>
> G) Improve merge-related portion of testsuite.
>
>    Notes: Intended to help test new merge strategy with more
>    confidence.  Will include approximately a dozen edge and corner
>    cases where merge-recursive currently falls short.  Started at [8];
>    see also [9].

Most items forward-reference "Depends on (<later letter>) up to here;
(H) seems independent, but might be a good first start.
(G) [8] is queued as origin/en/merge-recursive-tests, or do you mean
to expand (G) into a mini-framework of merge-testing? i.e. run the
mini test framework multiple times, each using a different
merge strategy, similar to submodule tests, e.g. see
t/lib-submodule-update.sh and one of its users, t1013.


> H) Miscellaneous code cleanups irritating me while working on other
>    changes[10].
>
>
> My current plan was to work roughly in reverse, from H to A.  Some questions:
>
>   * Does any of this look objectionable?

Going in order A-H seems slightly out-of-order to me, I'd think (H) and (G)
would go first;

(B) sounds like an independent feature, which could go in parallel?

>   * Should I post RFC questions on A and C earlier?

I would think so, it is easier to give feedback on code, I would think.

>   * Should I split D and G?  (Current plan was to keep D together, but
>     split G into five short slightly inter-dependent topics)

I would have expected to have tests (G) as a companion of (A) or (C)
rather than (D), as performance improvements would keep the test suite
unchanged?

>   * E is kind of big.  Are there any specific things folks would like to see
>     with how that is handled?

How much abstraction can be done ahead of time such that there is an
interface/API where you just plug in a new merge strategy and do not
need to duplicate a lot of code/tests?

Stefan