Re: RFC: Merge-related plans

context: https://public-inbox.org/git/CABPp-BFQJZHfCJZ1qvhvVcMd-_sOfi0Fkm5PexEwzzN+Zw2akw@mail.gmail.com/

Hi Elijah,

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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.

Sorry, I should have been more clear about G.

In more detail...

[...]

Yeah that is what I had in mind when talking about a mini test frame work
above, such that you can run the same test on any merge strategy easily.
Sorry for being confusing there.

quoted

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

B may sound like an independent feature, but it needs a merge
algorithm that doesn't mess with the working tree

I agree on that,

so it depends pretty strongly on E.

.. but not quite on the conclusion:
You could also make the current merge algorithm working
tree independent. Thomas Rast (cc'd) did so IIUC in
https://public-inbox.org/git/cover.1409860234.git.tr@thomasrast.ch/
(search the archive for "--remerge" if interested in this prior work)
which did not land upstream.

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  * 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.

If the idea is to give feedback on *code* rather than just
ideas/tradeoffs/pinpointing-buggy-lines, then it sounds like you're
actually suggesting posting the RFC later rather than earlier?

Some people have complained that they don't get feedback on
ideas/tradeoffs/pinpointing-buggy-lines, but did so after sending
RFC code. So I'd think a sure way to get feedback is to send actual
code as an RFC even if it misses some parts.

Also, the bigger question for me wasn't so much "should I ask the list
about these changes?" before making them, but rather: Do folks want me
to bring these things up before I work on D & E -- even if I end up
not getting back to incorporating their answers for months until D & E
are completed and merged?

I'd think A + C are worth asking early nevertheless, even if D & E are
away for some month; having these niche cases covered (in code and
tests) as well as a new UI/UX for user communication sound exciting
(The latter could result in some bike shedding, and then having enough
time before you spend time to do it one way or another in D&E sounds
favorable)

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  * 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?

Let me re-phrase: D and G are completely independent series, both of
which happen to be divisible.  Should either of them be split?

Sorry for my confusion. This clears it up

More background: D is only a handful of commits, so far; the main
reason to split it is to allow some of it to go first (maybe even
before G or H).  The downside is introducing extra churn and risk in
merge-recursive, when I'm planning to rewrite it soon anyway.  I was
trying to minimize merge-recursive changes, other than trying to make
sure that the new merge strategy and merge-recursive will give
identical results (modulo maybe fixing a few extra corner cases and
running faster).  Basically, I wanted it to be really easy to compare
old and new strategies, but otherwise wanted to leave merge-recursive
mostly alone.  It's not entirely clear how quickly I'll find time to
work on all of this, though, so maybe just-wait-for-the-rewrite is not
the right prioritization.

My experience is that smaller patch series are reviewed faster,
so if the cost of splitting them up is not prohibitive, I'd strongly
consider doing that.

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  * 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?

For avoiding duplicate tests, my plan was to use a variable (like
GIT_TEST_SPLIT_INDEX or GIT_FSMONITOR_TEST), which, when set, would
change the default merge strategy from "recursive" to the new one, and
also replace explicit requests for the "recursive" merge strategy with
the new one.

So your long term plan is to *replace* the whole merge recursive strategy
giving the same results [analogous to implementing refstable as a new refs
back end, that behaves the same except faster and easier code for corner
cases] and not offer a *new* strategy [analog here: different (merge/)diff
strategies, such as Myers vs minimal vs patience] that yields user visible
difference and it is unclear whether the user prefers one or the other?
That wasn't clear to me, thanks for clarifying.

My original reply was assuming a new "mode" of merge strategy which
would be a user visible knob to turn. The environment variable you mentioned
sounds like that is an implementation detail, that is just for a
smooth transition
period, but the current code would be deprecated and deleted eventually.

For avoiding duplicate code...well, Junio's suggestion was "[to
rewrite] without using much from the existing code in
merge-recursive.c at all" because he has "written off that code as
mostly unsalvageable"[4].

There are some functions that I think are worth leaving intact (e.g.
helpers like output(), the directory rename detection stuff, or the
merge_submodule code that you recently moved).  That stuff I was
planning to put in a "/* COPIED FROM merge-recursive.c VERBATIM */"
section.  And eventually, the idea would be to delete the old
merge-recursive and just use the replacement, making the duplication
concerns eventually go away.

This clears up even more, thanks for clarifying!

...or is your question more about how to abstract things so that
others can also write new merge strategies in the future and allow as
much code and test re-use as possible?

That was my original question as I thought an API would help with
doing that as it would help to not fiddle around with the rest of the
code base.

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[4] https://public-inbox.org/git/xmqqk1ydkbx0.fsf@gitster.mtv.corp.google.com/

Did you mean:
https://public-inbox.org/git/xmqqd147kpdm.fsf@gitster.mtv.corp.google.com/

So in that case my reply above ("..so it depends pretty strongly on E.."
I have a different conclusion) is void, as that *is* the new strategy?

Thanks,
Stefan