Re: [SCRIPT/RFC 0/3] git-commit --onto-parent (three-way merge, no working tree file changes)

Am 28.11.2017 um 02:15 schrieb Igor Djordjevic:

On 27/11/2017 22:54, Johannes Sixt wrote:

quoted

I my opinion, putting the focus on integration merge commits and the
desire to automate the re-merge step brings in a LOT of complexity in
the implementation for a very specific use-case that does not
necessarily help other cases.

It might seem more complex than it is, until you examine the guts to
see how it really works :)

Basic concept is pretty simple, as I actually don`t automate
anything, at least not in terms of what would manual user steps look
like - for example, there`s no real re-merge step in terms of
actually redoing the merge, I just reuse what was already there in a
very clean way, I would think (supported by my current, humble
knowledge, still).

The only merge that could possibly ever happen is upon committing
desired subset of changes onto parent, and that shouldn`t be too
complex by definition, otherwise that commit doesn`t really belong
there in the first place, if it can`t be meaningfully applied where
we want it (for now, at least).

That said, the whole operation of "posting on parent and re-merging
everything", the way it looks like from the outside, could end just
with a simple diff-apply-commit-commit internally, no merges at all.
Only if simple `git apply` fails, we try some trivial merging - and
all that inside separate (parent) index only, not touching original
HEAD index nor working tree, staying pristine for the whole process,
untouched.

Once done, you should be in the very same situation you started from,
nothing changed, just having your history tweaked a bit to tell a
different story on how you got there (now including a commit you
posted on your HEAD`s parent).

Ok, then please explain, how this process should work in my workflow and with the `commit --onto-parent` feature that you have in mind. I have an integration branch (which is a throw-away type, so you can mangle it in any way you want); it is a series of merges:

 ...A    ...C          <- topics A, C
     \       \
   ---o---o---o---o    <- integration
         /       /
     ...B    ...D      <- topics B, D

Now I find a bug in topic B. Assume that the merges of C and D have textual conflicts with the integration branch (but not with B) and/or may be evil. What should I do?

With git-post, I make a fixup commit commit on the integration branch, then `git post B && git merge B`:

 ...A    ...C                  <- topics A, C
     \       \
   ---o---o---o---o---f---F    <- integration
         /       /       /
     ...B    ...D       /      <- topic D
         \             /
          f'----------'        <- topic B

The merge F does not introduce any changes on the integration branch, so I do not need it, but it helps keep topic B off radar when I ask `git branch --no-merged` later.

Don`t let "usual/preferred/recommended" Git workflow distract you too
much - one of the reasons I made this is because it also allows _kind
of_ "vanilla Git" patch queue, where you can quickly work on top of
the merge head, pushing commits onto parents below, being tips of
your "queues", putting you up to speed without a need to ever switch
a branch (hypothetically), until satisfied with what you have, where
you can slow down and polish each branch separately, as usual.

Like working on multiple branches at the same time, in the manner
similar to what `git add --patch` allows in regards to working on
multiple commits at the same time. This just takes it on yet another
level... hopefully :)

'kay, I'm not eagerly waiting for this particular next level (I prefer to keep things plain and simple), but I would never say this were a broken workflow. ;)

quoted

In your scenario above, it would certainly not be too bad if you
forgo the automatic merge and have the user issue a merge command
manually. The resulting history could look like this:

(3)         o---o---A---X    (topicA)
            /         \   \
           /           M1--M2 (test, HEAD)
          /           /||
      ---o---o---M---' ||     (master)
          \   \       / |
           \   o-----B /      (topicB)
            \         /
             o---o---C        (topicC)

I.e., commit --onto-parent A produced commit X, but M2 was then a
regular manual merge. (Of course, I am assuming that the merge
commits are dispensible, and only the resulting tree is of
interest.)

I see - and what you`re asking for is what I already envisioned and
hoped to get some more feedback about, here`s excerpt from
[SCRIPT/RFC 3/3] git-commit--onto-parent.sh[1] (I guess you didn`t
have time to checked that one yet?):

I did have a brief look, but I stopped when I saw

	# Remove entry from HEAD reflog, not to pollute it with
	# uninteresting in-between steps we take, leaking implementation
	# details to end user.

It's a clear sign for me that's something wrong. It is not just reflogs that can become stale, but all operations that follow the `git commit` can fail. How do you clean up such a mess?

   For example, it might make sense to separate commit creation (on
   current HEAD`s parent) and its actual re-merging into integration
   test branch, where "--remerge" (or something) parameter would be used
   on top of "--onto-parent" to trigger both, if/when desired.
   
   Another direction to think in might be introducing more general
   "--onto" parameter, too (or instead), without "parent" restriction,
   allowing to record a commit on top of any arbitrary commit (other
   than HEAD). This could even be defaulted to "git commit <commit-ish>"
   (no option needed), where current "git commit" behaviour would then
   just be a special case of omitted <commit-ish> defaulting to HEAD,
   aligning well with other Git commands sharing the same behaviour.

So I definitely look forward decoupling these two ((1) commit to
parent and (2) remerge), with enough discussion flowing :)

Heck, even "to parent" is an artificial/imposed restriction now, in
reality you could commit on top of any other commit you want (without
switching branches)... but let`s take one step at a time.

Just note that omitting the remerge step is what actually makes the
logic more complex, as we now need to change the original situation,
too, both HEAD index and working tree, to remove changes which we
committed elsewhere (without "merging" back in).

The "complex situation" I had in mind was not so much about the user's view, but how the implementation has to work when there are errors.

quoted

From what I understand, your `git-post` makes a commit where you are _and copies_ it over to destination, so you end up with same changes

in two places (two commits). More, you seem to keep working on top of
your previous commit(s), which seems logical, later "posting" it
where desired - so after the fact. It also means you do still have
all your "fix" commits in HEAD while you work on them (and until you
git-post and abandon them, discarding the integration branch).

Just to clarify: git-post only copies existing commits, but does not make the original itself.

But `git commit --onto-parent` proposed here does the commit on
destination only, where later "remerge" is crucial part of still
keeping that commit changes in the current HEAD working tree as well,
so you can still keep working on top of them.

Could it be that you`re looking at `git commit --onto-parent` from an
unexpected perspective (through your `git-post`, being conceptually
different)...?

That may be the case. I'm very interested in your answer to my challenge above.

-- Hannes