Re: [PATCH v3 04/15] merge-tree: implement real merges

> On Thu, Feb 3, 2022 at 2:42 AM Johannes Altmanninger <aclopte@gmail.com> wrote:
> >
> > On Wed, Feb 02, 2022 at 04:18:39PM -0800, Elijah Newren wrote:
> > > On Wed, Feb 2, 2022 at 2:01 PM Junio C Hamano <gitster@pobox.com> wrote:
> > > >
> > > > Elijah Newren <newren@gmail.com> writes:
> > > >
> > > > > Yes, you are reading right.  I think the cherry-pick/rebase
> > > > > replacement actually deserves a separate command from what merges
> > > > > should use; replaying a sequence of commits just has a number of UI
> > > > > differences and abilities that I think pull it in a different
> > > > > direction.
> > > >
> > > > I completely disagree.  Each individual step in a sequence of
> > > > replaying commits in order (or in reverse order) should be
> > > > scriptable as a single merge-tree that takes "apply the change to go
> > > > from A^ to A on X".  Sequencing and placing UI around it is a job
> > > > for the script that drives merge-tree.
> > >
> > > Adding such an ability to merge-tree would be trivial -- it basically
> > > involves just two things: (1) accepting one extra argument, and (2)
> > > calling merge_incore_nonrecursive() instead of
> > > merge_incore_recursive().
> > >
> > > However, I think forking a subprocess for every merge of a series of
> > > commits is a completely unreasonable overhead, so even if we provide
> > > such an option to merge-tree, I still want a separate plumbing-ish
> > > tool that does non-worktree/non-index replaying of commits which is
> > > not written as a driver of merge-tree.  That other tool should just
> > > call merge_incore_nonrecursive() directly.  And such a tool, since it
> > > should handle an arbitrary number of commits, should certainly be able
> > > to handle just one commit.  From that angle, it feels like adding
> > > another mode to merge-tree would just be a partial duplication of the
> > > other tool.
> >
> > I wonder how the UI of a tool that does non-worktree/non-index cherry-picks
> > will look like.  I'd expect it to produce the same output as merge-tree,
> > except cherry-pick should probably output a commit OID, not a tree.
> >
> > Maybe we want a unified command that produces commits from any sequence of
> > merge/cherry-pick/revert/reword steps. The obvious UI would use something
> > like the rebase-todo list as input.  For example:
> >
> >         $ echo '
> >         pick commit1
> >         reword commit2  # edit commit message in $GIT_EDITOR
> >         merge commit3 -m "log message"
> >         ' | git create-commit commit0
> >         <OID of final commit>
> >
> > we start from commit0 and apply steps one-by-one. Obviously, one unsolved
> > problem is how to pass parameters like commit messages if no editor should
> > be invoked (my sketch uses -m).
> > If any of the steps fails when merging merge, then we get the tree with
> > conflicts
> >
> >         $ echo '
> >         pick commit1
> >         pick commit2
> >         pick commit-that-does-not-apply
> >         ' | git create-commit commit0
> >         <OID of commit after step 2>
> >         <OID of toplevel tree after failed merge>
> >         <Conflicted file info>
> >         <Informational messages>
> >
> > Replaying a series of commits might look like this:
> >
> >         $ echo 'pick commit1 ^commit0' | git create-commit new-base
> >
> > I'm concluding that this is a difficult UI problem
>
> I agree.  I've got a lot of thoughts on it, and some work in progress
> towards it (https://github.com/newren/git/tree/replay -- _very_ hacky,
> not even close to alpha quality, lots of fixup commits, todo comments,
> random brain dump files added to the tree, based on a previous round
> of this patch series, not updated for weeks, etc., etc.)

> > and having a merge-tree command that accepts a "common ancestor"
> > parameter could make it easier to experiment.  Of course that depends
> > on who is experimenting.
>
> I think that would result in experiments and eventually full-blown
> scripts designed around forking subprocesses for every merge, and
> pushes us back into the world of having a scripted-rebase again.  Yes,
> I know people can transliterate shell back to C; it seems to always be
> done as a half-way measure with the forking just being done from C or
> have other UI-warts guided by the shell design.  In fact, *that* was
> the primary reason for me not providing a merge-tree option based on
> merge_incore_nonrecursive(), despite how trivial it'd be to provide
> it.  If someone wanted a merge_incore_nonrecursive() mode for
> merge-tree for reasons other than attempting to build a
> rebase/cherry-pick replacement based on it, then I'd be much happier
> to provide it.
>
> If someone wants to experiment with what a plumbing-ish
> rebase/cherry-pick would look like, the _right_ way to do it would be
> making using of merge_incore_nonrecursive() directly.  If they want
> example code, I already provided some a year and a half ago and got it
> merged into git.git in the form of t/helper/test-fast-rebase.c.  My
> "replay" branch is based on that code, but (a) moves it from t/helper
> to a real builtin, (b) removes the hardcoded very strict input, (c)
> removes the line of code doing the index & working tree updates, and
> (d) modifies the output to be a more plumbing-ish style.