Re: Feature request: rebase -i inside of rebase -i

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi George,

On Sat, 21 Mar 2020, George Spelvin wrote:

> On Sat, Mar 21, 2020 at 11:51:10AM +0100, Johannes Schindelin wrote:
> > My biggest caveat is that I had to force-exit the rebase at some stage
> > due to reasons I only vaguely remember. It had something to do with the
> > replacement cache not being updated when an `exec` is executed that adds a
> > replacement object via `git replace` [*1*]. This issue might have
> > _nothing_ to do with nested rebases, but as I said, my recollection is
> > vague.
>
> This is the sort of internal implementation gotcha I worry about.

There's plenty more gotchas like that ;-)

> > There are a couple more concerns, of course, such as: what to do if the
> > user deletes the entire todo list (which is traditionally the only way to
> > abort a rebase)? My gut feeling is that it should go back to the
> > _previous_ version of the todo list.
>
> My assumption has been that, for simplicity, there would only be one
> commit in progress, and aborting it aborts everything.

But that does not necessarily make sense. Imagine that you rebase the
latest three commits, interactively. Then a merge conflict in the third
makes you realize that the first commit is no longer needed.

Enter the nested rebase. You manually re-schedule the failed `pick` via
`git rebase --edit-todo` and then run the nested rebase: `git reset --hard
&& git rebase -i --nested HEAD~2`.

Except that you made a typo and said `HEAD~3` instead of `HEAD~2`. You
delete the entire todo list to get a chance to restart the nested rebase.

But now the entire rebase gets aborted?

If that would happen to me, I would unleash a whole slew of rarely used
words in the vague direction of whoever implemented the nested rebase
feature...

> > Another big concern is what to do about `rebase.missingCommitsCheck`: with
> > nested rebases, this will get increasingly tricky. Like, imagine you are
> > rebasing 5 commits, the third of them results in merge conflicts, you
> > realize that it is obsolete and so is now the first, already rebased
> > commit. You do a nested rebase of the latest two commits to drop them, but
> > they don't have their original commit hashes any longer. So it gets a bit
> > finicky to keep track of what commit has been dropped on purpose and what
> > was forgotten to pick instead.
>
> This doesn't *seem* difficult, but I don't know how the current mechanism
> works.

The implementation details do not matter at this stage. You have to get
the design of the feature right.

I am unfamiliar with the design of the feature as it is implemented right
now, but I imagine that it needs to be adjusted for nested rebases, as we
no longer have a single original todo list to roll back to.

> It just checks that all commits that were on the to-do list when the
> editor started are still listed (possibly marked "drop") when it exits.
>
> When you do a nested commit, additional commits are prepended to the to-do
> list, you're dropped into the editor, and the same check is made when the
> editor returns.
>
> If rebase.missingCommitsCheck = error is triggered, you end up with the
> <upstream> tree state with nothing applied and may either --continue to
> ignore the error or --edit-todo to put back the missing commits.
>
>
> Let me give an example.  Suppose I have commits a-b-c-d-e, and start
> with rebase -i b.
>
> My to-do list starts out as c-d-e, but suppose I decide to cherry-pick
> z and add it to the list, so it's now z-c-d-e.
>
> So I start rebasing, and it turns out that d creates a merge conflict
> because I forgot a prerequisite patch y.  And I really want y and z before
> b, anyway.
>
> So the tree state is currently a-b-z'-c', with d in progress and e yet to
> do.  In my simple model, I have to resolve and commit d, so the tree
> state is a-b-z'-c'-d'.  Then I can rebase -i a, and am presented with
> a to-do list of b-z'-c'-d'-e.
>
> If I delete any of those five commits, then rebase.missingCommitsCheck
> will trigger.  If I put y in the list, save it, then change my mind and
> --edit-todo and delete y, it will also trigger.

As I said, I am not using that feature myself, so I do not even know what
"trigger" means in this context. It might totally be okay to use the
existing code as-is in the context of a nested rebase. That remains to be
verified, though, I think.

> Now, it sould be nice if there were a way to say "screw this mess; just
> check out HEAD and put d back on the to-do list", but that could
> create a bit of a mess if I've split d and already committed half of
> it.  If I used that after doung that, I'd have a to-do list of
> b-z'-c'-d'-d-e which might be awkward, but maybe it wouldn't be too bad.

There is all kind of opportunity for messes, all right.

Ciao,
Johannes