Re: [RFC] should `git rebase --keep-base` imply `--reapply-cherry-picks` ?

[Phillip Wood <phillip.wood123@gmail.com>]

Hi all

On 15/07/2020 04:20, Denton Liu wrote:
> Hi all,
> 
> On Tue, Jul 14, 2020 at 10:38:23PM +0200, Johannes Schindelin wrote:
>>> To me the question is why are we looking at the upstream commits at all
>>> with `--keep-base`? I had expected `rebase --keep-base` to be the same
>>> as `rebase $(git merge-base [--fork-point] @{upstream} HEAD)` but
>>> looking at the code it seems to be `rebase --onto $(git merge-base
>>> @{upstream} HEAD) @{upstream}`. I didn't really follow the development
>>> of this feature - is there a reason we don't just use the merge-base as
>>> the upstream commit?
> 
> It behaves this way mostly for unimportant reasons. The first is that my
> workflow before implementing this feature invoked running
> `git rebase --onto master... master` and I wanted to replicate that.
> 
> More importantly, one feature of using the upstream I considered is
> documented in t3431. Essentially, if we have the following graph,
> 
> 	A---B---D---E    (master)
> 	     \
> 	      C*---F---G (side)
> 	
> 	C was formerly part of master but master was rewound to remove C
> 
> running `git rebase --keep-base --fork-point master` would drop C.
> 
>> Those are interesting questions, indeed.
>>
>> And I dare to suspect that the answer is indeed: `--keep-base` really
>> should not only substitute `onto` but also `upstream` with the merge base.
> 
> I would be open to changing the behaviour since the commit dropping
> isn't really a feature that I use very often. However, I am worried
> about pulling the rug out from other people if they use it since this is
> a documented feature in git-rebase.txt.

I think changing it to behave like

   git rebase --onto $(git merge-base @{upstream} HEAD) \
                     $(git merge-base --fork-point @{upstream} HEAD)

when --fork-point is given would keep the existing behavior of dropping 
commits when @{upstream} has been rewound without dropping cherry-picks. 
--fork-point seems less useful when combined with --keep-base than when 
used with a normal rebase as if @{upstream} has rewritten the commit 
that the branch is based on rather than just dropping it we end up 
dropping the original commit without getting the new version from 
upstream as we would with a normal rebase. This could be surprising to 
users if we keep --fork-point on by default with --keep-base.

Best Wishes

Phillip

> 
> Thanks,
> Denton
> 
>> Ciao,
>> Dscho