Re: [PATCH v4 5/9] rebase: add coverage of other incompatible options

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

Hi Elijah

On 24/01/2023 15:41, Elijah Newren wrote:
> Ah, despite looking over the code multiple times to check my
> statements, I somehow kept missing this:
> 
>      if (keep_base && options.reapply_cherry_picks)
>          options.upstream = options.onto;
> 
> which is how --[no-]reapply-cherry-picks is supported in conjunction
> with --keep-base.  Thanks.
> 
>>>> For all four cases (A)-(D), the apply backend will ignore whatever
>>>> --[no-]reapply-cherry-picks flag is provided.
>>>
>>> For (D) the flag is respected, (C) errors out, the other cases
>>> correspond to the default so it's like saying
>>>
>>>       git rebase --merge --no-reapply-cherry-picks
>>>
>>> ignores the flag.
>>
>> On reflection that is only true for (B). I agree that we should error
>> out on (A) which we don't at the moment.
>>
>> I'd support a change that errors out on (A) and (C) but continues to
>> allow (B) and (D). I think we can do that with the diff below
>>
>>
>> diff --git a/builtin/rebase.c b/builtin/rebase.c
>> index 1481c5b6a5..66aef356b8 100644
>> --- a/builtin/rebase.c
>> +++ b/builtin/rebase.c
>> @@ -1230,12 +1230,6 @@ int cmd_rebase(int argc, const char **argv, const
>> char *prefix)
>>                    if (options.fork_point < 0)
>>                            options.fork_point = 0;
>>            }
>> -        /*
>> -         * --keep-base defaults to --reapply-cherry-picks to avoid losing
>> -         * commits when using this option.
>> -         */
>> -        if (options.reapply_cherry_picks < 0)
>> -                options.reapply_cherry_picks = keep_base;
>>
>>            if (options.root && options.fork_point > 0)
>>                    die(_("options '%s' and '%s' cannot be used
>> together"), "--root", "--fork-point");
>> @@ -1412,11 +1406,17 @@ int cmd_rebase(int argc, const char **argv,
>> const char *prefix)
>>            if (options.empty != EMPTY_UNSPECIFIED)
>>                    imply_merge(&options, "--empty");
>>
>> -        /*
>> -         * --keep-base implements --reapply-cherry-picks by altering
>> upstream so
>> -         * it works with both backends.
>> -         */
>> -        if (options.reapply_cherry_picks && !keep_base)
>> +        if (options.reapply_cherry_picks < 0)
>> +                /*
>> +                 * --keep-base defaults to --reapply-cherry-picks to
>> +                 * avoid losing commits when using this option.
>> +                 */
> 
> I know you were copying the previous comment, but this comment is
> really confusing to me.  Shouldn't it read "--reapply-cherry-picks
> defaults to --keep-base" instead of vice-versa?

Clearly this comment has not been as helpful as I indented it to be. I 
think maybe we should spell out the defaults with and without 
--keep-base. Perhaps something like

default to --no-reapply-cherry-picks unless --keep-base is given in 
which case default to --reapply-cherry-picks


>> +                options.reapply_cherry_picks = keep_base;
>> +        else if (!keep_base)
>> +                /*
>> +                 * --keep-base implements --reapply-cherry-picks by
> 
> Should this be --[no-]reapply-cherry-picks, to clarify that it handles
> both cases?  Especially given how many times I missed it?

This has obviously proved to be confusing. The aim was to explain that 
in order to work with the apply backend "[--reapply-cherry-picks] 
--keep-base" was doing something unusual with `upstream` to reapply 
cherry picks. "--no-reapply-cherry-picks --keep-base" does not do 
anything unusual with `upstream`. I don't think changing it to 
--[no-]reapply-cherry-picks quite captures that. I came up with

To support --reapply-cherry-picks (which is not supported by the apply 
backend) --keep-base alters upstream to prevent cherry picked commits 
from being dropped.

but it really needs to mention that --keep-base also supports 
--no-reapply-cherry-picks in the usual way

>> +                 * altering upstream so it works with both backends.
>> +                 */
>>                    imply_merge(&options, "--reapply-cherry-picks");
> 
> And perhaps this should be
> 
>      imply_merge(&options, options.reapply_cherry_picks ?
> "--reapply-cherry-picks" :
>           "--no-reapply-cherry-picks");

That's a good idea

> Also, the comment in git-rebase.txt about incompatibilities would become
> 
>       * --[no-]reapply-cherry-picks, when --keep-base is not provided

Yes, that's good

Thanks again for working on this

Phillip