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

[Elijah Newren <newren@gmail.com>]

Hi Phillip,

On Tue, Jan 24, 2023 at 5:16 AM Phillip Wood <phillip.wood123@gmail.com> wrote:
>
> >>>> Signed-off-by: Elijah Newren <newren@gmail.com>
> >>>> --- a/builtin/rebase.c
> >>>> +++ b/builtin/rebase.c
> >>>> @@ -1224,6 +1224,26 @@ int cmd_rebase(int argc, const char **argv,
> >>>> const char *prefix)
> >>>>                if (options.fork_point < 0)
> >>>>                        options.fork_point = 0;
> >>>>        }
> >>>> +     /*
> >>>> +      * The apply backend does not support
> >>>> --[no-]reapply-cherry-picks.
> >>>> +      * The behavior it implements by default is equivalent to
> >>>> +      * --no-reapply-cherry-picks (due to passing --cherry-picks to
> >>>> +      * format-patch), but --keep-base alters the upstream such
> >>>> that no
> >>>> +      * cherry-picks can be found (effectively making it act like
> >>>> +      * --reapply-cherry-picks).
> >>>> +      *
> >>>> +      * Now, if the user does specify --[no-]reapply-cherry-picks, but
> >>>> +      * does so in such a way that options.reapply_cherry_picks ==
> >>>> +      * keep_base, then the behavior they get will match what they
> >>>> +      * expect despite options.reapply_cherry_picks being ignored.  We
> >>>> +      * could just allow the flag in that case, but it seems better to
> >>>> +      * just alert the user that they've specified a flag that the
> >>>> +      * backend ignores.
> >>>> +      */
> >>>
> >>> I'm a bit confused by this. --keep-base works with either
> >>> --reapply-cherry-picks (which is the default if --keep-base is given) or
> >>> --no-reapply-cherry-picks. Just below this hunk we have
> >>>
> >>>          if (options.reapply_cherry_picks < 0)
> >>>                  options.reapply_cherry_picks = keep_base;
> >>>
> >>> So we only set options.reapply_cherry_picks to match keep_base if the
> >>> user did not specify -[-no]-reapply-cherry-picks on the commandline.
> >>
> >> options.reapply_cherry_picks is totally ignored by the apply backend,
> >> regardless of whether it's set by the user or the setup code in
> >> builtin/rebase.c.  And if we have an option which is ignored, isn't it
> >> nicer to provide an error message to the user if they tried to set it?
> >>
> >> Said another way, while users could start with these command lines:
> >>
> >>      (Y) git rebase --whitespace=fix
> >>      (Z) git rebase --whitespace=fix --keep-base
> >>
> >> and modify them to include flags that would be ignored, we could allow:
> >>
> >>      (A) git rebase --whitespace=fix --no-reapply-cherry-picks
> >>      (B) git rebase --whitespace=fix --keep-base --reapply-cherry-picks
> >>
> >> But we could not allow commands like
> >>
> >>      (C) git rebase --whitespace=fix --reapply-cherry-picks
> >>      (D) git rebase --whitespace=fix --keep-base
> >> --no-reapply-cherry-picks
> >
> > (C) is already an error
> > (D) is currently allowed and I think works as expected (--keep-base only
> > implies --reapply-cherry-picks, the user is free to override that with
> > --no-reapply-cherry-picks) so I don't see why we'd want to make it an
> > error.

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?

> +                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?

> +                 * 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");

Also, the comment in git-rebase.txt about incompatibilities would become

     * --[no-]reapply-cherry-picks, when --keep-base is not provided