Re: [PATCH 5/5] submodule: submodule_move_head omits old argument in forced case

[Stefan Beller <sbeller@google.com>]

On Tue, Dec 19, 2017 at 2:44 PM, Jonathan Nieder <jrnieder@gmail.com> wrote:
> Hi,
>
> I had trouble understanding what this fixes, so I'll try nitpicking a
> bit as a sideways way to address that.
>
> Stefan Beller wrote:
>
>> With the previous patch applied (fix of the same() function),
>
> This tripped me up a bit.  Usually commits assume that all previous
> patches have already been applied, since that allows the maintainer to
> apply the early part of a series on one day and the later part another
> day without losing too much context.
>
> I think this intends to say something like
>
>         Now that we allow recursing into an unchanged submodule (see
>         "unpack-trees: Fix same() for submodules", 2017-12-19), it is
>         possible for ...


yup

>
>>                                                               the
>> function `submodule_move_head` may be invoked with the same argument
>> for the `old` and `new` state of a submodule, for example when you
>> run `reset --hard --recurse-submodules` in the superproject that has no
>> change in the gitlink entry, but only worktree related change in the
>> submodule. The read-tree call in the submodule is not amused about
>> the duplicate argument.
>
> What is the symptom of read-tree being unamused?  Is that a sign that
> these patches are out of order (i.e. that we should prepare to handle an
> unchanged submodule first, and then adjust the caller to pass in
> unchanged submodules)?
>
>> It turns out that we can omit the duplicate old argument in all forced
>> cases anyway, so let's do that.
>
> What is the end-user visibile effect of such a change?  E.g. what
> becomes possible to a user that wasn't possible before?
>
> Among the commands you mentioned before:
>
>   checkout -f
>         I think I would expect this not to touch a submodule that
>         hasn't changed, since that would be consistent with its
>         behavior on files that haven't changed.
>
>   reset --hard
>         Nice!  Yes, recursing into unchanged submodules is a big part
>         of the psychological comfort of being able to say "you can
>         always use reset --hard <commit> to get back to a known
>         state".

I may have a different understanding of git commands than you do,
but a plain "checkout -f" with no further arguments is the same as
a hard reset IMHO, and could be used interchangeably?

Rehashing the "What is a submodule?" discussion, I would claim
that when its worktree is changed, we'd want checkout to restore
the submodules worktree back, so I disagree with your assessment
of checkout -f.

>   read-tree -u --reset
>         This is essentially the plumbing equivalent of reset --hard,
>         so it makes sense for them to be consistent.  Ok.
>
> Based on the checkout -f case, if I've understood correctly then patch
> 4/5 goes too far on it (but I could easily be convinced otherwise).

Without 4/5 we cannot implement hard reset recursing into submodules
as it is functionally the same as forced checkout except when we
differentiate them
on a higher layer?

>> Signed-off-by: Stefan Beller <sbeller@google.com>
>> ---
>>  submodule.c               | 4 +++-
>>  t/lib-submodule-update.sh | 2 +-
>>  2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/submodule.c b/submodule.c
>> index fa25888783..db0f7ac51e 100644
>> --- a/submodule.c
>> +++ b/submodule.c
>> @@ -1653,7 +1653,9 @@ int submodule_move_head(const char *path,
>>       else
>>               argv_array_push(&cp.args, "-m");
>>
>> -     argv_array_push(&cp.args, old ? old : EMPTY_TREE_SHA1_HEX);
>> +     if (!(flags & SUBMODULE_MOVE_HEAD_FORCE))
>> +             argv_array_push(&cp.args, old ? old : EMPTY_TREE_SHA1_HEX);
>
> Interesting.  What is the effect when old != new?

when the force flag is set we mostly pass in old="HEAD", which is
technically correct,
but not a sha1. (I assume you want to know what happens when two unequal sha1s
are given; for that it will perform a 2 way merge instead of a complete reset)

Thanks,
Stefan