Re: [PATCH v2] sequencer: do not squash 'reword' commits when we hit conflicts

[Phillip Wood <phillip.wood@talktalk.net>]

Hi Johannes

On 17/06/18 20:28, Johannes Schindelin wrote:
> Hi Phillip,
> 
> On Sun, 17 Jun 2018, Phillip Wood wrote:
> 
>> On 17/06/18 06:37, Elijah Newren wrote:
>>> Ever since commit 18633e1a22 ("rebase -i: use the rebase--helper builtin",
>>> 2017-02-09), when a commit marked as 'reword' in an interactive rebase
>>> has conflicts and fails to apply, when the rebase is resumed that commit
>>> will be squashed into its parent with its commit message taken.
>>>
>>> The issue can be understood better by looking at commit 56dc3ab04b
>>> ("sequencer (rebase -i): implement the 'edit' command", 2017-01-02), which
>>> introduced error_with_patch() for the edit command.  For the edit command,
>>> it needs to stop the rebase whether or not the patch applies cleanly.  If
>>> the patch does apply cleanly, then when it resumes it knows it needs to
>>> amend all changes into the previous commit.  If it does not apply cleanly,
>>> then the changes should not be amended.  Thus, it passes !res (success of
>>> applying the 'edit' commit) to error_with_patch() for the to_amend flag.
>>>
>>> The problematic line of code actually came from commit 04efc8b57c
>>> ("sequencer (rebase -i): implement the 'reword' command", 2017-01-02).
>>> Note that to get to this point in the code:
>>>    * !!res (i.e. patch application failed)
>>>    * item->command < TODO_SQUASH
>>>    * item->command != TODO_EDIT
>>>    * !is_fixup(item->command) [i.e. not squash or fixup]
>>> So that means this can only be a failed patch application that is either a
>>> pick, revert, or reword.  For any of those cases we want a new commit, so
>>> we should not set the to_amend flag.
>>
>> Unfortunately I'm not sure it's that simple. Looking and do_pick() sometimes
>> reword amends HEAD and sometimes it does not. In the "normal" case then the
>> commit is picked and committed with '--edit'. However when fast-forwarding the
>> code fast forwards to the commit to be reworded and then amends it. If the
>> root commit is being reworded it takes the same code path. While these cases
>> cannot fail with conflicts, it is possible for the user to cancel the commit
>> or for them to fail due to collisions with untracked files.
>>
>> If I remember correctly the shell version always picks the commit and then
>> calls 'git commit --amend' afterwards which is less efficient but consistent.
>>
>> I'm afraid I don't have a simple solution for fixing this, as currently
>> pick_commits() does not know if the commit was called with AMEND_MSG, I guess
>> that means adding some kind of flag for do_pick() to set.
> 
> Oh, you're right, the fast-forwarding path would pose a problem. I think
> there is an easy way to resolve this, though: in the case that we do want
> to amend the to-be-reworded commit, we simply have to see whether HEAD
> points to the very same commit mentioned in the `reword` command:

That's clever, I think to get it to work for rewording the root commit,
it will need to do something like comparing HEAD to squash-onto as well.

> -- snip --
> diff --git a/sequencer.c b/sequencer.c
> index 2dad7041960..99d33d4e063 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -3691,10 +3691,22 @@ static int pick_commits(struct todo_list
> *todo_list, struct replay_opts *opts)
>                                         intend_to_amend();
>                                 return error_failed_squash(item->commit, opts,
>                                         item->arg_len, item->arg);
> -                       } else if (res && is_rebase_i(opts) && item->commit)
> +                       } else if (res && is_rebase_i(opts) && item->commit) {
> +                               int to_amend = 0;
> +
> +                               if (item->command == TODO_REWORD) {
> +                                       struct object_id head;
> +
> +                                       if (!get_oid("HEAD", &head) &&
> +					    !oidcmp(&item->commit->object.oid,
> +                                                   &head))
> +                                               to_amend = 1;
> +                               }
> +
>                                 return res | error_with_patch(item->commit,
>                                         item->arg, item->arg_len, opts, res,
> -                                       item->command == TODO_REWORD);
> +                                       to_amend);
> +                       }
>                 } else if (item->command == TODO_EXEC) {
>                         char *end_of_arg = (char *)(item->arg + item->arg_len);
>                         int saved = *end_of_arg;
> -- snap --
> 
> Note that
> 
> - this patch is only compile-tested, and
> 
> - it is on top of my sequencer-shears branch thicket, so it might not
>   apply cleanly to master, and
> 
> - it could probably use a comment what we are doing here (see whether we
>   wanted to amend a fast-forwarded commit).

Yes that would be helpful for future readers I think.
> 
> What do you think about this approach?

I like it assuming it's easy to extend it to the 'reword the root
commit' case

Best Wishes

Phillip
> Dscho
>