Re: [PATCH v4 2/2] revision: implement `git log --merge` also for rebase/cherry-pick/revert

> Hi Philippe
> 
> On 10/02/2024 23:35, Philippe Blain wrote:
>> From: Michael Lohmann <mi.al.lohmann@gmail.com>
>>
>> 'git log' learned in ae3e5e1ef2 (git log -p --merge [[--] paths...],
>> 2006-07-03) to show commits touching conflicted files in the range
>> HEAD...MERGE_HEAD, an addition documented in d249b45547 (Document
>> rev-list's option --merge, 2006-08-04).
>>
>> It can be useful to look at the commit history to understand what lead
>> to merge conflicts also for other mergy operations besides merges, like
>> cherry-pick, revert and rebase.
>>
>> For rebases, an interesting range to look at is HEAD...REBASE_HEAD,
>> since the conflicts are usually caused by how the code changed
>> differently on HEAD since REBASE_HEAD forked from it.
>>
>> For cherry-picks and revert, it is less clear that
>> HEAD...CHERRY_PICK_HEAD and HEAD...REVERT_HEAD are indeed interesting
>> ranges, since these commands are about applying or unapplying a single
>> (or a few, for cherry-pick) commit(s) on top of HEAD. However, conflicts
>> encountered during these operations can indeed be caused by changes
>> introduced in preceding commits on both sides of the history.
> 
> I tend to think that there isn't much difference between rebase and cherry-pick here - they are both cherry-picking commits and it is perfectly possible to rebase a branch onto an unrelated upstream. The important part for me is that we're showing these commits because even though they aren't part of the 3-way merge they are relevant for investigating where any merge conflicts come from.
> 
> For revert I'd argue that the only sane use is reverting an ancestor of HEAD but maybe I'm missing something. In that case REVERT_HEAD...HEAD is the same as REVERT_HEAD..HEAD so it shows the changes since the commit that is being reverted which will be the ones causing the conflict.

>> Adjust the code in prepare_show_merge so it constructs the range
>> HEAD...$OTHER for each of OTHER={MERGE_HEAD, CHERRY_PICK_HEAD,
>> REVERT_HEAD or REBASE_HEAD}. Note that we try these pseudorefs in order,
>> so keep REBASE_HEAD last since the three other operations can be
>> performed during a rebase. Note also that in the uncommon case where
>> $OTHER and HEAD do not share a common ancestor, this will show the
>> complete histories of both sides since their root commits, which is the
>> same behaviour as currently happens in that case for HEAD and
>> MERGE_HEAD.
>>
>> Adjust the documentation of this option accordingly.
> 
> Thanks for the comprehensive commit message.
> 
>> diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
>> index 2bf239ff03..5b4672c346 100644
>> --- a/Documentation/rev-list-options.txt
>> +++ b/Documentation/rev-list-options.txt
>> @@ -341,8 +341,10 @@ See also linkgit:git-reflog[1].
>>   Under `--pretty=reference`, this information will not be shown at all.
>>     --merge::
>> -    After a failed merge, show refs that touch files having a
>> -    conflict and don't exist on all heads to merge.
>> +    Show commits touching conflicted paths in the range `HEAD...$OTHER`,
>> +    where `$OTHER` is the first existing pseudoref in `MERGE_HEAD`,
>> +    `CHERRY_PICK_HEAD`, `REVERT_HEAD` or `REBASE_HEAD`. Only works
>> +    when the index has unmerged entries.
> 
> Do you know what "and don't exist on all heads to merge" in the original is referring to? The new text doesn't mention anything that sounds like that but I don't understand what the original was trying to say.

> It might be worth adding a sentence explaining when this option is useful.
> 
>     This option can be used to show the commits that are relevant
>     when resolving conflicts from a 3-way merge
> 
> or something like that.

> 
>>   --boundary::
>>       Output excluded boundary commits. Boundary commits are
>> diff --git a/revision.c b/revision.c
>> index aa4c4dc778..36dc2f94f7 100644
>> --- a/revision.c
>> +++ b/revision.c
>> @@ -1961,11 +1961,31 @@ static void add_pending_commit_list(struct rev_info *revs,
>>       }
>>   }
>>   +static const char *lookup_other_head(struct object_id *oid)
>> +{
>> +    int i;
>> +    static const char *const other_head[] = {
>> +        "MERGE_HEAD", "CHERRY_PICK_HEAD", "REVERT_HEAD", "REBASE_HEAD"
>> +    };
>> +
>> +    for (i = 0; i < ARRAY_SIZE(other_head); i++)
>> +        if (!read_ref_full(other_head[i],
>> +                RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
>> +                oid, NULL)) {
>> +            if (is_null_oid(oid))
>> +                die("%s is a symbolic ref???", other_head[i]);
> 
> This would benefit from being translated and I think one '?' would suffice (I'm not sure we even need that - are there other possible causes of a null oid here?)

>> +            return other_head[i];
>> +        }
>> +
>> +    die("--merge without MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD or REBASE_HEAD?");
> 
> This is not a question and would also benefit from translation. It might be more helpful to say that "--merge" requires one of those pseudorefs.