Re: Limitiations of git rebase --preserve-merges --interactive

[Stefan Beller <sbeller@google.com>]

On Fri, Sep 23, 2016 at 2:13 PM, Johannes Schindelin
<Johannes.Schindelin@gmx.de> wrote:
> Hi Stefan,
>
> On Fri, 23 Sep 2016, Stefan Haller wrote:
>
>> Stefan Beller <sbeller@google.com> wrote:
>>
>> > On Thu, Sep 22, 2016 at 12:48 PM, Kevin Daudt <me@ikke.info> wrote:
>> > > On Thu, Sep 22, 2016 at 07:33:11PM +0000, Anatoly Borodin wrote:
>> > >> Hi Stefan,
>> > >>
>> > >> this section was added to the manual in the commit
>> > >> cddb42d2c58a9de9b2b5ef68817778e7afaace3e by "Jonathan Nieder"
>> > >> <jrnieder@gmail.com> 6 years ago. Maybe he remembers better?
>> > >>
>> > >
>> > > Just to make it clear, this section explicitly talks about 'bugs' with
>> > > preserve-merges and interactive rebase.  Without the --preserve-merges
>> > > option, those operations works as expected.
>> > >
>> > > The reason, as that section explains, is that it's not possible to store
>> > > the merge structure in the flat todo list. I assume this means git
>> > > internally remembers where the merge commit was, and then restores it
>> > > while rebasing.
>> > >
>> > > Changing the order, or dropping commits might then give unexpected
>> > > results.
>> > >
>> >
>> > The commit message may help as well:
>> >
>> >     rebase -i -p: document shortcomings
>> >
>> >     The rebase --preserve-merges facility presents a list of commits
>> >     in its instruction sheet and uses a separate table to keep
>> >     track of their parents.  Unfortunately, in practice this means
>> >     that with -p after most attempts to rearrange patches, some
>> >     commits have the "wrong" parent and the resulting history is
>> >     rarely what the caller expected.
>> >
>> >     Yes, it would be nice to fix that.  But first, add a warning to the
>> >     manual to help the uninitiated understand what is going on.
>>
>> Thanks, but all of this still talks about the issues in very generic
>> terms ("most attempts to rearrange patches"). I'm interested in more
>> details as to exactly what kind of attempts do or don't work. In
>> particular, I'm interested in fixup/squash commands (without reordering
>> anything else), or dropping (non-merge) commits.
>>
>> I could of course experiment with these and try to find out myself, but
>> I was hoping someone would just know the answer off the top of their
>> head, saving me some time.
>
> The fundamental problem here is the underlying design of bolting on the
> "recreate a merge" functionality onto the "pick" command.
>
> That is, if you try to rebase non-linear commit history, it will still
> generate a linear list of "pick <commit-name>" lines, as if it were
> linear, except that it will include the merge commits, too.

Which on a more fundamental design level would be ok.
(C.f. your shell history is a linear list of git commands, but it
deals just fine
with non linear DAGSs)

>
> It then will try to guess what you want to do by recording which commit
> was rewritten as which commit. And when it encounters a "pick" with a
> merge commit, it will try to merge the *rewritten* commit.

Instead of guessing we'd need to differentiate between "pick" and "pickmerge",
whereas the later describes creating commits with more than one parent (i.e.
the prior pick line).

I could imagine the "pickmerge" to list all additional parents (The
first parent being
the previously picked commit) via symbolic naming:

    pick 1234affe implement foo
    pickmerge 3456feed origin/js/new-feature-1 # Merge origin/js/new-feature-1
    pick 45678ead implement feature-2

The "pickmerge" would have first the merge tips, and then the old
subject line after
a # character.

>
> In other words, the design does not allow for changing the tip of any
> merged branch. Not reordering, not dropping.

I see how the current design is problematic as there is no argument
possible that
allows the user to correct the wrong guess.

>
> And I do not think that there is a way to fix that design. That is why I
> came up with the Git garden shears (see the link I sent elsewhere in this
> thread).

I'll look into that.

Thanks,
Stefan