Rebasing a branch that contains merges

Rebasing a branch that contains merges

[Andy Parkins]

Hello,

I've done some little experiments with rebasing a branch with merges on it.  I 
wanted to check that what I found is what is intended.

I made a repository like this:

 * -- * -- * (master)
      |\
      | A (branch1) -- M (merge)
       \              /
        B (branch2) --

It's a horribly unfair thing to do, but while on merge I did

 git rebase master

I had been hoping that both branches would rebase, but only one did.  I got:

 * -- * -- * (master) -- B -- M (merge)
      |\
      | A (branch1)
       \              
        B (branch2)

"M" effectively now contains all of branch1's changes.  If I hadn't referenced 
the merge head separately from the two branch heads, all the history in 
branch1 would now be represented by "M".

I don't think this is the right result.  Either
 * git should refuse to rebase because it would lose history
 * git should rebase both branches

The ideal result (perhaps unrealisticly) would have been

 * -- * -- * (master)
           |\
           | A (branch1) -- M (merge)
            \              /
             B (branch2) --

This is an unreasonable thing to expect a version control system to be able to 
do - it's a nasty, complicated wish.  git is its own worst enemy for things 
like this; it's getting to be so good at nasty unreasonable things that they 
don't seem like fantasy-land wishes.

A perfectly valid answer is "don't be so ridiculous Andy, go away". :-)


Andy
-- 
Dr Andy Parkins, M Eng (hons), MIEE
andyparkins@gmail.com

Re: Rebasing a branch that contains merges

[Junio C Hamano]

Andy Parkins <andyparkins@gmail.com> writes:

> This is an unreasonable thing to expect a version control
> system to be able to do ...

I do not necessarily think so.  If you have this:

       o---X
      /
  ---a---o---o---o
          \       \ 
          -o---o---M---Y

it is perfectly reasonable to want to have this:

       o---X---o---o---o
      /         \       \ 
  ---a           o---o---M'--Y'

when you say "rebase --onto X a Y".

However, you have to be careful.  The thing is, you do not want
to 'rebase' a merge if the other branch came from somewhere not
under your control.  If 'a', 'o', 'X', 'Y' and 'M' are under
your control, and 'x' are from somebody else's tree in the
following picture:

       o---X
      /
  ---a---o---o---o
      \           \
       x---x---x---M---Y

and if you say "rebase --onto X a Y", it is not useful to rebase
'x' when you want to rewrite your history.  They are somebody
else's history, and other people might have got it already, too.
Even if you _could_ rebase them, you would rather not.

And this is true even if 'x' are your own.  You might have
pushed out these 'x' to public repo although you haven't pushed
out 'M', 'Y', 'o' nor 'X', and would want to rebase the
still-private part to clean up the history before publishing.
In such a case, you would want to end up with something like
this:

       o---X---o---o---o
      /                 \
  ---a           .-------M'--Y'
      \         /
       x---x---x

This kind of rebase should be doable but you would need to tell
the tool which parents of merges you would want to rebase and
which ones you do not want to touch but recreate the merge by
merging with your rebased branches again.  In the above 'ideal',
picture, three 'o's have been rebased, and then merge 'M' was
recreated from the rebased rightmost 'o' and the rightmost 'x'
in the original picture.

Re: Rebasing a branch that contains merges

[Andy Parkins]

On Monday 2007 February 19 09:36, Junio C Hamano wrote:

> > This is an unreasonable thing to expect a version control
> > system to be able to do ...
>
> I do not necessarily think so.  If you have this:

Sorry; by "unreasonable", I meant "unreasonable to expect that your VCS is 
clever enough to do it".  It's an offhand compliment to git; I can't imagine 
any other VCS is capable of this sort of thing, so the discussion wouldn't 
even happen.

> However, you have to be careful.  The thing is, you do not want
> to 'rebase' a merge if the other branch came from somewhere not
> under your control.  If 'a', 'o', 'X', 'Y' and 'M' are under
> your control, and 'x' are from somebody else's tree in the
> following picture:

Absolutely.  We can take that as a given though can't we?  That is always true 
for a rebase, irrespective of whether its got merges in the branch.  As your 
diagrams show, dealing with merges during a rebase just makes things more 
complicated in that you can now have one parent of the merge being rebasable 
and one not.

My original query should have stated my assumption: that all the merged 
branches are mine to rebase as I see fit.

> and if you say "rebase --onto X a Y", it is not useful to rebase
> 'x' when you want to rewrite your history.  They are somebody
> else's history, and other people might have got it already, too.
> Even if you _could_ rebase them, you would rather not.

Coo; this obviously makes things much harder.  What if there are further 
merges on the merged branches?  It's going to get hard to specify which of 
the merge branches you want to rebase and which you don't.

> the tool which parents of merges you would want to rebase and
> which ones you do not want to touch but recreate the merge by
> merging with your rebased branches again.  In the above 'ideal',

So - rebase would need some sort of language to tell it which branch to 
favour.

 $ git checkout branch-with-merges
 $ git rebase other-branch
 rebase: Aborted because commit 123456abcdef has multiple parents:
 rebase:  - 123456789abcdef
 rebase:  - abcdef123456789
 $ git rebase --include-parent abcdef123456789 other-branch

Yuck.  I've made myself ill now.  It's not going to be pretty is it?

For now, I wanted to make git-rebase just abort if there are merge commits in 
the rebased branch, I had a quick look at git-rebase.sh to see if I could 
find the place were this could be detected and have got lost.  So I've failed 
again :-)

At the moment, git-rev-list --no-merges is used.  I don't think that, that's 
the correct thing to do.  That just filters out the merge.  Doesn't that risk 
loss of history as I showed in my first email?


Andy

-- 
Dr Andy Parkins, M Eng (hons), MIEE
andyparkins@gmail.com

Re: Rebasing a branch that contains merges

[A.J. Rossini]

On 2/19/07, Andy Parkins <andyparkins@gmail.com> wrote:

> So - rebase would need some sort of language to tell it which branch to
> favour.

The more I use git, the more I start thinking that a domain-specific
language to describe merges would not be a bad thing.  It's so easy
(painless, thought-free) to create a complex set of branches that
you'd like to rearrange with each other.

Of course, perhaps the right solution is to be simpler and old
fashioned and cease hyper-multitasking (i.e. shooting myself in the
foot).

(I'm sharing Andy's assumption that my repo is just mine, not really
shared -- add in sharing, or public use, and it gets far more complex,
and the language idea might be a good idea, and not just an idea.
Darcs has it's theory of patches, though it's not a language).

best,
-tony

blindglobe@gmail.com
Muttenz, Switzerland.
"Commit early,commit often, and commit in a repository from which we can easily
roll-back your mistakes" (AJR, 4Jan05).