Re: [PATCH] rebase: use reflog to find common base with upstream

[John Keeping <john@keeping.me.uk>]

On Sun, Oct 20, 2013 at 10:03:29PM -0700, Martin von Zweigbergk wrote:
> On Wed, Oct 16, 2013 at 11:53 AM, John Keeping <john@keeping.me.uk> wrote:
> > Commit 15a147e (rebase: use @{upstream} if no upstream specified,
> > 2011-02-09) says:
> >
> >         Make it default to 'git rebase @{upstream}'. That is also what
> >         'git pull [--rebase]' defaults to, so it only makes sense that
> >         'git rebase' defaults to the same thing.
> >
> > but that isn't actually the case.  Since commit d44e712 (pull: support
> > rebased upstream + fetch + pull --rebase, 2009-07-19), pull has actually
> > chosen the most recent reflog entry which is an ancestor of the current
> > branch if it can find one.
> 
> It is exactly this inconsistency between "git rebase" and "git pull
> --rebase" that confused me enough to make me send my first email to
> this list almost 4 years ago [1], so thanks for working on this! I
> finished that thread with:
> 
>   Would it make sense to teach "git rebase" the same tricks as "git
> pull --rebase"?
> 
> Then it took me a year before I sent a patch not unlike this one [2].
> To summarize, the patch did not get accepted then because it makes
> rebase a little slower (or a lot slower in some cases). "git pull
> --rebase" is of course at least as slow in the same cases, but because
> it often involves connecting to a remote host, people would probably
> blame the connection rather than git itself even in those rare (?)
> cases.
> 
> I think
> 
>   git merge-base HEAD $(git rev-list -g "$upstream_name")
> 
> is roughly correct and hopefully fast enough. That can lead to too
> long a command line, so I was planning on teaching merge-base a
> --stdin option, but never got around to it.

I'm not sure we should worry about the additional overhead here.  In the
common case, we should hit a common ancestor within the first couple of
reflog entries; and in the case that will be slow, it's likely that
there are a lot of differences between the branches so the cherry
comparison phase will take a while anyway.