Re: How does git follow branch history across a merge commit?

From: Björn Steinbrink <b.steinbrink@gmx.de>
Date: Aug 29, 2009, 00:25 UTC
Message-ID: <20090829002559.GA32479@atjola.homenet>
In-Reply-To: <831vmv5wh7.fsf@torus.sehlabs.com>

On 2009.08.28 19:50:44 -0400, Steven E. Harris wrote:

Show 19 quoted lines

> I missed the point that merge commits are not "predecessor neutral";
> they apparently have a bias indicating "/this branch/ received content
> from /that branch/ (or /those branches/)".
> 
> To try to recreate my confusing scenario, I tried this:
> 
> ,----
> | git checkout competition
> | git merge master
> | # This fast-forwarded "competition" be equivalent to "master".
> | git checkout 'HEAD^'
> | # This wound up again at "master"'s predecessor, not "competition"'s.
> `----
> 
> It seems that since fast-forward merges don't produce a commit, the
> merge record remains in place recording that branch "competition" came
> into branch "master".Even though we're checked out to branch
> "competition" here, following its history back in time requires some
> manual intervention. Do you concur, or is my example perhaps flawed?

As there is no merge commit, there's no "merge record" either. "competition" just got updated to reference the same commit as "master". Branch "competition" did not "come into" branch "master" either. "competition" and "master" are just the names of the branch heads. And those just reference commits, which actually form the history and have no record of belonging to any branch head on their own.

IOW: "master" and "competition" aren't branches as in "a series of
commits with a fixed relation to each other", but just branch heads,
referencing the tips where the branches grow.

Scenario A:

Given this history:

A---B---C (master)
 \
  D---E (competition)

If you now merge "master" to "competition" you get:

A---B---C (master)
 \       \
  D---E---M (competition)

Where M is a merge commit with two parents, E being the first one, C being the second one. And that's everything that git knows, that M is a merged of C and E. That E is the first parent is just a smart choice, nothing else.

Scenario B:

Given this history:

A (competition)
 \
  B---C (master)

If you ask git to merge "master" to "competition", it will see that there aren't any commits reachable through "competition" that aren't reachable through "master". So simply fast-forwarding "competition won't make you lose anything. Thus you get:

A---B---C (master) (competition)

There's no trace of any merge, because it wasn't required. You can't even tell that a fast-forward happened from that history. You could as well just have created "competition".

That said, if you want to force a merge commit (which can sometimes be useful), you can use the --no-ff flag:

git checkout competition git merge --no-ff master

And this leads to:

A------M (competition)
 \    /
  B---C (master)

With M's first parent being A, and the second parent being C.

(Assuming that "competition" is a topic branch, this would be a bit weird though, as you produce a totally pointless merge commit, before you even started working on your topic branch. OTOH doing this the other way around, i.e. forcing a merge commit when you merge a topic branch to "master" _can_ be useful. Don't over do it though).

HTH Björn