Strange git merge behaviour with empty branches

Hello git people,

I'm seeing some strange behaviour with git merge and wondered if you could help me to understand what's going on.

Say I have a master branch tracking upstream and a few local branches based off that:

$ git branch
  master
  foo
  bar
  baz

Furthermore, let's say that both foo and bar are currently at master (that is, they have no additional commits) but baz does have changes committed on top of master.

Now, if I make an integration branch:

$ git checkout master -b integration

I can then merge foo, bar and baz into this:

$ git merge foo bar baz

which works fine. However, specifying the non-empty branch first does:

$ git merge baz foo bar Already up-to-date.

and the changes from baz are not merged in. For extra weirdness, if you have another non-empty branch (qux) and do:

$ git merge baz foo bar qux

Then you end up with a branch that *doesn't* contain baz, *does* contain qux and has a weird looking merge-commit message that just reads "baz"!

Now, I appreciate that merging in empty branches might seem weird but it's sometimes useful if you have scripts that take a set of known branches, merge them and then do a build.

Any thoughts?

Cheers,

Will

Will Deacon <will.deacon@arm.com> writes:

What is recorded in Octopus merges is a rather torn issue. The original motivation was to allow recording anything the user throws at us, even though some are ancestors of others, without reducing them to the minimum set of parents that are necessary to represent the full history, because that is after all what the user wanted to do. You see some of that there.

On the other hand, there was an edict from the original maintainer that we shouldn't record useless merge parents, so there were some changes in that direction to cull obviously redundant parents from the parent list.

But over time, novelty of Octopus merges waned, and nobody serious uses it for real work these days, except when bored and being curious, playing around and seeing what it does.

An advice for an end-user is "Don't throw meaningless parents at Octopus. It will do whatever it does, and the only guarantee you are getting is that it does not record wrong parents, it may omit obviously redundant parents, and it will never discard an independent parent."

An advice for a developer is to update it to make sure only the independent parents are recorded; "show-branch --independent" may be a good tool to compute them