Re: git-svn: Use prefix by default?

From: Johan Herland <johan@herland.net>
Date: Sep 30, 2013, 22:39 UTC
Message-ID: <CALKQrgeXAtWeTedqkVtcTAUgbsBA9U6rbqUntu_ArmPgL9R3pg@mail.gmail.com>
In-Reply-To: <CAEcj5uVRcXF+XADFvy_q2mbQOdWPZkz57q1vePOqroMbQw4+HA@mail.gmail.com>

On Thu, Aug 22, 2013 at 11:02 AM, Thomas Ferris Nicolaisen <tfnico@gmail.com> wrote:

> I've recently been forced back into using git-svn, and while I was at
> it, I noticed that git-svn generally behaves a lot better when it is
> initialized using the --prefix option.

Thanks for raising this issue. Although I'm not using git-svn myself, I have been aware of how git-svn pollutes the refs/remote/* namespace for a while. In addition to being messy, it causes potential problems for the remote ref namespaces being discussed in other threads. The issue is even recognized by the git-svn author (see e.g. http://www.mail-archive.com/git@vger.kernel.org/msg26248.html ).

I have some bad news, and some good news for you. Read on...

Show 29 quoted lines

> For example, I make a standard-layout svn clone:
>
> $ git svn clone -s https://svn.company.com/repos/project-foo/
>
> .. and end up with this .gitconfig:
>
> [svn-remote "svn"]
>         url = https://svn.company.com/repos/
>         fetch = project-foo/trunk:refs/remotes/trunk
>         branches = project-foo/branches/*:refs/remotes/*
>         tags = project-foo/tags/*:refs/remotes/tags/*
>
> And my remote branches looks like this:
>     remotes/trunk
>     remotes/feat-bar
>
> Now, let's say I want to work on the feat-bar branch, so I attempt to
> create a tracking branch by the same name:
>
> $ git checkout feat-bar  # will detach head
> $ git checkout remotes/feat-bar  # will detach head
> $ git checkout -t remotes/feat-bar # fatal: Missing branch name; try -b
> $ git checkout -tb remotes/feat-bar # Branch remotes/feat-bar set up
> to track local branch master.
>
> Well, that's not what I wanted.. So I end up doing it the good
> old-fashioned way:
>
> $ git checkout -tb feat-bar remotes/feat-bar # works

The bad news is that the upstream relationship you're trying to create between your local feat-bar and git-svn's remotes/feat-bar is not a valid upstream relationship.

An upstream relationship in Git is encoded in the branch.foo.remote and branch.foo.merge configuration variables. These specify the name of the upstream remote, and which branch at that remote to pull from, respectively. However, in your case, there is no git remote associated with refs/remotes/feat-bar (nor with refs/remotes/mirror/feat-bar which you create below), and hence there is no valid remote name to assign to branch.feat-bar.remote.

Prior to v1.8.3.2 this still sort-of works (as you observe below), because the code fails to realize the remote is invalid, and falls back to setting branch.feat-bar.remote = "." (i.e. the current repo). This might seem like an ok practice until you realize that a "git push" back to that invalid upstream would happily overwrite refs/remotes/(mirror/)feat-bar, and thus break git-svn's internal state.

This bug was fixed in v1.8.3.2, more specifically 41c21f22 (branch.c: Validate tracking branches with refspecs instead of refs/remotes/*), and you can read more about the rationale in that commit message.

The end result for you is that setting up your local feat-bar to track git-svn's feat-bar will no longer be accepted once you upgrade to >= v1.8.3.2. The correct way to set up a local feat-bar branch to work on top of git-svn's feat-bar is instead to forgo the upstream relationship and simply do "git checkout -b feat-bar refs/remotes/(mirror/)feat-bar".

Show 21 quoted lines

> Now I am up and rolling, but I get this warning with every checkout or rebase:
>
>      warning: refname 'feat-bar' is ambiguous.
>
> So, let's see what happens when I create the svn clone using a
> --prefix=mirror/ instead. Here's the config:
>
> [svn-remote "svn"]
>         url = https://svn.company.com/repos/
>         fetch = project-foo/trunk:refs/remotes/mirror/trunk
>         branches = project-foo/branches/*:refs/remotes/mirror/*
>         tags = project-foo/tags/*:refs/remotes/mirror/tags/*
>
> Here are my remote branches:
>      remotes/mirror/trunk
>      remotes/mirror/feat-bar
>
> Let's create the tracking branch:
>
> $ git checkout feat-bar  # Branch feat-bar set up to track remote
> branch feat-bar from mirror.

As I explained above, this no longer works in >= v1.8.3.2...

> Voila, worked on the first try. And no more warnings about ambiguous refnames.

...but the warning about ambiguous refnames can and should be fixed, by making git-svn's --prefix default to adding an extra level below refs/remotes.

> So now my question is: If using a prefix is so healthy for git's
> branch tracking conventions, why doesn't git-svn default to use some
> prefix like 'origin' or something when initializing a git svn clone?

Exactly, and the good news is that I finally got around to writing that patch, which I'll post in a minute...

Hope this helps,

...Johan

-- Johan Herland, <johan@herland.net> www.herland.net