git-svn, and which branch am I on?

G'day.

I recently got asked a question about git-svn that I had no idea how to answer, and which I am actually curious to know how to find out.

The general question was: in git, how do I identify where this branch came from?

Specifically, this was about 'git svn', but also generally how to identify this information in git.

So, with a repository branch layout like this:

  master        (local)
  testing       (local)
  trunk         (remote)
  v100          (remote)

How would I find out which remote branch master and trunk came from?

To restate that, because I am not sure if that is clear, given this layout of branches:

     trunk (remote)
     |
 o---o---o---o---o  branch master
  \
   \
    o---o---o---o branch testing
    |
    v100 (remote)

How can I identify that 'testing' came from the 'v100' branch, and that master came from the 'trunk' branch?

Ideally, I would like to work this out on the command line, without needing to reference gitk or another graphical tool, but even a solution that used them would be fine.

Initially I figured there would be some equivalent of the Mercurial 'glog' output available, showing this; for reference the second and third examples here are what I was envisaging: http://www.selenic.com/mercurial/wiki/index.cgi/GraphlogExtension

(from that display I could infer where testing and master came from,
 rather than directly getting the answer, but that is just fine.)

...and, finally, is the reason that I am finding it hard to explain this because I have an expectation of how things work that doesn't match up with git? In other words, is the question actually meaningless?

Regards,
        Daniel

On 2009.02.28 19:50:08 +1100, Daniel Pittman wrote:

"git svn info" tells you, besides other things, the URL your checked out branch is based upon WRT svn. Use it with "--url" to show only the URL.

If you use git-svn, it's actually meaningful, because it controls what svn rebase and svn dcommit do. For a pure git repo, it's usually not that interesting.

Björn

On Sat, Feb 28, 2009 at 3:50 AM, Daniel Pittman wrote:

You've hit the nail on the head. The question is meaningless. Git tracks a DAG, and a branch is just that - a branch (or 'tip', if you will) of the DAG.

Take, for example,

o--o--o--o master
 \
  o--o--o--o feature
      \
       o--o working

Which branch did 'working' come from? 'feature' or 'master'? Say I later delete branch 'feature', since it was going in the wrong direction. Does the answer change? What if I simply redraw the above so that feature and working are swapped? Does the answer change? It's still the same tree.

You can, however, answer related questions.

'git svn info' will tell you which branch a 'git svn dcommit' will commit to, which is usually the question you actually wanted to answer.

'git branch -r' will tell you which remote branch your local branch is tracking. ie. which branch, if any, a plain 'git pull' will fetch and merge from.

See also http://thread.gmane.org/gmane.comp.version-control.git/77730/focus=77823 and surrounding thread.

Peter Harris

Daniel Pittman <daniel@rimspace.net> writes:

In general, you cannot. In specific cases, you can.
See below for details.
 

[...]

On the plumbing level, or on the level of graph of commits, git does not store information "where this branch came from". For git, from the point of view of commit objects, the following two pictures are totally equivalent (by design):

        /-o---o      branch a
       /
  o---*---o---o---o  branch b

and
  
  o---*---o---o      branch a
       \
        \-o---o---o  branch b

What you _can_ get on this level is to find common ancestor (or common ancestors) of branches 'a' and 'b', using "git merge-base a b"; this would return commit marked '*'.

However, when creating a branch, you can tell git that you want for newly created branch to track branch you are based on (with --track) option. By default saving tracking information is done when branching off remote-tracking branches.

  When a local branch is started off a remote branch, git sets up the
  branch so that 'git-pull' will appropriately merge from
  the remote branch. This behavior may be changed via the global
  `branch.autosetupmerge` configuration flag. That setting can be
  overridden by using the `--track` and `--no-track` options.

  [...]

  --track::
      When creating a new branch, set up configuration so that 'git-pull'
      will automatically retrieve data from the start point, which must be
      a branch. Use this if you always pull from the same upstream branch
      into the new branch, and if you don't want to use "git pull
      <repository> <refspec>" explicitly. This behavior is the default
      when the start point is a remote branch. Set the
      branch.autosetupmerge configuration variable to `false` if you want
      'git-checkout' and 'git-branch' to always behave as if '--no-track' were
      given. Set it to `always` if you want this behavior when the
      start-point is either a local or remote branch.

And git-config(1)

  branch.<name>.remote::
      When in branch <name>, it tells 'git-fetch' which remote to fetch.
      If this option is not given, 'git-fetch' defaults to remote "origin".

  branch.<name>.merge::
      When in branch <name>, it tells 'git-fetch' the default
      refspec to be marked for merging in FETCH_HEAD. The value is
      handled like the remote part of a refspec, and must match a
      ref which is fetched from the remote given by
      "branch.<name>.remote".
      The merge information is used by 'git-pull' (which at first calls
      'git-fetch') to lookup the default branch for merging. Without
      this option, 'git-pull' defaults to merge the first refspec fetched.
      Specify multiple values to get an octopus merge.
      If you wish to setup 'git-pull' so that it merges into <name> from
      another branch in the local repository, you can point
      branch.<name>.merge to the desired branch, and use the special setting
      `.` (a period) for branch.<name>.remote

In this case you can get _name_ of the branch this branch "came from" with "git config branch.<branchname>.merge".

HTH.