[PATCHv2 4/5] t3200: Add test demonstrating minor regression in 41c21f2

In 41c21f2 (branch.c: Validate tracking branches with refspecs instead of refs/remotes/*), we changed the rules for what is considered a valid tracking branch (a.k.a. upstream branch). We now use the configured remotes and their refspecs to determine whether a proposed tracking branch is in fact within the domain of a remote, and we then use that information to deduce the upstream configuration (branch.<name>.remote and branch.<name>.merge).

However, with that change, we also check that - in addition to a matching refspec - the result of mapping the tracking branch through that refspec (i.e. the corresponding ref name in the remote repo) happens to start with "refs/heads/". In other words, we require that a tracking branch refers to a _branch_ in the remote repo.

Now, consider that you are e.g. setting up an automated building/testing infrastructure for a group of similar "source" repositories. The build/test infrastructure consists of a central scheduler, and a number of build/test "slave" machines that perform the actual build/test work. The scheduler monitors the group of similar repos for changes (e.g. with a periodic "git fetch"), and triggers builds/tests to be run on one or more slaves. Graphically the changes flow between the repos like this:

  Source #1 -------v          ----> Slave #1
                             /
  Source #2 -----> Scheduler -----> Slave #2
                             \
  Source #3 -------^          ----> Slave #3

        ...                           ...

The scheduler maintains a single Git repo with each of the source repos set up as distinct remotes. The slaves also need access to all the changes from all of the source repos, so they pull from the scheduler repo, but using the following custom refspec:

  remote.origin.fetch = "+refs/remotes/*:refs/remotes/*"

This makes all of the scheduler's remote-tracking branches automatically available as identical remote-tracking branches in each of the slaves.

Now, consider what happens if a slave tries to create a local branch with one of the remote-tracking branches as upstream:

  git branch local_branch --track refs/remotes/source-1/some_branch

Git now looks at the configured remotes (in this case there is only "origin", pointing to the scheduler's repo) and sees refs/remotes/source-1/some_branch matching origin's refspec. Mapping through that refspec we find that the corresponding remote ref name is "refs/remotes/source-1/some_branch". However, since this remote ref name does not start with "refs/heads/", we discard it as a suitable upstream, and the whole command fails.

This patch adds a testcase demonstrating this failure by creating two source repos ("a" and "b") that are forwarded through a scheduler ("c") to a slave repo ("d"), that then tries create a local branch with an upstream. See the next patch in this series for the exciting conclusion to this story...

Reported-by: Per Cederqvist <cederp@opera.com>
Signed-off-by: Johan Herland <johan@herland.net>
---
 t/t3200-branch.sh | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)