Why do I get a svn: Illegal repository URL '' error when using maven's release plugin?

Question

I was using

mvn release:branch ...

And I got this cryptic error:

[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 27.391s
[INFO] Finished at: Mon Jan 07 17:22:46 PST 2013
[INFO] Final Memory: 7M/245M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-release-plugin:2.3.2:branch (default-cli) on project XXX: Unable to commit files
[ERROR] Provider message:
[ERROR] The svn command failed.
[ERROR] Command output:
[ERROR] svn: Commit failed (details follow):
[ERROR] svn: Illegal repository URL ''
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException

What is causing this?

Answer 1

This actually has very little to do with maven. The answer is in this other stack overflow answer. The maven release plugin modifies all the poms in your project to update the version numbers then commits them. If one of those poms is in an svn extern, it tries to commit the local and external modifications in one revision and gives this very cryptic error message. Hopefully this will help someone else.

Unfortunately, the only work around I know to this is to stop using externs or stop using the release plugin.