You can put "." in the module field when using Subversion to avoid checking out
everything into a subdir, which is just annoying. But then the first checkout prints

Checking out a fresh workspace because Hudson failed to detect the current
workspace .../jobs/.../workspace
ERROR: svn: '.../jobs/.../workspace' is not a working copy
org.tmatesoft.svn.core.SVNException: svn: '.../jobs/.../workspace' is not a
working copy
at
org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:64)
at
org.tmatesoft.svn.core.internal.wc.admin.SVNWCAccess.probeOpen(SVNWCAccess.java:296)
at
org.tmatesoft.svn.core.internal.wc.admin.SVNWCAccess.probeOpen(SVNWCAccess.java:275)
at org.tmatesoft.svn.core.wc.SVNWCClient.crawlEntries(SVNWCClient.java:3027)
at org.tmatesoft.svn.core.wc.SVNWCClient.doInfo(SVNWCClient.java:2271)
at org.tmatesoft.svn.core.wc.SVNWCClient.doInfo(SVNWCClient.java:2647)
at hudson.scm.SubversionSCM.parseSvnInfo(SubversionSCM.java:721)
at hudson.scm.SubversionSCM.access$200(SubversionSCM.java:149)
at hudson.scm.SubversionSCM$IsUpdatableTask.invoke(SubversionSCM.java:835)
at hudson.scm.SubversionSCM$IsUpdatableTask.invoke(SubversionSCM.java:813)
at hudson.FilePath.act(FilePath.java:552)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:468)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:418)
at hudson.model.AbstractProject.checkout(AbstractProject.java:808)
at
hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:314)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:266)
at hudson.model.Run.run(Run.java:899)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:234)
at hudson.model.ResourceController.execute(ResourceController.java:93)
at hudson.model.Executor.run(Executor.java:119)

Subsequent updates work fine with no error message, hence P4 - alarming but
apparently harmless.