Failure in testNoJUnit4ClasspathIfUsingMaven: StaleMutableProjectRegistryException

[briandealwis]

Seen on Photon (link to log)
Haven't seen this error logged before.

The full code in m2e's ProjectRegistryManager is:

      if(newState.isStale() || (syncRefreshThread != null && syncRefreshThread != Thread.currentThread())) {
        throw new StaleMutableProjectRegistryException();
      }

syncRefreshThread is called set by ProjectRegistryManager#refresh() which obtains a workspace root rule.

Log:

[Watchdog] > testJstl12JarIfNonMavenProject(com.google.cloud.tools.eclipse.appengine.newproject.flex.CreateAppEngineFlexWtpProjectTest)



!ENTRY org.eclipse.wst.sse.core 1 1 2018-09-06 14:58:57.325

!MESSAGE Indexing project testproject0.8738283321394694 for JSP Index Manager

[Watchdog] > testNoJUnit4ClasspathIfUsingMaven(com.google.cloud.tools.eclipse.appengine.newproject.flex.CreateAppEngineFlexWtpProjectTest)
[...]
testNoJstl12JarIfMavenProject(com.google.cloud.tools.eclipse.appengine.newproject.flex.CreateAppEngineFlexWtpProjectTest)  Time elapsed: 16.365 sec  <<< ERROR!
org.eclipse.m2e.core.internal.project.registry.StaleMutableProjectRegistryException: null
	at org.eclipse.m2e.core.internal.project.registry.ProjectRegistryManager.refresh(ProjectRegistryManager.java:457)
	at org.eclipse.m2e.core.internal.project.registry.ProjectRegistryManager.refresh(ProjectRegistryManager.java:351)
	at org.eclipse.m2e.core.internal.project.registry.ProjectRegistryManager.refresh(ProjectRegistryManager.java:298)
	at org.eclipse.m2e.core.internal.project.ProjectConfigurationManager.configureNewMavenProjects(ProjectConfigurationManager.java:259)
	at org.eclipse.m2e.core.internal.project.ProjectConfigurationManager$4.run(ProjectConfigurationManager.java:528)
	at org.eclipse.m2e.core.internal.embedder.AbstractRunnable.call(AbstractRunnable.java:26)
	at org.eclipse.m2e.core.internal.embedder.AbstractRunnable.call(AbstractRunnable.java:1)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.executeBare(MavenExecutionContext.java:177)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.execute(MavenExecutionContext.java:151)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.execute(MavenExecutionContext.java:99)
	at org.eclipse.m2e.core.internal.embedder.MavenImpl.execute(MavenImpl.java:1355)
	at org.eclipse.m2e.core.internal.project.ProjectConfigurationManager.enableMavenNature(ProjectConfigurationManager.java:525)
	at com.google.cloud.tools.eclipse.appengine.newproject.CreateAppEngineWtpProject.enableMavenNature(CreateAppEngineWtpProject.java:193)
	at com.google.cloud.tools.eclipse.appengine.newproject.CreateAppEngineWtpProject.addAdditionalDependencies(CreateAppEngineWtpProject.java:147)
	at com.google.cloud.tools.eclipse.appengine.newproject.flex.CreateAppEngineFlexWtpProject.addAdditionalDependencies(CreateAppEngineFlexWtpProject.java:99)
	at com.google.cloud.tools.eclipse.appengine.newproject.CreateAppEngineWtpProject.execute(CreateAppEngineWtpProject.java:138)
	at com.google.cloud.tools.eclipse.appengine.newproject.CreateAppEngineWtpProjectTest.testNoJstl12JarIfMavenProject(CreateAppEngineWtpProjectTest.java:273)
[...]