Exception upon build failure in parallel builds

Arnaud Héritier reports seeing the following stacktrace upon (a failing) parallel build. The build was failing due to some other cause, and the race condition is due to a problem in the fail logic:

>
> Exception in thread "pool-1-thread-4" java.lang.NullPointerException
> at java.io.DeleteOnExitHook.add(DeleteOnExitHook.java:34)
> at java.io.File.deleteOnExit(File.java:939)
> at org.apache.maven.repository.legacy.DefaultWagonManager.getRemoteFile(DefaultWagonManager.java:293)
> at org.apache.maven.repository.legacy.DefaultWagonManager.getArtifact(DefaultWagonManager.java:99)
> at org.apache.maven.repository.legacy.DefaultWagonManager.getArtifact(DefaultWagonManager.java:150)
> at org.apache.maven.artifact.resolver.DefaultArtifactResolver.resolve(DefaultArtifactResolver.java:251)
> at org.apache.maven.artifact.resolver.DefaultArtifactResolver.access$000(DefaultArtifactResolver.java:73)
> at org.apache.maven.artifact.resolver.DefaultArtifactResolver$ResolveTask.run(DefaultArtifactResolver.java:689)
> at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
> at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
> at java.lang.Thread.run(Thread.java:637)

The race condition is believed to be caused because MavenCli calls System.exit(), even though other parallel-build modules may still be actively downloading artifacts; the check for isHalted (LifecycleThreadedBuilder line 69 (2x?!)) and similar in Weave mode needs to wait for all started modules to catch up with the "abort" status.