Seems like both milestone-5 and milestone-6 (I haven't tried M4) are having concurrency issues with cobertura during testRuntime if test forking and module dependencies are present at the same time.

Here's a simple project that reproduces the issue:
[1]https://github.com/johnburbridge/grad...

Note that this works fine when commenting out either this:

test {
forkEvery = 1;
maxParallelForks = 2;
}

or this:

axisGenAntTask module('org.apache.axis:axis:1.4') {
dependency 'javax.xml:jaxrpc-api:1.1';
dependency 'axis:axis-wsdl4j:1.5.1';
dependency 'commons-discovery:commons-discovery:0.2';
dependency 'commons-codec:commons-codec:1.3';
dependency 'javax.mail:mail:1.3.1';
}

But the combination of both causes this:

FAILURE: Build failed with an exception.

• What went wrong:
  Execution failed for task ':test'.
  Cause: Could not resolve all dependencies for configuration ':testRuntime'.
  Cause: Cannot lock the artifact cache, as it is already locked by this process.

Also, interesting that setting forkEvery = 0 doesn't help any though one would expect that should help.

This works fine under milestone-3 provided you update mavenRepo url to urls.

HTH!
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[1] https://github.com/johnburbridge/gradleBug/blob/master/build.gradle