Portable Validates Runner Tests on Flink flaky after update to gradle5

After upgrade to gradle 5 [1], the two portable runner test projects on jenkins

• beam_PostCommit_Java_PVR_Flink_Streaming [2]
• beam_PostCommit_Java_PVR_Flink_Batch [3]

became flaky.

First investigation seems to point the tests to be failing on direct buffer memory (e.g. [4]) while staging files.

Although I am unsure, whether this is really the root cause, or something that shows up after some other failure.

INFO: Transport failed
org.apache.beam.vendor.grpc.v1p13p1.io.netty.util.internal.OutOfDirectMemoryError: failed to allocate 16777216 byte(s) of direct memory (used: 1895825695, max: 1908932608)

As far as I know, we do not set `-XX:MaxDirectMemorySize` anywhere in our setup, neither does gradle itself. At least on my machine both gradle 4 and gradle 5 stick to the same jvm default

###########################################
sun.misc.VM.maxDirectMemory(): 1908932608 Bytes
sun.misc.VM.maxDirectMemory(): 1820 MB
###########################################

Unfortunately this does not reproduce on (my) local machine. We might try to workaround here by increasing ` -XX:MaxDirectMemorySize==3G` but this would probably only hide the problem? But might still be helpful to increase temporarily on branch just to be sure, that this is indeed the root cause?

[1] https://issues.apache.org/jira/browse/BEAM-6630
[2] https://builds.apache.org/job/beam_PostCommit_Java_PVR_Flink_Streaming/
[3] https://builds.apache.org/job/beam_PostCommit_Java_PVR_Flink_Batch/
[4] https://scans.gradle.com/s/tpo3yffjznfxa/tests/yobvrae4rwsg4-go44ti5iq45vq