Flink 1.12 job server error: Trying to access closed classloader.

When running multiple jobs in local mode on the Flink 1.12 job server, pipeline construction for subsequent jobs fails with the error below. Classloader leakage is being detected by the wrapper classloader (see FLINK-16245).

The classloader is probably being leaked by GRPC threads that we can't shut down cleanly.

RuntimeError: java.lang.IllegalStateException: Trying to access closed classloader. Please check if you store classloaders directly or indirectly in static fields. If the stacktrace suggests that the leak occurs in a third party library and cannot be fixed immediately, you can disable this check with the configuration 'classloader.check-leaked-classloader'. at org.apache.flink.runtime.execution.librarycache.FlinkUserCodeClassLoaders$SafetyNetWrapperClassLoader.ensureInner(FlinkUserCodeClassLoaders.java:161) at org.apache.flink.runtime.execution.librarycache.FlinkUserCodeClassLoaders$SafetyNetWrapperClassLoader.loadClass(FlinkUserCodeClassLoaders.java:168) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:348) at org.apache.beam.sdk.options.PipelineOptionsFactory.findCallersClassName(PipelineOptionsFactory.java:418) at org.apache.beam.sdk.options.PipelineOptionsFactory.access$100(PipelineOptionsFactory.java:122) at org.apache.beam.sdk.options.PipelineOptionsFactory$Builder.(PipelineOptionsFactory.java:212) at org.apache.beam.sdk.options.PipelineOptionsFactory$Builder.(PipelineOptionsFactory.java:208) at org.apache.beam.sdk.options.PipelineOptionsFactory$Builder.(PipelineOptionsFactory.java:199) at org.apache.beam.sdk.options.PipelineOptionsFactory.create(PipelineOptionsFactory.java:131) at org.apache.beam.sdk.Pipeline.create(Pipeline.java:146) at org.apache.beam.sdk.expansion.service.ExpansionService.expand(ExpansionService.java:394) at org.apache.beam.sdk.expansion.service.ExpansionService.expand(ExpansionService.java:491) at org.apache.beam.model.expansion.v1.ExpansionServiceGrpc$MethodHandlers.invoke(ExpansionServiceGrpc.java:232) at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.stub.ServerCalls$UnaryServerCallHandler$UnaryServerCallListener.onHalfClose(ServerCalls.java:172) at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.halfClosed(ServerCallImpl.java:331) at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1HalfClosed.runInContext(ServerImpl.java:817) at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37) at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:123) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748)

https://ci-beam.apache.org/job/beam_PreCommit_Python_PVR_Flink_Commit/1707/testReport/junit/apache_beam.runners.portability.flink_runner_test/FlinkRunnerTestStreaming/test_external_transform/

Imported from Jira [BEAM-11570](https://issues.apache.org/jira/browse/BEAM-11570). Original Jira may contain additional context.
Reported by: ibzib.