BigtableSession is never closed by Reader causing "ManagedChannel allocation site" exceptions

[pdex]

This isn't a major issue, but it is causing exceptions to be logged that may confuse someone who isn't familiar with grpc-java/netty.

Code example

The BigtableSession created here is never closed.

Stack trace

java.lang.RuntimeException: ManagedChannel allocation site
	at com.google.bigtable.repackaged.io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.<init>(ManagedChannelOrphanWrapper.java:93)
	at com.google.bigtable.repackaged.io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:53)
	at com.google.bigtable.repackaged.io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:44)
	at com.google.bigtable.repackaged.io.grpc.internal.AbstractManagedChannelImplBuilder.build(AbstractManagedChannelImplBuilder.java:517)
	at com.google.bigtable.repackaged.com.google.cloud.bigtable.grpc.BigtableSession.createNettyChannel(BigtableSession.java:630)
	at com.google.bigtable.repackaged.com.google.cloud.bigtable.grpc.BigtableSession.<init>(BigtableSession.java:367)
	at com.google.cloud.bigtable.beam.CloudBigtableIO$Reader.initializeScanner(CloudBigtableIO.java:583)
	at com.google.cloud.bigtable.beam.CloudBigtableIO$Reader.start(CloudBigtableIO.java:573)
	at org.apache.beam.runners.dataflow.worker.WorkerCustomSources$BoundedReaderIterator.start(WorkerCustomSources.java:607)
	at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation$SynchronizedReaderIterator.start(ReadOperation.java:361)
	at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.runReadLoop(ReadOperation.java:194)
	at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.start(ReadOperation.java:159)
	at org.apache.beam.runners.dataflow.worker.util.common.worker.MapTaskExecutor.execute(MapTaskExecutor.java:77)
	at org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.executeWork(BatchDataflowWorker.java:417)
	at org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.doWork(BatchDataflowWorker.java:386)
	at org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.getAndPerformWork(BatchDataflowWorker.java:311)
	at org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.doWork(DataflowBatchWorkerHarness.java:140)
	at org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:120)
	at org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:107)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

[pdex]

Just to bring everyone up to speed in one place #2782 was reverted by #2873
citing this comment in #2871

#2913 is incorrect there is not a leak, the underlying grpc-netty library is informing the user that they haven't properly closed a channel. This isn't a big issue for Dataflow jobs.

The ideal solution will close the session when the worker shuts down. But again, if that doesn't happen you can ignore this log message in your Dataflow job.