ConcurrentModificationException from RowCoderGenerator lazy caching

Summary:

RowCoderGenerator caches a delegate Coder<Row> once encode or decode is exercised, but there's not an API for caching this delegate eagerly.

Use Case:

When creating several PCollections to perform distinct reads with the same schema, you might create one RowCoder.of(schema) before creating the list of PCollections / PCollectionsList. However, once the pipeline begins and rows arrive for encoding, these pipelines will simultaneously try to cache a delegate coder for the row's schema. 

Workaround:

You can force the eager caching of the code by exercising encode in the main application before creating PCollections using the RowCoder:

try {
 myRowCoder.encode(null, null);
 } catch (IOException | NullPointerException e) {
 // do nothing
}

Context:

I've only encountered this during development with the direct runner.