join on cats-effect fiber doesn't raise inner error

Question

A cats-effect fiber, once started, a reference kept of it, some other code executed and then rejoined, won't raise the errors that happen inside.

Do you know why .join doesn't throw an error and why my application doesn't quit. Why will a thread remain running and the application stays hanging while also hiding the original error?

In my code

(for {
  startedStreamsFiber <- List(
        stream1,
        stream2,
      ).parTraverse_(_.compile.drain).toResource.start
  _ <- logger.info("Application has started").toResource
  _ <- startedStreamsFiber.join
} yield ()).use_

if one of the inner streams raises an error such as

org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint ...

the application won't fail, but stay on and keep logging:

Non-daemon threads currently preventing JVM termination: - 34: Thread[KQueueEventLoopGroup-2-1,10,main]
 -  - 38: Thread[DestroyJavaVM,5,main]
Non-daemon threads currently preventing JVM termination: - 34: Thread[KQueueEventLoopGroup-2-1,10,main]
 -  - 38: Thread[DestroyJavaVM,5,main]

Answer 1

Seems like the solution is to use startedStreamsFiber.joinWithUnit. Simple as that. .join doesn't have the same error handling as .joinWith, .joinNever or .joinWithUnit. It's the only one that doesn't propagate errors. This way it works and I get the nice stacktrace and the application quits.