The new consumer is currently using very coarse-grained synchronization. For most methods this isn't a problem since they finish quickly once the lock is acquired, but poll() might run for a long time (and commonly will since polling with long timeouts is a normal use case). This means any operations invoked from another thread may block until the poll() call completes.
Some example use cases where this can be a problem:
- A shutdown hook is registered to trigger shutdown and invokes close(). It gets invoked from another thread and blocks indefinitely.
- User wants to manage offset commit themselves in a background thread. If the commit policy is not purely time based, it's not currently possibly to make sure the call to commit() will be processed promptly.
Two possible solutions to this:
1. Make sure a lock is not held during the actual select call. Since we have multiple layers (KafkaConsumer -> NetworkClient -> Selector -> nio Selector) this is probably hard to make work cleanly since locking is currently only performed at the KafkaConsumer level and we'd want it unlocked around a single line of code in Selector.
2. Wake up the selector before synchronizing for certain operations. This would require some additional coordination to make sure the caller of wakeup() is able to acquire the lock promptly (instead of, e.g., the poll() thread being woken up and then promptly reacquiring the lock with a subsequent long poll() call).