Remove Sync from SendableRecordBatchStream #1614

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Currently SendableRecordBatchStream imposes Sync constraints.

pub type SendableRecordBatchStream = Pin<Box<dyn RecordBatchStream + Send + Sync>>;

This is unfortunate as not all streams have Sync bounds, furthermore it is unclear why Sync is particularly useful for a stream.

A type is Sync if a reference to it can be shared across multiple threads, as opposed to Send which just requires that it can be sent to another thread. However, you can only poll a stream with a mutable, and therefore exclusive reference. The only thing you can therefore do with a &SendableRecordBatchStream is get the schema which I doubt is being exploited anywhere.

Describe the solution you'd like

Remove the Sync constraint

Describe alternatives you've considered

Keep the sync constraint

Additional context

This constraint currently forces a Mutex in FlightDataStream even though it should be unnecessary - see here