Copies data in both directions between
This function returns a future that will read from both streams, writing any data read to the opposing stream. This happens in both directions concurrently.
If an EOF is observed on one stream,
shutdown() will be invoked on
the other, and reading from that stream will stop. Copying of data in
the other direction will continue.
The future will complete successfully once both directions of communication has been shut down.
A direction is shut down when the reader reports EOF,
at which point
shutdown() is called on the corresponding writer. When finished,
it will return a tuple of the number of bytes copied from a to b
and the number of bytes copied from b to a, in that order.
The future will immediately return an error if any IO operation on
b returns an error. Some data read from either stream may be lost (not
written to the other stream) in this case.
Returns a tuple of bytes copied
b and bytes copied