the Pico port does not have a separate USB_VCP object so we resort to use sys stdio. while receiving works, transmitting blocks or crashes on writing. other ports like the pyboard support this. It would be good if this works so that uasyncio communication can be done via USB too.

Here is a minimal example for testing:
echo.py.txt

• It works on a pyboard (pybv11-20210302-unstable-v1.14-82-gcdaec0dca)
• It works on the unix port (though with sys.stdout instead of sys.stdout.buffer)
• It fails on the pico (rp2-pico-20210301-unstable-v1.14-82-gcdaec0dca)
• [edit] it fails on the esp32 (esp32-20210302-unstable-v1.14-82-gcdaec0dca)

Output when it fails (drain never outputs anything on pico and esp32):

# ready
.# input b'hello\r\n'
# drain
.........Traceback (most recent call last):
[...]
KeyboardInterrupt: 

And there is an exception on the pico (not on esp32) when the feedback_task is not started, pressing RETURN twice results in:

# ready
# input b'hello\r\n'
# drain
Traceback (most recent call last):
  File "<stdin>", line 32, in <module>
  File "<stdin>", line 29, in echo
  File "uasyncio/core.py", line 1, in run_forever
  File "uasyncio/core.py", line 1, in run_until_complete
IndexError: empty heap

Compared to an output on the pyboard, where it echoes and goes back to reading as expected:

# ready
...# input b'hallo\r\n'
# drain
.hallo
# ready
[...]