RP2: Poll-for-write on usys.stdout.buffer blocks

[smurfix]

import uasyncio, usys
def can_write(s):
    yield uasyncio.core._io_queue.queue_write(s)

async def write_stdout(data):
    await can_write(usys.stdout.buffer)
    usys.stdout.buffer.write(data.encode())

uasyncio.run(write_stdout('hello\n'))

This blocks indefinitely on RP2.

The corresponding code for reading works.

[dpgeorge]

Thanks for the report. Inspecting the code it seems that mp_hal_stdio_poll() only implements MP_STREAM_POLL_RD handling for stdin handling (USB CDC and REPL UART incoming streams). This is also the case for other ports using TinyUSB, eg mimxrt and samd.

To fix, it's easy enough to support MP_STREAM_POLL_WR. But what also needs to be done is for sys.stdout.buffer.write(data) to only write data that it can, not block, and return the number of bytes actually written. That's a bit tricky in the face of os.dupterm().

What's the reason to use sys.buffer.stdout in this case? Is it because you want to write to the USB serial output as part of an asyncio app?

[smurfix]

That's a bit tricky in the face of os.dupterm().

Right, but should be possible when there's just one stream.

What's the reason to use sys.buffer.stdout in this case?

I'm running a binary protocol (msgpack, basically) on USB-CDC. There's a multiplexer on the server side and modular code on the RP2, so I can basically run a bunch of code (including LittleFs file system access) in parallel.

[dpgeorge]

See #11091 for a fix.

[dpgeorge]

Fixed by 38e7b84