Introduction：

I just used the uasyncio library on microPython (upy) playform for my ESP32 project. As I was implementing a simple web service using the server provided by this library, the example code below didn’t work on my code sketch:

Error appearance:

import uasyncio as asyncio
@asyncio.coroutine
def serve(reader, writer):
    print(reader, writer)
    print("================")
    print((yield from reader.read()))
    yield from writer.awrite("HTTP/1.0 200 OK\r\n\r\nHello.\r\n")
    print("After response write")
    yield from writer.aclose()
    print("Finished processing request")

loop = asyncio.get_event_loop()
loop.call_soon(asyncio.start_server(serve, "127.0.0.1", 8081))
loop.run_forever()
loop.close()

It said:

File “/lib/uasyncio/__init__.py”, line 60, in remove_writer

TypeError: function takes 2 positional arguments but 3 were given

every time I call yield from writer.aclose() in my code.

#(uasyncio, writer.aclose(), remove_writer)

In order to fix the bug, I dug into the source code to find its defect.

Install uasyncio

——How I installed the uasyncio library on micropython ESP32 build?

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import upip
upip.install('micropython-uasyncio')
import uasyncio

With an internet connection, the board will search the web with its builtin (but not loaded by default) package manager upip. It will download the library to /lib/uasyncio/ directory on your ESP32’s file system.

Finding the bug

For a long-time struggle, I finally located where the error occurs. This is the function in __init__.py of uasyncio library:

import uselect as select
class PollEventLoop(EventLoop):
    self.poller = select.poll()
    ...
    def remove_writer(self, sock):
        if DEBUG and __debug__:
            log.debug("remove_writer(%s)", sock)
        self.objmap.pop(id(sock), None)
        # StreamWriter.awrite() first tries to write to a socket,
        # and if that succeeds, yield IOWrite may never be called
        # for that socket, and it will never be added to poller. So,
        # ignore such error.

        self.poller.unregister(sock, False)

As I looked up the documentation, the self.poller.unregister() only receives 2 arguments: (self, fd).

So, as the code tries to pass 3 arguments (self.poller, sock, FALSE) to the poller, the program crashes.

Fix:

My solution, may be a dirty hack:

def remove_writer(self, sock):
    if DEBUG and __debug__:
        log.debug("remove_writer(%s)", sock)
    self.objmap.pop(id(sock), None)
    # StreamWriter.awrite() first tries to write to a socket,
    # and if that succeeds, yield IOWrite may never be called
    # for that socket, and it will never be added to poller. So,
    # ignore such error.
    self.poller.unregister(sock)

By just removing the False flag argument, the code worked like a charm (at least for my small piece of example code).

Another solution from Github:

def remove_writer(self, sock):
    if DEBUG and __debug__:
        log.debug("remove_writer(%s)", sock)
    try:
        self.poller.unregister(sock)
        self.objmap.pop(id(sock), None)
    except OSError as e:
            # StreamWriter.awrite() first tries to write to a socket,
            # and if that succeeds, yield IOWrite may never be called
            # for that socket, and it will never be added to poller. So,
            # ignore such error.
        if e.args[0] != uerrno.ENOENT:
            raise

Perhaps this one is better a solution.