Asyncio StreamReader occasionally reads 0 bytes - python-3.x

Question

I have a simple streamreader that is listening to a TCP port for a websocket implementation. The stream occasionally reads (every 30-300 second) blank data and throws an error.
loop = asyncio.get_event_loop()
coro = asyncio.start_server(handle_echo, 'not.real.ip.address', 8888, loop=loop)
server = loop.run_until_complete(coro)
server.close()
loop.run_until_complete(server.wait_closed())
loop.close()
#asyncio.coroutine
def handle_echo(reader, writer):
#Starts the connection with upgrade handshake
while True:
#read frame data
first_byte = yield from reader.readexactly(1)
opcode = first_byte & 0xf
second_byte = yield from reader.readexactly(1)
second_byte = ord(second_byte)
mask = second_byte >> 7
len1 = second_byte & 0x7f
if len1 == 126:
len2 = yield from reader.readexactly(2)
length = int.from_bytes(len2, byteorder='big')
elif len1 == 127:
len2 = yield from reader.readexactly(8)
length = int.from_bytes(len2, byteorder='big')
else:
length = len1
if mask:
masking_key = yield from reader.readexactly(4)
if length:
payload = yield from reader.readexactly(length)
payload = bytearray(payload)
if mask:
for i,b in enumerate(payload):
payload[i] = payload[i] ^ masking_key[i%4]
if opcode & 0x1:
payload = payload.decode("UTF-8")
Every once in a while, the following error is thrown:
Future/Task exception was never retrieved
future: Task(<handle_echo>)<exception=IncompleteReadError('0 bytes read on a total of 1 expected bytes',)>
Traceback (most recent call last):
File "/usr/lib/python3.4/asyncio/tasks.py", line 300, in _step
result = coro.send(value)
File "server.py", line 76, in handle_echo
first_byte = yield from reader.readexactly(1)
File "/usr/lib/python3.4/asyncio/streams.py", line 464, in readexactly
raise IncompleteReadError(partial, len(partial) + n)
asyncio.streams.IncompleteReadError: 0 bytes read on a total of 1 expected bytes
I am having a hard time tracking down what is causing this. I have used tcptrack to watch the port, and nothing is coming through. Am I improperly reading from the port, is there some sort of clean up that I need to do, or does the reader occasionally misread? I have tried other read functions (read and readline), and they throw similar errors occasionally.

Answer 1

readexactly() is a picky method. From the documentation:
Read exactly n bytes. Raise an IncompleteReadError if the end of the stream is reached before n can be read, the IncompleteReadError.partial attribute of the exception contains the partial read bytes.
Since the server seems to be sending empty bytes strings sometimes, it is probably a good idea for you to put the readexactly() call on a try / except block:
try:
first_byte = yield from reader.readexactly(1)
except asyncio.IncompleteReadError:
pass
else:
# Rest of your code
You could even check if you received more than 0 (zero) bytes and buffer them if your use case requires it:
except asyncio.IncompleteReadError as e:
if len(e.partial) > 0:
# Handle the bytes read
else:
pass