No data available to recv after select

Question

Situation

I write a python client using nonblocking udp sockets. Sometimes I get

[Errno 11] Resource temporarily unavailable

As far as I know, this happens because I

Use nonblocking sockets
recv() on a nonblocking socket that has no data available

My code is this:

def doRequestReply(self, addr, msg, readTimeout=2, writeTimeout=2):
    # Create Socket
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.setblocking(0)

    ...
    # Send Request
    ...

    # Wait for Reply
    readable, _, exceptional = select.select([s], [], [s], readTimeout)
    if s in readable:
        data = s.recv(1024)

    ...

    s.close()

I sometimes get:

Traceback (most recent call last):
  File "/home/steffen/Desktop/shared/Monitor/CrawlerWorker.py", line 159, in run
    if not self.DoPeerExchange(peer, log=True):
  File "/home/steffen/Desktop/shared/Monitor/CrawlerWorker.py", line 106, in DoPeerExchange
    msg_rep = self.doRequestReply(addr, msg_req)
  File "/home/steffen/Desktop/shared/Monitor/CrawlerWorker.py", line 53, in doRequestReply
    data = s.recv(1024)
error: [Errno 11] Resource temporarily unavailable

Question

How is it possible that the exception can be raised. I mean select ensures data is ready to be read from this socket. I should not get this error.

I dont find any details in the docs. I dont want to just except the error as long as I dont know why it happens or what it means. Does anyone knows about this behaviour?

Thanks,

Steffen

Update 1

The server part also uses udp nonblocking sockets. Only one thread is reading from this socket, using select before receiving data.

# Create a TCP/IP socket
server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
server.setblocking(0)

while running:
    readable, _, exceptional = select.select([server], [], [server], 1)
    if server in readable:
        # Handle incoming connections
        data, client_address = server.recvfrom(1024)
        ...

After about an hour the got again:

Traceback (most recent call last):
  File "Sensor.py", line 87, in main
    data, client_address = server.recvfrom(1024)
error: [Errno 11] Resource temporarily unavailable

Does select actually ensures data will be available? What could be a reason for this exception?

I think this cannot be. As you see I create the socket in the function and close it before leaving. The socket is not shared — Steffen
I saw that, but sometimes the code people post here is abbreviated from the real thing. — Barmar
I now finished the server part. Again, I see the same issue. Only the main thread is reading from the socket in a loop! — Steffen

bosnjak bosnjak · Accepted Answer · 2014-12-27T13:33:19

Try:

select.select([s], [], [], readTimeout)

instead of

select.select([s], [], [s], readTimeout)

From the documentation:

select.select(rlist, wlist, xlist[, timeout])

rlist: wait until ready for reading

wlist: wait until ready for writing

xlist: wait for an “exceptional condition” (see the manual page for what your system considers such a condition)

It might me that an "exceptional condition" is causing this, depending on your operating system. You should select only on "ready for reading".

No data available to recv after select

Situation

Question

Update 1

1 Answers