condition variable not holding wait across two threads

Question

Stumped on this thread sync I'm encountering. Basically, I'm writing to a out buffer, and waiting on a condition variable until the read buffer is populated with a response from a socket. It's an incredibly simple thread sync.

def write_wait_response(self, buffer, timeout=30):
        '''
            Write and wait for response
            Params:
                Buffer BYTE encoded data
                Timeout timeout to wait for response
            Returns: 
                response str if successful
        '''
        self.buffer = buffer

        if self.waitLock(timeout):
            # condition var was signaled, we can return a response
            readbuf = bytes(self.readbuffer)
            self.readbuffer = b''
            return readbuf
        else:
            print("AsyncClientSocket: No response recieved from {} in {} seconds, dumping buffer".format(
                    self.sa, timeout))
            self.buffer = ''
            raise TimeoutError("AsyncClientSocket Timed Out")

def handle_read(self):
        self.readbuffer, address = self.recvfrom(2048)
        print(self.readbuffer)
        print("notifying")
        self.cond.notifyAll() 

Seems straightforward enough, right? There's 1 thread waiting on the condition variable, and 1 thread (asyncore async callback loop) that will populate self.readbuffer and notify on the condition variable. Even more curious : if I do a time.sleep() instead of using a condition variable, I get a perfectly populated self.readbuffer on the calling thread of write_wait_response(). Obviously this is not a solution I can accept.

Here's what I'm expecting is happening:

1. Call to write_wait_response(buffer), this writes to buffer and waits on self.cond
2. asyncore callback loop calls handle_write, writes bytes to socket.
3. Server receives bytes, writes response.
4. asyncore callback loop sees bytes on the socket, reads into self.readbuffer, notifies cv
5. ?????????? write_wait_response should unblock?

Console output:

waiting <- thread 1 waiting on CV
AsyncClientSocket: writing 5 bytes <- thread 2: handle_write
b'200,2' <- thread 2: that's the server response
notifying <- thread 2: that's handle_read attempting to notify the held CV

error: uncaptured python exception, closing channel <my_socket_stuff.AsyncClientSocket connected 127.0.0.1:50000 at 0x1051bf438> (<class 'RuntimeError'>:cannot notify on un-acquired lock

Note: at the end of this log, thread 1 is STILL waiting on self.cond. What's going on?

Full class:

class AsyncClientSocket(asyncore.dispatcher):
    def __init__(self, socketargs):
        asyncore.dispatcher.__init__(self)
        family, type, proto, canonname, sa = socketargs
        self.sa = sa
        self.create_socket(family, type)

        if type == socket.SOCK_STREAM:
            self.connect( sa )
        elif type == socket.SOCK_DGRAM:
            pass

        self.buffer = b''
        self.lock = threading.Lock()
        self.cond = threading.Condition(self.lock)
        self.readbuffer = b''

    def write_wait_response(self, buffer, timeout=30):
        '''
            Write and wait for response
            Params:
                Buffer BYTE encoded data
                Timeout timeout to wait for response
            Returns: 
                response str if successful
        '''
        self.buffer = buffer

        if self.waitLock(timeout):
            # condition var was signaled, we can return a response
            readbuf = bytes(self.readbuffer)
            self.readbuffer = b''
            return readbuf
        else:
            print("AsyncClientSocket: No response recieved from {} in {} seconds, dumping buffer".format(
                    self.sa, timeout))
            self.buffer = ''
            raise TimeoutError("AsyncClientSocket Timed Out")

    def waitLock(self, timeout):
        '''
            Wait for timeout seconds on CV
        '''
        try:
            self.cond.acquire()
            print("waiting")
            return self.cond.wait(timeout)
        finally:
            self.cond.release()


    def handle_connect(self):
        pass

    def handle_close(self):
        self.close()

    def handle_read(self):
        self.readbuffer, address = self.recvfrom(2048)
        print(self.readbuffer)
        print("notifying")
        self.cond.notifyAll() 

    def writable(self):
        return (len(self.buffer) > 0)

    def handle_write(self):
        print("AsyncClientSocket: writing {} bytes".format(len(self.buffer)))
        self.readbuffer = b''
        sent = self.sendto(self.buffer, self.sa)
        self.buffer = self.buffer[sent:]

Answer 1

Figured it out. This isn't related to asyncore. I was just signaling the condition variable incorrectly. The python3 threading api doc says the calling thread of notify() must acquire the underlying lock which makes sense, wouldn't want two producers to notify on the same condition variable. Would want one to block on the critical section while the other performs its task.

def handle_read(self):
    try:
        self.cond.acquire()
        self.readbuffer, address = self.recvfrom(2048)
        print(self.readbuffer)
        self.cond.notify() 
    finally:
        self.cond.release()