Interesting Thread Gotcha

[Hendrik van Rooyen]

I thought I would share this nasty little gotcha with the group.

Consider the following code fragment:

<start>
print 'starting kbd thread'
keyboard_thread = thread.start_new_thread(kbd_driver (port_q,kbd_q))
print 'starting main loop'
error = Mainloop(s,port_q,active_q_list)
<end>

It produces, as output, the following:

starting kbd thread
we get here - a

It does not print 'starting main loop', the Mainloop routine
is never executed, and no exceptions are raised.

Here is the offending routine that seems to capture the control:

<start>
def kbd_driver(out_q,in_q):
    """
    thread to look for keyboard input and to put it on the queue out_q
    also looks for replies on in_q and prints them
    """

    kbdname = '/dev/stdin'

    kbd  = open(kbdname,'r+',1) # Reading, line buffered

    unblock(kbd)     # Call the magic to unblock keyboard
    print 'we get here - a'
    while True:

        try:
            d = kbd.readline()  # see if any kbd input
        except:
            IOError
            try:
                msg=in_q.get(block=False)
            except Queue.Empty:
                time.sleep(0.1)
                continue
            print msg
            time.sleep(0.1)
            continue
        d = d.rstrip()    # get rid of line feed
        out_q.put([d + '\r',in_q]) # add a carriage return and return q and send
to port
<end>


The unblock is a routine that unblocks a port using fcntl - it
is not the problem.  In case you don't believe me, here it is:

def unblock(f):
 """Given file 'f', sets its unblock flag to true."""

    fcntl.fcntl(f.fileno(), fcntl.F_SETFL, os.O_NONBLOCK)

I will post the solution tomorrow when I read my mail,
if no one has spotted it by then.

- Hendrik