Problem with signals & exceptions...

[Eric Lee Green]

I'm having trouble catching my thrown exceptions under FreeBSD 3.2. I am
using the Python 1.5.2 that came with FreeBSD 3.2 (well, on disk 1 of
the <n>-disk set). The following program works properly under Red Hat
Linux version 6.0 running a self-compiled Python 1.5.2 (in place of the
1.5.1 which comes with Red Hat 6.0, which does not properly handle
signals during socket i/o calls). How it works: it listens to port 6664.
When it gets a connection, it then sets an alarm handler and alarm and
then tries to read from the socket. If it does not get data within 10
seconds, it then times out, at which point the alarm handler throws an
exception and aborts the socket read.  

The test:
  In one shell window, type:
     python testalrm.py
  in the other shell window type:
     telnet localhost 6664
type NOTHING just let it time out...

The output in the first window:

bash-2.03$ python testalarm.py
Got alarm!
READ_TIMEOUT
Traceback (innermost last):
  File "testalarm.py", line 30, in ?
    raise IOError, why
IOError: READ_TIMEOUT
bash-2.03$ 

The program:
----------------------------testalarm.py------------------

# A routine to test the 'alarm' signal handling.

import socket
import signal

READ_TIMEOUT="READ_TIMEOUT"

def alarmhandler(signum,stackframe):
    print "Got alarm!"
    raise IOError,READ_TIMEOUT


s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
addr=("",6664)
s.bind(addr)
s.listen(5)

connection,address=s.accept()

oldhandler=signal.signal(signal.SIGALRM,alarmhandler)

signal.alarm(10)  # wait 10 seconds for an alarm!
try:
        str=connection.recv(4096) # get a string
except IOError, why:
        print why
        if why == READ_TIMEOUT :
                print "why = read timeout"
        else:
                raise IOError, why
signal.alarm(0)
print str

----------------end program-------------------------------------