problems with threaded socket app

[Gordon Messmer]

On Mon, 15 Sep 2003 09:58:50 +0200, Anthony McDonald wrote:
> 
>         if rpipe not in ready_pipes[0]:
>             # Time to cancel this SMTP conversation
>             smtpi.close()
>             # The dialback thread will now write a failure message to
>             # its status pipe, and we'll need to clear that out.
>             os.read( rpipe, 1024 )
>             continue
> 
> The code creates a "race" condition. To work correctly it requires the
> worker thread to raise and handle an exception, and to write that result
> onto the pipe BEFORE your main thread attempts to read the pipe.
> 
> If the worker thread loses the race, the next MX result you process will
> recieve the last MX's results 400 error code, and your left with 1 thread at
> the end of the sequence which can't terminate as it stays active until what
> its written to the pipe is read from the pipe.
> 
> Simple enough to fix, just add a select call between closing the SMTP
> connection and reading the expected 400 error response.


I can do that, but can you explain why, if the monitor thread loses the
race, it wouldn't block until the worker thread writes its status to the
pipe?

The code below should have the same race condition, but it works properly.

[root at deerhunter root]# python2
Python 2.2.2 (#1, Jan 30 2003, 21:26:22)
[GCC 2.96 20000731 (Red Hat Linux 7.3 2.96-112)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> import thread
>>> import time
>>> def write_stat( wpipe ):
...     time.sleep(20)
...     os.write( wpipe, 'my status' )
...
>>> (rpipe, wpipe) = os.pipe()
>>> thread.start_new_thread( write_stat, (wpipe,) )
1026
>>> test = os.read( rpipe, 1024)
>>> print test
my status
>>>