[mod_python] ioerrors with worker thread mpm

Michael Barton mike at weirdlooking.com
Thu Dec 11 00:55:00 EST 2008

On Dec 10, 2008, at 10:33 PM, "Graham Dumpleton" <graham.dumpleton at gmail.com 
 > wrote:

> 2008/12/11 Michael Barton <mike at weirdlooking.com>:
>> Yes, sorry.  The exceptions I've seen are raised by sendall and recv
>> on socket objects inside httplib, which is connected to a service.  I
>> can reproduce this with a very simple handler like,
>> from mod_python import apache
>> from socket import socket
>> def handler(req):
>>     s = socket()
>>     s.connect(('localhost', 80))
>>     s.recv(1024)
>>     return apache.OK
>> Make a request to that handler, which will obviously not return any
>> time soon.  Then hit the server enough times that it reaches
>> MaxRequestsPerChild.  And,
>> ...
>> File "/home/redbo/test/modpython_handler.py", line 7, in handler\n
>> s.recv(1024)
>> error: (4, 'Interrupted system call')
> Apache doesn't appear to use SA_RESTART when registering signal
> handlers, so yes, it means that system calls interrupted by a signal
> will not be restarted. This is quite normal behaviour and typically
> programs should be written to detect an EINTR and retry the system
> library function call being done if they need to be robust in the face
> of such an event. That is, code which would run in an unknown
> environment, can't assume system calls will be restarted.

I guess i'm really just frustrated that there's no way to recover a  
non-fatal exception in httplib without restarting the entire request,  
which just isn't possible in the case of this case.

Maybe I can monkey patch around it or something.

It's a python problem though, not a modpython problem.

> The EAGAIN error is different and doubtful it is cause by signals.
> What EAGAIN means will depend on what system library call is being
> used. For recv() it means:
>     [EAGAIN]           The socket is marked non-blocking, and the  
> receive
>                        operation would block, or a receive timeout  
> had been
>                        set, and the timeout expired before data were
>                        received.

It's definitely also happening when apache hits maxrequests.  I wonder  
if this has something to do with python's recv using select to block.  
I don't have the python source in front of me to see what happens if  
that select is interrupted.

Ah well, thanks for thinking about it.


> Graham
>> On Wed, Dec 10, 2008 at 9:19 PM, Graham Dumpleton
>> <graham.dumpleton at gmail.com> wrote:
>>> Blocking i/o operations against what? Sockets you have created off  
>>> to
>>> something else????
>>> 2008/12/11 Michael Barton <mike at weirdlooking.com>:
>>>> Hey list!
>>>> I'm having a problem adapting a mod_python-based app to work with
>>>> Apache's worker thread mpm.
>>>> When the process hits MaxRequestsPerChild under a high  
>>>> concurrency of
>>>> requests, blocking i/o operations in other threads on that child  
>>>> raise
>>>> exceptions (mostly EINTR or EAGAIN IOErrors in httplib).
>>>> It looks like system calls are being interrupted by signals Apache
>>>> uses to control its children.  But that seems like a problem that
>>>> would have come up before, and I can't find much by searching the  
>>>> web
>>>> or list archives.
>>>> I figured I'd punt to the list and see if anyone has other  
>>>> thoughts.
>>>> My setup is: debian, apache 2.2.3, mod_python 3.3.1, python 2.4.4
>>>> --Mike
>>>> _______________________________________________
>>>> Mod_python mailing list
>>>> Mod_python at modpython.org
>>>> http://mailman.modpython.org/mailman/listinfo/mod_python

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