On Oct 22, 2008, at 11:37 AM, Jesse Noller wrote:
On Wed, Oct 22, 2008 at 11:06 AM, Philip Semanchuk <[EMAIL PROTECTED]
> wrote:
One oversight I noticed the multiprocessing module docs is that a
semaphore's acquire() method shouldn't have a timeout on OS X as
sem_timedwait() isn't supported on that platform. (You note OS X's
lack of
support for sem_getvalue() elsewhere.)
Please file a ticket or update http://bugs.python.org/issue4012 so I
don't loose it, my memory is increasingly lossy. Good catch.
I updated issue4012.
A question - how do you handle the difference in error messages on
various
platforms? For instance, sem_trywait() raises error 35, "Resource
temporarily unavailable" under OS X but error 11 under Ubuntu.
Right now I'm
just passing these up to the (Python) caller as OSErrors. This
makes it
really hard for the Python programmer to write cross-platform code.
If you look at the code, we're pretty much raising OSError - it's
possible we could enhance this in later versions, but given MP is
supposed to be a cross-platform as possible and protect the user from
the seedy underbelly of semaphores/pipes/etc - when an OSError does
occur, it's generally a bug in our code, not the users.
Gotcha. I had a look at the code, and you're testing for errno ==
EAGAIN when sem_trywait() fails. This is correct for OS X and Ubuntu
(the platforms I mentioned above) and probably most other Unices. I
just don't have confidence that it will be true across all platforms,
esp. ones to which I don't have access like AIX and big-iron systems
that support POSIX. Maybe I am just taking defensive programming too
far.
The only solution I can think of (which I haven't coded yet) is to
compile &
run a series of small C programs during setup.py that test things
like
sem_trywait() to see what errors occur, and provide those constants
to my
main .c module so that it can detect those errors exactly and wrap
them into
a specific, custom error for the Python caller.
Any thoughts on this?
That's actually (while feeling hacky) a possibly sensible idea, the
problem is is that you'd need to maintain documentation to tell users
the exceptions for their platform.
If I pass all errors up as OSError, yes, that will be true. But that's
actually the situation I was trying to avoid.
By generating the error at install time, I can see exactly what
platform X returns in that situation (e.g. 11). Then I can create a
#define something like this:
#define ERRNO_WHEN_CALLING_SEM_TRYWAIT_ON_A_LOCKED_SEMAPHORE 11
My main .c module can then test for errno ==
ERRNO_WHEN_CALLING_SEM_TRYWAIT_ON_A_LOCKED_SEMAPHORE and wrap that in
a custom error, like posix_ipc.SemaphoreBusyError.
Again, maybe I'm just taking defensive programming too far. I've been
bitten by other cross-platform inconsistencies at the C API level and
I'm trying to get the jump on them here.
Cheers
Philip
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