[Python-Dev] PEP 446: Add new parameters to configure the inherance of files and for non-blocking sockets

[Victor Stinner]

2013/7/7 Charles-François Natali <cf.natali at gmail.com>:
> 2013/7/7 Cameron Simpson <cs at zip.com.au>:
>> On 06Jul2013 11:23, Charles-François Natali <cf.natali at gmail.com> wrote:
>> | > I've read your "Rejected Alternatives" more closely and Ulrich
>> | > Drepper's article, though I think the article also supports adding
>> | > a blocking (default True) parameter to open() and os.open(). If you
>> | > try to change that default on a platform where it doesn't work, an
>> | > exception should be raised.
>> |
>> | Contrarily to close-on-exec, non-blocking only applies to a limited
>> | type of files (e.g. it doesn't work for regular files, which represent
>> | 90% of open() use cases).
>>
>> sockets, pipes, serial devices, ...
>
> How do you use open() on a socket (which are already covered by
> socket(blocking=...)? Also, I said *regular files* - for which
> O_NONBLOCK doesn't make sense - represent 90% of io.open() use cases,
> and stand by this claim. Nothing prevents you from setting the FD
> non-blocking manually.

Ok, I think that the best consensus here is to add a new
os.set_blocking() function. The function would not be available on
Windows, but should be available on all UNIX platforms. The advantage
of os.set_blocking() over fcntl is that it may be implemented using
ioctl (FIONBIO) to use 1 syscall instead of 2. It is also easier to
remember calling "os.set_blocking(fd, True)" instead of
"fcntl.fcntl(fd, fcntl.F_SETFL, fcntl.fcntl(fcntl.F_GETFL, fd) |
os.O_NONBLOCK)".

See the diff:
http://hg.python.org/peps/rev/ce61588d244c

@Charles-François and Cameron: Do you prefer to new version of the PEP?
http://www.python.org/dev/peps/pep-0446/
(will be updated in a few minutes)

I'm not sure that many people are concerned by the number of syscalls,
but there is at least Peter Portante who opened a thread
"Modules/socketmodule.c: avoiding second fcntl() call worth the
effort?" on this mailing list, last January:
http://mail.python.org/pipermail/python-dev/2013-January/123661.html
"OpenStack Swift using the Eventlet module, which sets the accepted socket
non-blocking, resulting in twice the number of fcntl() calls. Not a killer
on performance, but it seems simple enough to save a system call here."

(This specific issue will be solved by the new blocking parameter
added to socket constructor ;-))

Victor