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Adding the SIO_LOOPBACK_FAST_PATH option to socket.ioctl on Windows allows Windows users to enable the loopback fast path option available on Windows 8+. This allows for much better TCP loopback performance on Windows.
For more information on TCP Loopback Fast Path, see:
 * https://msdn.microsoft.com/en-us/library/windows/desktop/jj841212%28v=vs.85%29.aspx
 * http://blogs.technet.com/b/wincat/archive/2012/12/05/fast-tcp-loopback-performance-and-low-latency-with-windows-server-2012-tcp-loopback-fast-path.aspx 

The "Lifecycle of a Patch" document recommends pinging an issue after a month of no review. I do not see any special ping option, so I am assuming that means to post a comment.
To help with code review I tried to keep this change very similar to a previous similar change: http://bugs.python.org/issue6971 

Thanks for pinging, Daniel. I'm not a Windows user so I can't test the patch, but I left some review comments on Rietveld: http://bugs.python.org/review/26536/ 

Thank you for the review. I have made the changes suggested in the code and I have uploaded a new patch.

I have added a zip containing a simple server and client to test the patch with. With loopback fast path enabled, I get 3-5x more throughput on my Windows 10 computer with an AMD 900 series chipset.

Thanks for the test script. I left some minor comments about loopback_fast_path_v2.patch on Rietveld.

I have uploaded a new patch with the suggested changes, thanks for the review.

New changeset f8957c755c7a by Steve Dower in branch 'default':
Issue #26536: socket.ioctl now supports SIO_LOOPBACK_FAST_PATH. Patch by Daniel Stokes.
https://hg.python.org/cpython/rev/f8957c755c7a 

All looked good to me, and as far as I could see all of Berker's feedback was addressed, so consider it in!

This is failing on Windows 7 buildbots:
http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/7849/steps/test/logs/stdio
======================================================================
ERROR: test_sio_loopback_fast_path (test.test_socket.GeneralModuleTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "C:\buildbot.python.org3円.x.kloth-win64\build\lib\test\test_socket.py", line 1226, in test_sio_loopback_fast_path
 s.ioctl(socket.SIO_LOOPBACK_FAST_PATH, True)
OSError: [WinError 10045] The attempted operation is not supported for the type of object referenced

New changeset e8b10ed64e63 by Berker Peksag in branch 'default':
Issue #26536: Skip test_sio_loopback_fast_path under Windows 7
https://hg.python.org/cpython/rev/e8b10ed64e63 

It looks like SIO_LOOPBACK_FAST_PATH is also defined in older Windows versions. I updated the test to skip if SIO_LOOPBACK_FAST_PATH is defined and exc.winerror is WSAEOPNOTSUPP. Buildbot is now green: http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/7855
Please reopen if I did something wrong.

New changeset 213c240cce0a by Berker Peksag in branch 'default':
Issue #26536: Use spaces instead of tabs
https://hg.python.org/cpython/rev/213c240cce0a 

That'll handle the test fine, though I wonder whether we should try and conditionally define the constant at runtime?
Probably at least want to add a note to the docs about Win 8 being the minimum, especially since so many people are still on 7.

> That'll handle the test fine, though I wonder whether we should try and conditionally define the constant at runtime?
+1. Is it safe to use IsWindows8OrGreater()?

That's the best function to use.