FAQ
*** Submitted as
http://code.google.com/p/go/source/detail?r=8087f34d11d6 ***

net: fix connection resets when closed on windows

It is common to close network connection while another goroutine is
blocked reading on another goroutine. This sequence corresponds to
windows calls to WSARecv to start io, followed by
GetQueuedCompletionStatus
that blocks until io completes, and, finally, closesocket called from
another thread. We were expecting that closesocket would unblock
GetQueuedCompletionStatus, and it does, but not always
(http://code.google.com/p/go/issues/detail?id=4170#c5). Also that
sequence
results in connection is being reset.

This CL inserts CancelIo between GetQueuedCompletionStatus and
closesocket,
and waits for both WSARecv and GetQueuedCompletionStatus to complete
before
proceeding to closesocket. This seems to fix both connection resets and
issue 4170. It also makes windows code behave similar to unix version.

Unfortunately, CancelIo needs to be called on the same thread as
WSARecv.
So we have to employ strategy we use for connections with deadlines to
every connection now. It means, there are 2 unavoidable thread switches
for every io. Some newer versions of windows have new CancelIoEx api
that
doesn't have these drawbacks, and this CL uses this capability when
available.
As time goes by, we should have less of CancelIo and more of CancelIoEx
systems. Computers with CancelIoEx are also not affected by issue 4195
anymore.

Fixes issue 3710
Fixes issue 3746
Fixes issue 4170
Partial fix for issue 4195

R=golang-dev, mikioh.mikioh, bradfitz, rsc
CC=golang-dev
http://codereview.appspot.com/6604072


http://codereview.appspot.com/6604072/

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