Description:
On Windows we can't use alarms or signals to enforce net_(read|write)_timeout settings
as the WinSock send() call is not interruptable, so we have to rely on socket options to
request proper timeouts from WinSock.

Function net_real_write() uses vio_timeout() to set the WinSock write timeout ot
net_write_timeout seconds. But vio_timeout() does only set the SO_RCVTIMEO 
option, not SO_SNDTIMEO as needed for writes.

This leads to unkillable server threads hanging in "Writing to Net" state if a client
stalls
while reading results (write buffer fills up, client doesn't read fast enough or not at
all
but socket still exists). 

How to repeat:
hard to repeat at will, you need a stalling client and a windows server to reproduce this

Suggested fix:

As a quick fix i added a set_sockopt call to set SO_SNDTIMEO together with SO_RCVTIMEO in
vio_socket (effectively setting both to the same values), this did solve the locking
problems for the customer.

A clean solution would probably require seperate vio_readtimeout() and vio_writetimeout()
funcions or an aditional parameter to vio_timeout() to
specify which value(s) to set.

Fixed in 4.1.12 & 5.0.6

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/internals/24587

Noted in 4.1.12, 5.0.6 changelogs.