Re: /dev/windows and select() [was Re: Slow response to keypresses in xorg-server-1.8.0-1]

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Aug 29 14:39, Jon TURNEY wrote:
> On 08/08/2010 12:04, Andy Koppe wrote:
> >On 7 August 2010 23:07, Jon TURNEY wrote:
> >>Hmmm, looking again at the implementation of select(), I don't immediately
> >>see that when waiting on /dev/windows, it checks that the message queue has
> >>old messages on it before waiting.  The MSDN documentation for
> >>MsgWaitForMultipleObjects() seems to says that messages which had arrived
> >>before the last PeekMessage() etc. aren't considered new and so don't end
> >>the wait?
> >
> >I think you're right, a call to PeekMessage is needed for proper
> >select() semantics: it shouldn't block if data is available for
> >reading.
> 
> Attached is a small test-case which seems to demonstrate this problem.
> 
> Run ./dev-windows-select-test and observe select() blocks for the
> full timeout, despite the fact that the /dev/windows fd is ready for
> reading (and it reported as such as the end of the timeout)
> 
> If you run './dev-windows-select-test -skip' to skip the
> PeekMessage(), select() returns immediately, indicating the
> /dev/windows fd is ready for reading.
> [...]

Thanks for the testcase.  I examined this and I think I have a
workaround.  MSDN states that there's a flag QS_ALLPOSTMESSAGE for
MsgWaitForMultipleObjects, which is not cleared by PeekMessage, if the
wMsgFilterMin and wMsgFilterMax arguments are not both 0.  So, what I
did was to add the QS_ALLPOSTMESSAGE flag to the
MsgWaitForMultipleObjects call in select.cc, and to change the
PeekMessage call in select.cc:peek_windows() from

  PeekMessage (&m, (HWND) h, 0, 0, PM_NOREMOVE)

to

  PeekMessage (&m, (HWND) h, 1, UINT_MAX, PM_NOREMOVE)

Same in your above test application.  This appears to do the trick.
However, I'm not exactly sure if that's a valid fix.  Patch below.


Corinna


Index: select.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/select.cc,v
retrieving revision 1.160
diff -u -p -r1.160 select.cc
--- select.cc	2 Apr 2010 22:36:44 -0000	1.160
+++ select.cc	29 Aug 2010 14:16:18 -0000
@@ -287,7 +287,8 @@ select_stuff::wait (fd_set *readfds, fd_
       if (!windows_used)
 	wait_ret = WaitForMultipleObjects (m, w4, FALSE, ms);
       else
-	wait_ret = MsgWaitForMultipleObjects (m, w4, FALSE, ms, QS_ALLINPUT);
+	wait_ret = MsgWaitForMultipleObjects (m, w4, FALSE, ms,
+					      QS_ALLINPUT | QS_ALLPOSTMESSAGE);
 
       switch (wait_ret)
       {
@@ -1531,7 +1532,7 @@ peek_windows (select_record *me, bool)
   if (me->read_selected && me->read_ready)
     return 1;
 
-  if (PeekMessage (&m, (HWND) h, 0, 0, PM_NOREMOVE))
+  if (PeekMessage (&m, (HWND) h, 1, UINT_MAX, PM_NOREMOVE))
     {
       me->read_ready = true;
       select_printf ("window %d(%p) ready", me->fd, me->fh->get_handle ());


-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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