[Interest] [PROGRESS] Long pause with 5.7

[william.crocker at analog.com]

On 11/07/2016 07:22 PM, william.crocker at analog.com wrote:
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>>>
>>> I am porting my app to Qt 5.7
>>> When I run it, it appears on the screen,
>>> but there is a long pause (~10 secs.) before it is responsive.
>>> I ran it with strace and there is a point where it makes
>>> 100K calls to the poll() function, which accounts for the long pause.
>>> I tried running it in a debugger, but the app does not exhibit the pause in
>>> that context.
>>
>> Sounds like a threading race condition and running in the debugger changes the
>> timing just enough.
>>
>
> Gave the debugger another try. On some occasions, it would
> take the long pause. (The pause is more like 5 seconds.)
> If I interrupt it, this is the stack trace.
> Note that it is in the call to show() which precedes
> my call to exec().
>
> Bill
>
> #0  0x0000003b0a0df113 in poll () from /lib64/libc.so.6
> #1  0x0000003b0bc449f9 in ?? () from /lib64/libglib-2.0.so.0
> #2  0x0000003b0bc44e4c in g_main_context_iteration () from /lib64/libglib-2.0.so.0
> #3  0x00007ffff55b2740 in
> QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
>     from
> /nobackup/whc/qt/5.7.0-shared-debug-64Bit-RH6-NoOpenGL/qtbase/lib/libQt5Core.so.5
> #4  0x00007ffff5565229 in
> QCoreApplication::processEvents(QFlags<QEventLoop::ProcessEventsFlag>, int) ()
>     from
> /nobackup/whc/qt/5.7.0-shared-debug-64Bit-RH6-NoOpenGL/qtbase/lib/libQt5Core.so.5
> #5  0x000000000098c60b in MyProg::show (this=0x7fffffffb8d0) at prog.cpp:562
> #6  0x0000000000986551 in MyMain (argc=@0x7fffffffc15c: 1, argv=0x7fffffffc2f8)
> at prog.cpp:151
> #7  0x0000000000986ad9 in main (argc=1, argv=0x7fffffffc2f8) at yoda.cpp:221
>

I did not notice the call to show() was a call *MY* show().
My bad, but there is still something funny going on.

Since the beginning of time I have been searching for the way to restore window
positions and sizes when a user reenters my app. This is especially tricky
under X windows with the separate client/server, various window managers
and window decorations. Following is part of my solution.

It appears that with Qt 5.7, SOMETIMES, my call to processEvents waits
for the full 5 secs even though I do not believe there is anything to do.
That is why my app is SOMETIMES unresponsive for 5 seconds when I start it up.

void
MyApp::show() {

     QSize sz; QPoint pos;
     {
         QSettings settings;
         sz = settings.value("MainWindowWSize",QSize(1200,700)).toSize();
         pos = settings.value("MainWindowPos",QPoint(180,190)).toPoint();
     }

     if( sz.width() > 0 && sz.height() > 0 ) {
         resize(sz);
         move(pos);
     }

     BaseClass::show();
     qApp->flush();

     // It appears that with Qt 5.7, SOMETIMES, this call to processEvents waits
     // for the full 5 secs though I do not believe there is anything to do.
     qApp->processEvents(QEventLoop::ExcludeUserInputEvents,5000);

     if( sz.width() > 0 && sz.height() > 0 ) {
         resize(sz);
         move(pos);
     }

     raise();
}