Rewrite Qt4 based nested yield mutex locking.

The Qt event loop may start a nested event loop, when checking for
clipboard and Drag'n'Drop events.

Previously this was handled by running this nested yield loop
inside the main glib loop using
  qApp->clipboard()->property( "useEventLoopWhenWaiting" );

But this results in nested paint events which crash LO:
  QWidget::repaint: Recursive repaint detected

To prevend yield mutex deadlocks, check for nested event loops
and always release the yield lock before starting the nested Yield
event loop.

This fixes fdo#69002.

Change-Id: I7e827abd3489783053ec7123372742a32555875d
Reviewed-on: https://gerrit.libreoffice.org/6685
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>

vcl/unx/kde4/KDE4FilePicker.cxx

@@ -58,6 +58,8 @@
 
 #undef Region
 
+#include "generic/geninst.h"
+
 using namespace ::com::sun::star;
 using namespace ::com::sun::star::ui::dialogs;
 using namespace ::com::sun::star::ui::dialogs::TemplateDescription;
@@ -253,28 +255,16 @@ sal_Int16 SAL_CALL KDE4FilePicker::execute()
     _dialog->setFilter(_filter);
     _dialog->filterWidget()->setEditable(false);
 
-    // At this point, SolarMutex is held. Opening the KDE file dialog here
-    // can lead to QClipboard asking for clipboard contents. If LO core
-    // is the owner of the clipboard content, this will block for 5 seconds
-    // and timeout, since the clipboard thread will not be able to acquire
-    // SolarMutex and thus won't be able to respond. If the event loops
-    // are properly integrated and QClipboard can use a nested event loop
-    // (see the KDE VCL plug), then this won't happen, but otherwise
-    // simply release the SolarMutex here. The KDE file dialog does not
-    // call back to the core, so this should be safe (and if it does,
-    // SolarMutex will need to be re-acquired).
-    long mutexrelease = 0;
-    if( !qApp->clipboard()->property( "useEventLoopWhenWaiting" ).toBool())
-        mutexrelease = Application::ReleaseSolarMutex();
-    //block and wait for user input
+    // We're entering a nested loop.
+    // Release the yield mutex to prevent deadlocks.
     int result = _dialog->exec();
+
     // HACK: KFileDialog uses KConfig("kdeglobals") for saving some settings
     // (such as the auto-extension flag), but that doesn't update KGlobal::config()
     // (which is probably a KDE bug), so force reading the new configuration,
     // otherwise the next opening of the dialog would use the old settings.
     KGlobal::config()->reparseConfiguration();
-    if( !qApp->clipboard()->property( "useEventLoopWhenWaiting" ).toBool())
-        Application::AcquireSolarMutex( mutexrelease );
+
     if( result == KFileDialog::Accepted)
         return ExecutableDialogResults::OK;
 

vcl/unx/kde4/KDEXLib.cxx

@@ -225,9 +225,6 @@ void KDEXLib::setupEventLoop()
         eventLoopType = GlibEventLoop;
         old_gpoll = g_main_context_get_poll_func( NULL );
         g_main_context_set_poll_func( NULL, gpoll_wrapper );
-        // set QClipboard to use event loop, otherwise the main thread will hold
-        // SolarMutex locked, which will prevent the clipboard thread from answering
-        m_pApplication->clipboard()->setProperty( "useEventLoopWhenWaiting", true );
         return;
     }
 #endif
@@ -244,9 +241,6 @@ void KDEXLib::setupEventLoop()
         eventLoopType = QtUnixEventLoop;
         QInternal::callFunction( QInternal::GetUnixSelectFunction, reinterpret_cast< void** >( &qt_select ));
         QInternal::callFunction( QInternal::SetUnixSelectFunction, reinterpret_cast< void** >( lo_select ));
-        // set QClipboard to use event loop, otherwise the main thread will hold
-        // SolarMutex locked, which will prevent the clipboard thread from answering
-        m_pApplication->clipboard()->setProperty( "useEventLoopWhenWaiting", true );
         return;
     }
 #endif
@@ -300,6 +294,9 @@ void KDEXLib::socketNotifierActivated( int fd )
 
 void KDEXLib::Yield( bool bWait, bool bHandleAllCurrentEvents )
 {
+    // Nested yield loop counter.
+    static int loop_depth = 0;
+
     if( eventLoopType == LibreOfficeEventLoop )
     {
         if( qApp->thread() == QThread::currentThread())
@@ -310,13 +307,24 @@ void KDEXLib::Yield( bool bWait, bool bHandleAllCurrentEvents )
         }
         return SalXLib::Yield( bWait, bHandleAllCurrentEvents );
     }
+
     // if we are the main thread (which is where the event processing is done),
     // good, just do it
-    if( qApp->thread() == QThread::currentThread())
+    if( qApp->thread() == QThread::currentThread()) {
+        // Release the yield lock before entering a nested loop.
+        if (loop_depth > 0)
+            SalYieldMutexReleaser aReleaser;
+        loop_depth++;
         processYield( bWait, bHandleAllCurrentEvents );
-    else
-    { // if this deadlocks, event processing needs to go into a separate thread
-      // or some other solution needs to be found
+        loop_depth--;
+    }
+    else {
+        // we were called from another thread;
+        // release the yield lock to prevent deadlock.
+        SalYieldMutexReleaser aReleaser;
+
+        // if this deadlocks, event processing needs to go into a separate
+        // thread or some other solution needs to be found
         Q_EMIT processYieldSignal( bWait, bHandleAllCurrentEvents );
     }
 }