prevent deadlocks between main thread and other thread processing a Qt event

This part of 13a34f4c6307d1bd2443cbf3fbd83bfdd8cdbafb was correct. The main
thread will deadlock on the yield mutex if the calling thread does not release
it before waiting on the main thread.

Change-Id: I6692473457258b791c131e8bdb459b06b01550f7
(cherry picked from commit f09d4bc2853be2fa3faa0502b8efe94ad9719731)

1 files changed, 6 insertions, 2 deletions

diff --git a/vcl/unx/kde4/KDEXLib.cxx b/vcl/unx/kde4/KDEXLib.cxx
index 1c09058f08ee..86663b5fac45 100644
--- a/vcl/unx/kde4/KDEXLib.cxx
+++ b/vcl/unx/kde4/KDEXLib.cxx
@@ -297,8 +297,12 @@ void KDEXLib::Yield( bool bWait, bool bHandleAllCurrentEvents )
     if( qApp->thread() == QThread::currentThread())
         processYield( bWait, bHandleAllCurrentEvents );
     else
-    { // if this deadlocks, event processing needs to go into a separate thread
-      // or some other solution needs to be found
+    {
+        // we were called from another thread;
+        // release the yield lock to prevent deadlock with the main thread
+        // (it's ok to release it here, since even normal processYield() would
+        // temporarily do it while checking for new events)
+        SalYieldMutexReleaser aReleaser;
         Q_EMIT processYieldSignal( bWait, bHandleAllCurrentEvents );
     }
 }