tdf#100337 Message boxes showup empty with white background

the bug here was that we had, on the stack, going into the scheduler
THREE times. Two of those were idles.

The original code would end up always picking the first idle from inside
    ImplSchedulerData::GetMostImportantTask
and then
    ImplSchedulerData::Invoke
would just return because we were still inside that Idle, and the second
Idle would never get executed

Since the second Idle was responsible for painting the dialog in the
bug, sometimes the dialog would never get painted.

Reviewed-on: https://gerrit.libreoffice.org/31785
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>
(cherry picked from commit 19a4eaab9a55a2ecb33b727bad6307c5a2badc23)
Reviewed-on: https://gerrit.libreoffice.org/31798
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
(cherry picked from commit 3be163c72925824eeadf5e75f6e0cf6229d8ceab)

Change-Id: Ia15b98a06e231c8e1c29450e05a76ad427e41e36

2 files changed, 37 insertions, 1 deletions

diff --git a/vcl/qa/cppunit/timer.cxx b/vcl/qa/cppunit/timer.cxx
index 0bc86667f09d..b8d5285920cd 100644
--- a/vcl/qa/cppunit/timer.cxx
+++ b/vcl/qa/cppunit/timer.cxx
@@ -68,6 +68,7 @@ public:
 #endif
     void testRecursiveTimer();
     void testSlowTimerCallback();
+    void testTriggerIdleFromIdle();
 
     CPPUNIT_TEST_SUITE(TimerTest);
     CPPUNIT_TEST(testIdle);
@@ -82,6 +83,7 @@ public:
 #endif
     CPPUNIT_TEST(testRecursiveTimer);
     CPPUNIT_TEST(testSlowTimerCallback);
+    CPPUNIT_TEST(testTriggerIdleFromIdle);
 
     CPPUNIT_TEST_SUITE_END();
 };
@@ -366,6 +368,39 @@ void TimerTest::testSlowTimerCallback()
         Application::Yield();
 }
 
+
+class TriggerIdleFromIdle : public Idle
+{
+    bool* mpTriggered;
+    TriggerIdleFromIdle* mpOther;
+public:
+    explicit TriggerIdleFromIdle( bool* pTriggered, TriggerIdleFromIdle* pOther ) :
+        Idle(), mpTriggered(pTriggered), mpOther(pOther)
+    {
+    }
+    virtual void Invoke() override
+    {
+        Start();
+        if (mpOther)
+            mpOther->Start();
+        Application::Yield();
+        if (mpTriggered)
+            *mpTriggered = true;
+    }
+};
+
+void TimerTest::testTriggerIdleFromIdle()
+{
+    bool bTriggered1 = false;
+    bool bTriggered2 = false;
+    TriggerIdleFromIdle aTest2( &bTriggered2, nullptr );
+    TriggerIdleFromIdle aTest1( &bTriggered1, &aTest2 );
+    aTest1.Start();
+    Application::Yield();
+    CPPUNIT_ASSERT_MESSAGE("idle triggered", bTriggered1);
+    CPPUNIT_ASSERT_MESSAGE("idle triggered", bTriggered2);
+}
+
 CPPUNIT_TEST_SUITE_REGISTRATION(TimerTest);
 
 CPPUNIT_PLUGIN_IMPLEMENT();
diff --git a/vcl/source/app/scheduler.cxx b/vcl/source/app/scheduler.cxx
index efb65ca110d2..1310c239e0e0 100644
--- a/vcl/source/app/scheduler.cxx
+++ b/vcl/source/app/scheduler.cxx
@@ -26,6 +26,7 @@
 
 void ImplSchedulerData::Invoke()
 {
+    assert(!mbInScheduler);
     if (mbDelete || mbInScheduler )
         return;
 
@@ -49,7 +50,7 @@ ImplSchedulerData *ImplSchedulerData::GetMostImportantTask( bool bTimerOnly )
     sal_uInt64 nTimeNow = tools::Time::GetSystemTicks();
     for ( ImplSchedulerData *pSchedulerData = pSVData->mpFirstSchedulerData; pSchedulerData; pSchedulerData = pSchedulerData->mpNext )
     {
-        if ( !pSchedulerData->mpScheduler || pSchedulerData->mbDelete ||
+        if ( !pSchedulerData->mpScheduler || pSchedulerData->mbDelete || pSchedulerData->mbInScheduler ||
              !pSchedulerData->mpScheduler->ReadyForSchedule( bTimerOnly, nTimeNow ) ||
              !pSchedulerData->mpScheduler->IsActive())
             continue;