Workaround static Task destruction error

A task has to get the SchedulerLock to remove itself from the
Scheduler list. This doesn't work, if the Task is static, as the
static Scheduler might be destroyed earlier. In this case we fail
with the following backtrace:

 #0 SchedulerMutex::acquire
 #1 Task::~Task
 #2 __run_exit_handlers

Thanks to Michael Stahl to catching this backtrace.

As a workaround this marks static tasks, so they ignore the
SchedulerMutex in the destructor, We also mark all scheduled Tasks
as "static" in DeInitScheduler, as their cleanup was already done.

In the end all Tasks should be removed from static objects.

Change-Id: I38be3206378b9449193efaccbc96896ac8de9478
Reviewed-on: https://gerrit.libreoffice.org/42574
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>

include/vcl/task.hxx

@@ -47,6 +47,7 @@ class VCL_DLLPUBLIC Task
     const sal_Char    *mpDebugName;     ///< Useful for debugging
     TaskPriority       mePriority;      ///< Task priority
     bool               mbActive;        ///< Currently in the scheduler
+    bool               mbStatic;        ///< Is a static object
 
 protected:
     static void StartTimer( sal_uInt64 nMS );
@@ -88,6 +89,15 @@ public:
     void            Stop();
 
     bool            IsActive() const { return mbActive; }
+
+    /**
+     * This function must be called for static tasks, so the Task destructor
+     * ignores the SchedulerMutex, as it may not be available anymore.
+     * The cleanup is still correct, as it has already happened in
+     * DeInitScheduler call well before the static destructor calls.
+     */
+    void            SetStatic() { mbStatic = true; }
+    bool            IsStatic() const { return mbStatic; }
 };
 
 #endif // INCLUDED_VCL_TASK_HXX

svx/source/svdraw/svdetc.cxx

@@ -112,6 +112,7 @@ OLEObjCache::OLEObjCache()
     pTimer = new AutoTimer( "svx OLEObjCache pTimer UnloadCheck" );
     pTimer->SetInvokeHandler( LINK(this, OLEObjCache, UnloadCheckHdl) );
     pTimer->SetTimeout(20000);
+    pTimer->SetStatic();
 }
 
 OLEObjCache::~OLEObjCache()

vcl/README.scheduler

@@ -172,6 +172,13 @@ Since the Scheduler is always handled by the system message queue, there is
 really no more reasoning to stop after 100 events to prevent LO Scheduler
 starvation.
 
+== Drop static inherited or composed Task objects ==
+
+The sequence of destruction of static objects is not defined. So a static Task
+can not be guaranteed to happen before the Scheduler. When dynamic unloading
+is involved, this becomes an even worse problem. This way we could drop the
+mbStatic workaround from the Task class.
+
 == Run the LO application in its own thread ==
 
 This would probably get rid of most of the MacOS and Windows implementation

vcl/source/app/scheduler.cxx

@@ -160,6 +160,7 @@ void Scheduler::ImplDeInitScheduler()
                 pTask->mbActive = false;
             }
             pTask->mpSchedulerData = nullptr;
+            pTask->SetStatic();
         }
         ImplSchedulerData* pDeleteSchedulerData = pSchedulerData;
         pSchedulerData = pSchedulerData->mpNext;
@@ -548,6 +549,7 @@ Task::Task( const sal_Char *pDebugName )
     , mpDebugName( pDebugName )
     , mePriority( TaskPriority::DEFAULT )
     , mbActive( false )
+    , mbStatic( false )
 {
 }
 
@@ -556,16 +558,22 @@ Task::Task( const Task& rTask )
     , mpDebugName( rTask.mpDebugName )
     , mePriority( rTask.mePriority )
     , mbActive( false )
+    , mbStatic( false )
 {
     if ( rTask.IsActive() )
         Start();
 }
 
 Task::~Task() COVERITY_NOEXCEPT_FALSE
+{
+    if ( !IsStatic() )
     {
         SchedulerGuard aSchedulerGuard;
         if ( mpSchedulerData )
             mpSchedulerData->mpTask = nullptr;
     }
+    else
+        assert( nullptr == mpSchedulerData );
+}
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

vcl/win/gdi/salbmp.cxx

@@ -80,7 +80,7 @@ public:
         maEntries()
     {
         SetTimeout(1000);
-        Stop();
+        SetStatic();
     }
 
     ~GdiPlusBuffer() override