Fix crash painting deleted cels while applying filter (fix #4991)

This could happen in the Editor::onPaint() or Timeline::onPaint().

laf

@@ -1 +1 @@
-Subproject commit de781a5066732d700ac4520ee5fc9034e92875ad
+Subproject commit 39706c11063fb53cf4c8e865102c6f71e2606906

src/app/commands/filters/filter_manager_impl.cpp

@@ -325,10 +325,15 @@ void FilterManagerImpl::applyToTarget()
 void FilterManagerImpl::initTransaction()
 {
   ASSERT(!m_tx);
-  m_writer.reset(new ContextWriter(m_reader));
+  m_writer = std::make_unique<ContextWriter>(m_reader);
   m_tx.reset(new Tx(*m_writer, m_filter->getName(), ModifyDocument));
 }
 
+void FilterManagerImpl::updateWriterThread()
+{
+  document()->updateWriterThread();
+}
+
 bool FilterManagerImpl::isTransaction() const
 {
   return (m_tx != nullptr);

src/app/commands/filters/filter_manager_impl.h

@@ -93,6 +93,7 @@ public:
   void applyToTarget();
 
   void initTransaction();
+  void updateWriterThread();
   bool isTransaction() const;
   void commitTransaction();
 

src/app/commands/filters/filter_worker.cpp

@@ -118,7 +118,13 @@ FilterWorker::~FilterWorker()
 
 void FilterWorker::run()
 {
-  // Initialize writting transaction
+  // Initialize writing transaction from the main thread. This is
+  // required to get the activeSite() from the UIContext from
+  // CmdTransaction::calcSpritePosition().
+  //
+  // The document will keep the UI thread associated as the "writer"
+  // thread, but that will be updated later in
+  // applyFilterInBackground() with the worker thread ID.
   m_filterMgr->initTransaction();
 
   std::thread thread;
@@ -182,6 +188,11 @@ bool FilterWorker::isCancelled()
 void FilterWorker::applyFilterInBackground()
 {
   try {
+    // This background thread is the new writer. This is required to
+    // avoid read-locking from the UI thread from Editor and Timeline
+    // onPaint() events.
+    m_filterMgr->updateWriterThread();
+
     // Apply the filter
     m_filterMgr->applyToTarget();
 

src/app/doc.cpp

@@ -124,6 +124,11 @@ Doc::LockResult Doc::upgradeToWrite(int timeout)
   return res;
 }
 
+void Doc::updateWriterThread()
+{
+  m_rwLock.updateWriterThread();
+}
+
 void Doc::downgradeToRead(LockResult lockResult)
 {
   DOC_TRACE("DOC: downgradeToRead", this, (int)lockResult);

src/app/doc.h

@@ -82,6 +82,7 @@ public:
   LockResult readLock(int timeout);
   LockResult writeLock(int timeout);
   LockResult upgradeToWrite(int timeout);
+  void updateWriterThread();
   void downgradeToRead(LockResult lockResult);
   void unlock(LockResult lockResult);