Create the ContextWriter+Tx from UI thread on filters (related to #2122)

When we create a Tx the active site is queried, and the UIContext
needs access to the UI to calculate the active site correctly.

src/app/commands/filters/filter_manager_impl.cpp

@@ -280,10 +280,6 @@ void FilterManagerImpl::applyToTarget()
     return;
   }
 
-  // Initialize writting operation
-  ContextWriter writer(m_reader);
-  m_tx.reset(new Tx(writer.context(), m_filter->getName(), ModifyDocument));
-
   m_progressBase = 0.0f;
   m_progressWidth = (cels.size() > 0 ? 1.0f / cels.size(): 1.0f);
 
@@ -322,9 +318,18 @@ void FilterManagerImpl::applyToTarget()
   m_oldPalette.reset(nullptr);
 }
 
+void FilterManagerImpl::initTransaction()
+{
+  ASSERT(!m_tx);
+  m_writer.reset(new ContextWriter(m_reader));
+  m_tx.reset(new Tx(m_writer->context(),
+                    m_filter->getName(),
+                    ModifyDocument));
+}
+
 bool FilterManagerImpl::isTransaction() const
 {
-  return m_tx != nullptr;
+  return (m_tx != nullptr);
 }
 
 // This must be executed in the main UI thread.
@@ -333,6 +338,7 @@ void FilterManagerImpl::commitTransaction()
 {
   ASSERT(m_tx);
   m_tx->commit();
+  m_writer.reset();
 }
 
 #ifdef ENABLE_UI

src/app/commands/filters/filter_manager_impl.h

@@ -90,6 +90,8 @@ namespace app {
     void end();
     bool applyStep();
     void applyToTarget();
+
+    void initTransaction();
     bool isTransaction() const;
     void commitTransaction();
 
@@ -141,6 +143,7 @@ namespace app {
 #endif
 
     ContextReader m_reader;
+    std::unique_ptr<ContextWriter> m_writer;
     Site& m_site;
     Filter* m_filter;
     doc::Cel* m_cel;

src/app/commands/filters/filter_worker.cpp

@@ -135,6 +135,9 @@ FilterWorker::~FilterWorker()
 
 void FilterWorker::run()
 {
+  // Initialize writting transaction
+  m_filterMgr->initTransaction();
+
 #ifdef ENABLE_UI
   std::thread thread;
   // Open the alert window in foreground (this is modal, locks the main thread)

src/app/ui_context.cpp

@@ -28,6 +28,7 @@
 #include "app/ui/workspace_tabs.h"
 #include "base/mutex.h"
 #include "doc/sprite.h"
+#include "ui/system.h"
 
 #include <algorithm>
 
@@ -282,6 +283,12 @@ void UIContext::onRemoveDocument(Doc* doc)
 
 void UIContext::onGetActiveSite(Site* site) const
 {
+  // We can use the activeView only from the UI thread.
+#ifdef _DEBUG
+  if (isUIAvailable())
+    ui::assert_ui_thread();
+#endif
+
   DocView* view = activeView();
   if (view) {
     view->getSite(site);