Enable stateful kernels in G-API OCV Backend

modules/gapi/src/executor/gexecutor.cpp

@@ -265,3 +265,11 @@ void cv::gimpl::GExecutor::reshape(const GMetaArgs& inMetas, const GCompileArgs&
     passes::inferMeta(ctx, true);
     m_ops[0].isl_exec->reshape(g, args);
 }
+
+void cv::gimpl::GExecutor::prepareForNewStream()
+{
+    for (auto &op : m_ops)
+    {
+        op.isl_exec->handleNewStream();
+    }
+}

modules/gapi/src/executor/gexecutor.hpp

@@ -91,6 +91,8 @@ public:
     bool canReshape() const;
     void reshape(const GMetaArgs& inMetas, const GCompileArgs& args);
 
+    void prepareForNewStream();
+
     const GModel::Graph& model() const; // FIXME: make it ConstGraph?
 };
 

modules/gapi/src/executor/gstreamingexecutor.cpp

@@ -800,6 +800,7 @@ void cv::gimpl::GStreamingExecutor::setSource(GRunArgs &&ins)
             }
         }
     };
+    bool islandsRecompiled = false;
     const auto new_meta = cv::descr_of(ins); // 0
     if (gm.metadata().contains<OriginalInputMeta>()) // (1)
     {
@@ -821,6 +822,8 @@ void cv::gimpl::GStreamingExecutor::setSource(GRunArgs &&ins)
             }
             update_int_metas(); // (7)
             m_reshapable = util::make_optional(is_reshapable);
+
+            islandsRecompiled = true;
         }
         else // (8)
         {
@@ -929,7 +932,15 @@ void cv::gimpl::GStreamingExecutor::setSource(GRunArgs &&ins)
         for (auto &&out_eh : op.nh->outNodes()) {
             out_queues.push_back(reader_queues(*m_island_graph, out_eh));
         }
-        op.isl_exec->handleNewStream();
+
+        // If Island Executable is recompiled, all its stuff including internal kernel states
+        // are recreated and re-initialized automatically.
+        // But if not, we should notify Island Executable about new started stream to let it update
+        // its internal variables.
+        if (!islandsRecompiled)
+        {
+            op.isl_exec->handleNewStream();
+        }
 
         m_threads.emplace_back(islandActorThread,
                                op.in_objects,