view-backend: dispatch pending frame callbacks after a surface is unr…

…egistered

This was causing WebKit to stop painting after provisional navigation
fails with PSON enabled. It happened in the following situation:

1. Normal load, view backend registers surface for bridge 1
2. A frame is requested for bridge 1
3. CORS load starts, a new process is launched and view backend
   registers surface for bridge 2
4. Frame for bridge 1 is rendered, but current bridge is 2 so
   ViewBackend::dispatchFrameCallbacks() does nothing.
5. CORS load fails and the provisional load is destroyed so surface for
   bridge 2 is unregistered
6. No more updates because there's still the frame callback pending for
   surface for bridge 1.

This always happens when WebKit starts a CORS navigation that is
converted to a download, because in that case the provisional load fails
with cancelled error. It was never committed and nothing was rendered.
So, while it's ok to stop painting new frames during the provisional
load to avoid flashing, we should check if there's any pending callback
once the current surface is unregistered.

src/view-backend-private.cpp

@@ -105,8 +105,8 @@ void ViewBackend::commitDmabufPoolEntry(struct wpe_dmabuf_pool_entry* entry)
 void ViewBackend::dispatchFrameCallbacks()
 {
     if (G_LIKELY(!m_bridgeIds.empty())) {
-        WS::Instance::singleton().dispatchFrameCallbacks(m_bridgeIds.back());
-        wpe_view_backend_dispatch_frame_displayed(m_backend);
+        if (WS::Instance::singleton().dispatchFrameCallbacks(m_bridgeIds.back()))
+            wpe_view_backend_dispatch_frame_displayed(m_backend);
     }
 }
 
@@ -130,6 +130,9 @@ void ViewBackend::unregisterSurface(uint32_t bridgeId)
 
     m_bridgeIds.erase(it);
     WS::Instance::singleton().unregisterViewBackend(bridgeId);
+    // Dispatch frame callbacks in case there's any pending callback from previous bridge.
+    if (!m_bridgeIds.empty())
+        dispatchFrameCallbacks();
 }
 
 void ViewBackend::didReceiveMessage(uint32_t messageId, uint32_t messageBody)

src/ws.cpp

@@ -659,17 +659,18 @@ void Instance::unregisterSurface(Surface* surface)
     }
 }
 
-void Instance::dispatchFrameCallbacks(uint32_t bridgeId)
+bool Instance::dispatchFrameCallbacks(uint32_t bridgeId)
 {
     auto it = m_viewBackendMap.find(bridgeId);
     if (it == m_viewBackendMap.end()) {
         g_warning("Instance::dispatchFrameCallbacks(): "
                   "Cannot find surface with bridgeId %" PRIu32 " in view "
                   "backend map. Probably the associated surface is gone "
                   "due to a premature exit in the client side", bridgeId);
-    } else {
-        it->second->dispatchFrameCallbacks();
+        return false;
     }
+
+    return it->second->dispatchFrameCallbacks();
 }
 
 } // namespace WS

src/ws.h

@@ -94,7 +94,7 @@ struct Surface {
         wl_list_insert(m_pendingFrameCallbacks.prev, wl_resource_get_link(resource));
     }
 
-    void dispatchFrameCallbacks()
+    bool dispatchFrameCallbacks()
     {
         struct wl_resource* resource;
         struct wl_resource* tmp;
@@ -107,8 +107,11 @@ struct Surface {
             wl_resource_destroy(resource);
         }
 
-        if (client)
-            wl_client_flush(client);
+        if (!client)
+            return false;
+
+        wl_client_flush(client);
+        return true;
     }
 
 private:
@@ -151,7 +154,7 @@ class Instance {
     void unregisterSurface(Surface*);
     void registerViewBackend(uint32_t, APIClient&);
     void unregisterViewBackend(uint32_t);
-    void dispatchFrameCallbacks(uint32_t);
+    bool dispatchFrameCallbacks(uint32_t);
 
     using VideoPlaneDisplayDmaBufCallback = std::function<void(struct wpe_video_plane_display_dmabuf_export*, uint32_t, int, int32_t, int32_t, int32_t, int32_t, uint32_t)>;
     using VideoPlaneDisplayDmaBufEndOfStreamCallback = std::function<void(uint32_t)>;