LibWebView+WebContent: Let the WebView client broadcast when it painted

When the WebContent process has painted to its shared bitmaps, it sends
a synchronous IPC to the browser process to let the chrome paint. It is
synchronous to ensure the WC process doesn't paint onto the backing
bitmap again while it is being displayed.

However, this can cause a crash at exit if the browser process quits
while the WC process is waiting for a response to this IPC.

This patch makes the painting logic asynchronous by letting the browser
process broadcast when it has finished handling the paint IPC. The WC
process will not paint anything again until it receives that message. If
it had tried to repaint while waiting for that message, that paint will
be deferred until it arrives.

Userland/Libraries/LibWebView/ViewImplementation.cpp

@@ -58,6 +58,8 @@ void ViewImplementation::server_did_paint(Badge<WebContentClient>, i32 bitmap_id
         if (on_ready_to_paint)
             on_ready_to_paint();
     }
+
+    client().async_ready_to_paint();
 }
 
 void ViewImplementation::load(AK::URL const& url)

Userland/Services/WebContent/ConnectionFromClient.cpp

@@ -148,6 +148,11 @@ void ConnectionFromClient::add_backing_store(i32 front_bitmap_id, Gfx::Shareable
     m_backing_stores.back_bitmap = *const_cast<Gfx::ShareableBitmap&>(back_bitmap).bitmap();
 }
 
+void ConnectionFromClient::ready_to_paint()
+{
+    page().ready_to_paint();
+}
+
 void ConnectionFromClient::process_next_input_event()
 {
     if (m_input_event_queue.is_empty())

Userland/Services/WebContent/ConnectionFromClient.h

@@ -69,6 +69,7 @@ private:
     virtual void key_down(i32, unsigned, u32) override;
     virtual void key_up(i32, unsigned, u32) override;
     virtual void add_backing_store(i32 front_bitmap_id, Gfx::ShareableBitmap const& front_bitmap, i32 back_bitmap_id, Gfx::ShareableBitmap const& back_bitmap) override;
+    virtual void ready_to_paint() override;
     virtual void debug_request(ByteString const&, ByteString const&) override;
     virtual void get_source() override;
     virtual void inspect_dom_tree() override;

Userland/Services/WebContent/PageClient.cpp

@@ -64,7 +64,9 @@ PageClient::PageClient(PageHost& owner, u64 id)
         auto& backing_stores = client().backing_stores();
         swap(backing_stores.front_bitmap, backing_stores.back_bitmap);
         swap(backing_stores.front_bitmap_id, backing_stores.back_bitmap_id);
-        client().did_paint(viewport_rect.to_type<int>(), backing_stores.front_bitmap_id);
+
+        m_paint_state = PaintState::WaitingForClient;
+        client().async_did_paint(viewport_rect.to_type<int>(), backing_stores.front_bitmap_id);
     });
 
 #ifdef HAS_ACCELERATED_GRAPHICS
@@ -76,10 +78,23 @@ PageClient::PageClient(PageHost& owner, u64 id)
 
 void PageClient::schedule_repaint()
 {
+    if (m_paint_state != PaintState::Ready) {
+        m_paint_state = PaintState::PaintWhenReady;
+        return;
+    }
+
     if (!m_repaint_timer->is_active())
         m_repaint_timer->start();
 }
 
+void PageClient::ready_to_paint()
+{
+    auto old_paint_state = exchange(m_paint_state, PaintState::Ready);
+
+    if (old_paint_state == PaintState::PaintWhenReady)
+        schedule_repaint();
+}
+
 void PageClient::visit_edges(JS::Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);

Userland/Services/WebContent/PageClient.h

@@ -65,6 +65,8 @@ public:
 
     void set_user_style(String source);
 
+    void ready_to_paint();
+
 private:
     PageClient(PageHost&, u64 id);
 
@@ -149,6 +151,13 @@ private:
     bool m_should_show_line_box_borders { false };
     bool m_has_focus { false };
 
+    enum class PaintState {
+        Ready,
+        WaitingForClient,
+        PaintWhenReady,
+    };
+
+    PaintState m_paint_state { PaintState::Ready };
     RefPtr<Web::Platform::Timer> m_repaint_timer;
 
     Web::CSS::PreferredColorScheme m_preferred_color_scheme { Web::CSS::PreferredColorScheme::Auto };

Userland/Services/WebContent/WebContentClient.ipc

@@ -17,7 +17,7 @@ endpoint WebContentClient
     did_request_navigate_back() =|
     did_request_navigate_forward() =|
     did_request_refresh() =|
-    did_paint(Gfx::IntRect content_rect, i32 bitmap_id) => ()
+    did_paint(Gfx::IntRect content_rect, i32 bitmap_id) =|
     did_request_cursor_change(i32 cursor_type) =|
     did_layout(Gfx::IntSize content_size) =|
     did_change_title(ByteString title) =|

Userland/Services/WebContent/WebContentServer.ipc

@@ -24,6 +24,7 @@ endpoint WebContentServer
     load_html(ByteString html) =|
 
     add_backing_store(i32 front_bitmap_id, Gfx::ShareableBitmap front_bitmap, i32 back_bitmap_id, Gfx::ShareableBitmap back_bitmap) =|
+    ready_to_paint() =|
 
     set_viewport_rect(Web::DevicePixelRect rect) =|