Don't create 2D canvas contexts arbitrarily

Sometimes, the canvas still has no rendering context, in this case it represents
a transparent black rectangle.

components/canvas/canvas_paint_thread.rs

@@ -132,15 +132,21 @@ impl<'a> CanvasPaintThread <'a> {
                 self.canvas(canvas_id).is_point_in_path(x, y, fill_rule, chan)
             },
             Canvas2dMsg::DrawImage(
-                mut imagedata,
+                imagedata,
                 image_size,
                 dest_rect,
                 source_rect,
                 smoothing_enabled,
             ) => {
-                byte_swap(&mut imagedata);
+                let data = match imagedata {
+                    None => vec![0; image_size.width as usize * image_size.height as usize * 4],
+                    Some(mut data) => {
+                        byte_swap(&mut data);
+                        data.into()
+                    },
+                };
                 self.canvas(canvas_id).draw_image(
-                    imagedata.into(),
+                    data,
                     image_size,
                     dest_rect,
                     source_rect,

components/canvas_traits/canvas.rs

@@ -38,7 +38,7 @@ pub struct CanvasImageData {
 pub enum Canvas2dMsg {
     Arc(Point2D<f32>, f32, f32, f32, bool),
     ArcTo(Point2D<f32>, Point2D<f32>, f32),
-    DrawImage(ByteBuf, Size2D<f64>, Rect<f64>, Rect<f64>, bool),
+    DrawImage(Option<ByteBuf>, Size2D<f64>, Rect<f64>, Rect<f64>, bool),
     DrawImageSelf(Size2D<f64>, Rect<f64>, Rect<f64>, bool),
     DrawImageInOther(
         CanvasId, Size2D<f64>, Rect<f64>, Rect<f64>, bool),

components/script/dom/canvasrenderingcontext2d.rs

@@ -27,7 +27,7 @@ use dom::canvasgradient::{CanvasGradient, CanvasGradientStyle, ToFillOrStrokeSty
 use dom::canvaspattern::CanvasPattern;
 use dom::element::Element;
 use dom::globalscope::GlobalScope;
-use dom::htmlcanvaselement::HTMLCanvasElement;
+use dom::htmlcanvaselement::{CanvasContext, HTMLCanvasElement};
 use dom::imagedata::ImageData;
 use dom::node::{Node, NodeDamage, window_from_node};
 use dom_struct::dom_struct;
@@ -315,7 +315,8 @@ impl CanvasRenderingContext2D {
         result
     }
 
-    fn draw_html_canvas_element(&self,
+    fn draw_html_canvas_element(
+        &self,
         canvas: &HTMLCanvasElement,
         sx: f64,
         sy: f64,
@@ -324,8 +325,8 @@ impl CanvasRenderingContext2D {
         dx: f64,
         dy: f64,
         dw: Option<f64>,
-                                dh: Option<f64>)
-                                -> ErrorResult {
+        dh: Option<f64>,
+    ) -> ErrorResult {
         // 1. Check the usability of the image argument
         if !canvas.is_valid() {
             return Err(Error::InvalidState);
@@ -339,7 +340,8 @@ impl CanvasRenderingContext2D {
 
         let image_size = Size2D::new(canvas_size.width as f64, canvas_size.height as f64);
         // 2. Establish the source and destination rectangles
-        let (source_rect, dest_rect) = self.adjust_source_dest_rects(image_size,
+        let (source_rect, dest_rect) = self.adjust_source_dest_rects(
+            image_size,
             sx,
             sy,
             sw,
@@ -347,7 +349,8 @@ impl CanvasRenderingContext2D {
             dx,
             dy,
             dw,
-                                                                     dh);
+            dh,
+        );
 
         if !is_rect_valid(source_rect) || !is_rect_valid(dest_rect) {
             return Ok(());
@@ -355,29 +358,40 @@ impl CanvasRenderingContext2D {
 
         let smoothing_enabled = self.state.borrow().image_smoothing_enabled;
 
+        if let Some(context) = canvas.context() {
+            match *context {
+                CanvasContext::Context2d(ref context) => {
                     if self.canvas.as_ref().map_or(false, |c| &**c == canvas) {
                         self.send_canvas_2d_msg(Canvas2dMsg::DrawImageSelf(
-                image_size, dest_rect, source_rect, smoothing_enabled));
+                            image_size,
+                            dest_rect,
+                            source_rect,
+                            smoothing_enabled,
+                        ));
                     } else {
-            let context = match canvas.get_or_init_2d_context() {
-                Some(context) => context,
-                None => return Err(Error::InvalidState),
-            };
-
-            let msg = CanvasMsg::Canvas2d(
+                        context.get_ipc_renderer().send(CanvasMsg::Canvas2d(
                             Canvas2dMsg::DrawImageInOther(
                                 self.get_canvas_id(),
                                 image_size,
                                 dest_rect,
                                 source_rect,
-                    smoothing_enabled
+                                smoothing_enabled,
                             ),
-                context.get_canvas_id()
-            );
-
-            let renderer = context.get_ipc_renderer();
-            renderer.send(msg).unwrap();
-        };
+                            context.get_canvas_id(),
+                        )).unwrap();
+                    }
+                },
+                _ => return Err(Error::InvalidState),
+            }
+        } else {
+            self.send_canvas_2d_msg(Canvas2dMsg::DrawImage(
+                None,
+                image_size,
+                dest_rect,
+                source_rect,
+                smoothing_enabled,
+            ));
+        }
 
         self.mark_as_dirty();
         Ok(())
@@ -447,7 +461,7 @@ impl CanvasRenderingContext2D {
 
         let smoothing_enabled = self.state.borrow().image_smoothing_enabled;
         self.send_canvas_2d_msg(Canvas2dMsg::DrawImage(
-            image_data.into(),
+            Some(image_data.into()),
             image_size,
             dest_rect,
             source_rect,
@@ -1205,8 +1219,6 @@ impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
                     .ok_or(Error::InvalidState)?
             },
             CanvasImageSource::HTMLCanvasElement(ref canvas) => {
-                let _ = canvas.get_or_init_2d_context();
-
                 canvas.fetch_all_data().ok_or(Error::InvalidState)?
             },
             CanvasImageSource::CSSStyleValue(ref value) => {

components/script/dom/htmlcanvaselement.rs

@@ -37,9 +37,10 @@ use js::jsapi::JSContext;
 use js::rust::HandleValue;
 use offscreen_gl_context::GLContextAttributes;
 use profile_traits::ipc;
+use ref_filter_map;
 use script_layout_interface::{HTMLCanvasData, HTMLCanvasDataSource};
 use servo_config::prefs::PREFS;
-use std::iter::repeat;
+use std::cell::Ref;
 use style::attr::{AttrValue, LengthOrPercentageOrAuto};
 
 const DEFAULT_WIDTH: u32 = 300;
@@ -174,18 +175,22 @@ impl LayoutHTMLCanvasElementHelpers for LayoutDom<HTMLCanvasElement> {
 
 
 impl HTMLCanvasElement {
-    pub fn get_or_init_2d_context(&self) -> Option<DomRoot<CanvasRenderingContext2D>> {
-        if self.context.borrow().is_none() {
+    pub fn context(&self) -> Option<Ref<CanvasContext>> {
+        ref_filter_map::ref_filter_map(self.context.borrow(), |ctx| ctx.as_ref())
+    }
+
+    fn get_or_init_2d_context(&self) -> Option<DomRoot<CanvasRenderingContext2D>> {
+        if let Some(ctx) = self.context() {
+            return match *ctx {
+                CanvasContext::Context2d(ref ctx) => Some(DomRoot::from_ref(ctx)),
+                _ => None,
+            };
+        }
         let window = window_from_node(self);
         let size = self.get_size();
         let context = CanvasRenderingContext2D::new(window.upcast::<GlobalScope>(), self, size);
         *self.context.borrow_mut() = Some(CanvasContext::Context2d(Dom::from_ref(&*context)));
-        }
-
-        match *self.context.borrow().as_ref().unwrap() {
-            CanvasContext::Context2d(ref context) => Some(DomRoot::from_ref(&*context)),
-            _   => None,
-        }
+        Some(context)
     }
 
     #[allow(unsafe_code)]
@@ -194,20 +199,18 @@ impl HTMLCanvasElement {
         cx: *mut JSContext,
         options: HandleValue,
     ) -> Option<DomRoot<WebGLRenderingContext>> {
-        if self.context.borrow().is_none() {
+        if let Some(ctx) = self.context() {
+            return match *ctx {
+                CanvasContext::WebGL(ref ctx) => Some(DomRoot::from_ref(ctx)),
+                _ => None,
+            };
+        }
         let window = window_from_node(self);
         let size = self.get_size();
         let attrs = Self::get_gl_attributes(cx, options)?;
-            let maybe_ctx = WebGLRenderingContext::new(&window, self, WebGLVersion::WebGL1, size, attrs);
-
-            *self.context.borrow_mut() = maybe_ctx.map( |ctx| CanvasContext::WebGL(Dom::from_ref(&*ctx)));
-        }
-
-        if let Some(CanvasContext::WebGL(ref context)) = *self.context.borrow() {
-            Some(DomRoot::from_ref(&*context))
-        } else {
-            None
-        }
+        let context = WebGLRenderingContext::new(&window, self, WebGLVersion::WebGL1, size, attrs)?;
+        *self.context.borrow_mut() = Some(CanvasContext::WebGL(Dom::from_ref(&*context)));
+        Some(context)
     }
 
     #[allow(unsafe_code)]
@@ -219,20 +222,18 @@ impl HTMLCanvasElement {
         if !PREFS.is_webgl2_enabled() {
             return None
         }
-        if self.context.borrow().is_none() {
+        if let Some(ctx) = self.context() {
+            return match *ctx {
+                CanvasContext::WebGL2(ref ctx) => Some(DomRoot::from_ref(ctx)),
+                _ => None,
+            };
+        }
         let window = window_from_node(self);
         let size = self.get_size();
         let attrs = Self::get_gl_attributes(cx, options)?;
-            let maybe_ctx = WebGL2RenderingContext::new(&window, self, size, attrs);
-
-            *self.context.borrow_mut() = maybe_ctx.map( |ctx| CanvasContext::WebGL2(Dom::from_ref(&*ctx)));
-        }
-
-        if let Some(CanvasContext::WebGL2(ref context)) = *self.context.borrow() {
-            Some(DomRoot::from_ref(&*context))
-        } else {
-            None
-        }
+        let context = WebGL2RenderingContext::new(&window, self, size, attrs)?;
+        *self.context.borrow_mut() = Some(CanvasContext::WebGL2(Dom::from_ref(&*context)));
+        Some(context)
     }
 
     /// Gets the base WebGLRenderingContext for WebGL or WebGL 2, if exists.
@@ -289,9 +290,7 @@ impl HTMLCanvasElement {
                 // TODO: add a method in WebGL2RenderingContext to get the pixels.
                 return None;
             },
-            None => {
-                repeat(0xffu8).take((size.height as usize) * (size.width as usize) * 4).collect()
-            }
+            None => vec![0; size.height as usize * size.width as usize * 4]
         };
 
         Some((data, size))