canvas: ensure there is a subpath in PathBuilderRef (#37251)

This is also required by spec:
https://html.spec.whatwg.org/multipage/canvas.html#ensure-there-is-a-subpath
and if we not ensure it vello will trigger debug asserts. Enforcing this
at `PathBuilderRef` is the lowest possible level that avoids misuse (and
avoids IPC messages if we were to do this in content process) while
still keeping it from backend.

Testing: There are tests in WPT
Split of https://github.com/servo/servo/pull/36821
try run: 1544904469

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/canvas/canvas_data.rs

@@ -154,7 +154,18 @@ struct PathBuilderRef<'a, B: Backend> {
 }
 
 impl<B: Backend> PathBuilderRef<'_, B> {
+    /// <https://html.spec.whatwg.org/multipage#ensure-there-is-a-subpath>
+    fn ensure_there_is_a_subpath(&mut self, point: &Point2D<f32>) {
+        if self.builder.get_current_point().is_none() {
+            self.builder.move_to(*point);
+        }
+    }
+
+    /// <https://html.spec.whatwg.org/multipage#dom-context-2d-lineto>
     fn line_to(&mut self, pt: &Point2D<f32>) {
+        // 2. If the object's path has no subpaths, then ensure there is a subpath for (x, y).
+        self.ensure_there_is_a_subpath(pt);
+
         let pt = self.transform.transform_point(*pt);
         self.builder.line_to(pt);
     }
@@ -182,14 +193,22 @@ impl<B: Backend> PathBuilderRef<'_, B> {
         self.move_to(&first);
     }
 
+    /// <https://html.spec.whatwg.org/multipage#dom-context-2d-quadraticcurveto>
     fn quadratic_curve_to(&mut self, cp: &Point2D<f32>, endpoint: &Point2D<f32>) {
+        // 2. Ensure there is a subpath for (cpx, cpy).
+        self.ensure_there_is_a_subpath(cp);
+
         self.builder.quadratic_curve_to(
             &self.transform.transform_point(*cp),
             &self.transform.transform_point(*endpoint),
         )
     }
 
+    /// <https://html.spec.whatwg.org/multipage#dom-context-2d-beziercurveto>
     fn bezier_curve_to(&mut self, cp1: &Point2D<f32>, cp2: &Point2D<f32>, endpoint: &Point2D<f32>) {
+        // 2. Ensure there is a subpath for (cp1x, cp1y).
+        self.ensure_there_is_a_subpath(cp1);
+
         self.builder.bezier_curve_to(
             &self.transform.transform_point(*cp1),
             &self.transform.transform_point(*cp2),
@@ -210,6 +229,7 @@ impl<B: Backend> PathBuilderRef<'_, B> {
             .arc(center, radius, start_angle, end_angle, ccw);
     }
 
+    /// <https://html.spec.whatwg.org/multipage#dom-context-2d-arcto>
     fn arc_to(&mut self, cp1: &Point2D<f32>, cp2: &Point2D<f32>, radius: f32) {
         let cp0 = if let (Some(inverse), Some(point)) =
             (self.transform.inverse(), self.builder.get_current_point())
@@ -218,6 +238,9 @@ impl<B: Backend> PathBuilderRef<'_, B> {
         } else {
             *cp1
         };
+
+        // 2. Ensure there is a subpath for (x1, y1) is done by one of self.line_to calls
+
         if (cp0.x == cp1.x && cp0.y == cp1.y) || cp1 == cp2 || radius == 0.0 {
             self.line_to(cp1);
             return;

components/canvas/raqote_backend.rs

@@ -710,7 +710,7 @@ impl GenericPathBuilder<RaqoteBackend> for PathBuilder {
             PathOp::MoveTo(point) | PathOp::LineTo(point) => Some(Point2D::new(point.x, point.y)),
             PathOp::CubicTo(_, _, point) => Some(Point2D::new(point.x, point.y)),
             PathOp::QuadTo(_, point) => Some(Point2D::new(point.x, point.y)),
-            PathOp::Close => None,
+            PathOp::Close => path.ops.first().and_then(get_first_point),
         })
     }
 
@@ -733,6 +733,15 @@ impl GenericPathBuilder<RaqoteBackend> for PathBuilder {
     }
 }
 
+fn get_first_point(op: &PathOp) -> Option<euclid::Point2D<f32, euclid::UnknownUnit>> {
+    match op {
+        PathOp::MoveTo(point) | PathOp::LineTo(point) => Some(Point2D::new(point.x, point.y)),
+        PathOp::CubicTo(point, _, _) => Some(Point2D::new(point.x, point.y)),
+        PathOp::QuadTo(point, _) => Some(Point2D::new(point.x, point.y)),
+        PathOp::Close => None,
+    }
+}
+
 pub trait ToRaqoteStyle {
     type Target;
 

tests/wpt/meta/css/css-borders/tentative/corner-shape/corner-shape-render-precise.html.ini

@@ -37,9 +37,6 @@
 [corner-shape-render-precise.html?corner-top-left-shape=bevel&border-width=10px&border-color=black]
   expected: FAIL
 
-[corner-shape-render-precise.html?corner-bottom-right-shape=bevel&corner-bottom-left-shape=bevel]
-  expected: FAIL
-
 [corner-shape-render-precise.html?corner-shape=superellipse(8)&border-radius=10px&box-shadow=10px 10px 0 10px black]
   expected: FAIL
 
@@ -91,11 +88,5 @@
 [corner-shape-render-precise.html?corner-shape=superellipse(-2)&border-top-left-radius=40%&border-width=20px]
   expected: FAIL
 
-[corner-shape-render-precise.html?corner-top-left-shape=bevel&border-width=10px]
-  expected: FAIL
-
 [corner-shape-render-precise.html?corner-shape=notch&border-radius=30px&border-width=30px]
   expected: FAIL
-
-[corner-shape-render-precise.html?corner-top-right-shape=bevel&border-width=10px]
-  expected: FAIL