Add teardown phase to the start of each layout operation that manually breaks cycles between flows, boxes, fonts, shapers, and textruns on stale layout data.

src/components/servo-gfx/font.rs

@@ -177,6 +177,10 @@ pub impl FontGroup {
         }
     }
 
+    fn teardown(&mut self) {
+        self.fonts = ~[];
+    }
+
     fn create_textrun(&self, text: ~str) -> TextRun {
         assert!(self.fonts.len() > 0);
 
@@ -283,6 +287,11 @@ pub impl Font {
         return result;
     }
 
+    fn teardown(&mut self) {
+        self.shaper = None;
+        self.azure_font = None;
+    }
+
     // TODO: this should return a borrowed pointer, but I can't figure
     // out why borrowck doesn't like my implementation.
 

src/components/servo-gfx/text/text_run.rs

@@ -51,6 +51,10 @@ pub impl<'self> TextRun {
         return run;
     }
 
+    fn teardown(&self) {
+        self.font.teardown();
+    }
+
     fn compute_potential_breaks(text: &str, glyphs: &mut GlyphStore) {
         // TODO(Issue #230): do a better job. See Gecko's LineBreaker.
 

src/components/servo/layout/aux.rs

@@ -18,6 +18,14 @@ impl LayoutAuxMethods for AbstractNode {
     /// box in the COW model) and populates it with an empty style object.
     fn initialize_layout_data(self) -> Option<@mut LayoutData> {
         if self.has_layout_data() {
+            {
+                let layout_data = &mut self.layout_data().flow;
+                match *layout_data {
+                  Some(ref flow) => flow.teardown(),
+                  None => ()
+                }
+            }
+            self.layout_data().flow = None;
             None
         } else {
             let data = @mut LayoutData::new();

src/components/servo/layout/block.rs

@@ -45,6 +45,14 @@ impl BlockFlowData {
             is_root: true
         }
     }
+
+    pub fn teardown(&mut self) {
+        self.common.teardown();
+        for self.box.each |box| {
+            box.teardown();
+        }
+        self.box = None;
+    }
 }
 
 pub trait BlockLayout {

src/components/servo/layout/box.rs

@@ -59,6 +59,15 @@ pub enum RenderBox {
     UnscannedTextRenderBoxClass(@mut UnscannedTextRenderBox),
 }
 
+impl RenderBox {
+    pub fn teardown(&self) {
+        match *self {
+            TextRenderBoxClass(box) => box.teardown(),
+            _ => ()
+        }
+    }
+}
+
 /// A box that represents a (replaced content) image and its accompanying borders, shadows, etc.
 pub struct ImageRenderBox {
     base: RenderBoxBase,
@@ -88,6 +97,12 @@ pub struct TextRenderBox {
     text_data: TextBoxData,
 }
 
+impl TextRenderBox {
+    fn teardown(&self) {
+        self.text_data.teardown();
+    }
+}
+
 /// The data for an unscanned text box.
 pub struct UnscannedTextRenderBox {
     base: RenderBoxBase,

src/components/servo/layout/flow.rs

@@ -67,6 +67,43 @@ impl Clone for FlowContext {
     }
 }
 
+impl FlowContext {
+    pub fn teardown(&self) {
+        match *self {
+          AbsoluteFlow(data) |
+          FloatFlow(data) |
+          InlineBlockFlow(data) |
+          TableFlow(data) => data.teardown(),
+          BlockFlow(data) => data.teardown(),
+          InlineFlow(data) => data.teardown()
+        }
+    }
+}
+
+impl FlowData {
+    pub fn teardown(&mut self) {
+        // Under the assumption that all flows exist in a tree,
+        // we must restrict ourselves to finalizing flows that
+        // are descendents and subsequent siblings to ourselves,
+        // or we risk dynamic borrow failures.
+        self.parent = None;
+
+        for self.first_child.each |flow| {
+            flow.teardown();
+        }
+        self.first_child = None;
+
+        self.last_child = None;
+
+        for self.next_sibling.each |flow| {
+            flow.teardown();
+        }
+        self.next_sibling = None;
+
+        self.prev_sibling = None;
+    }
+}
+
 impl TreeNodeRef<FlowData> for FlowContext {
     fn with_base<R>(&self, callback: &fn(&FlowData) -> R) -> R {
         match *self {

src/components/servo/layout/inline.rs

@@ -638,6 +638,14 @@ impl InlineFlowData {
             elems: ElementMapping::new(),
         }
     }
+
+    pub fn teardown(&mut self) {
+        self.common.teardown();
+        for self.boxes.each |box| {
+            box.teardown();
+        }
+        self.boxes = ~[];
+    }
 }
 
 pub trait InlineLayout {

src/components/servo/layout/text.rs

@@ -21,6 +21,10 @@ impl TextBoxData {
             range: range,
         }
     }
+
+    pub fn teardown(&self) {
+        self.run.teardown();
+    }
 }
 
 pub fn adapt_textbox_with_range(mut base: RenderBoxBase, run: @TextRun, range: Range)