Auto merge of #18514 - asajeffrey:layout-dont-panic-if-no-iframe-bc, r=emilio

Remove sources of panic when laying out an iframe without a nested browsing context

<!-- Please describe your changes on the following line: -->

At the moment, layout panics if it discovers an iframe without a nested browsing context. Under normal circumstances, this is reasonable, but it requires very tight synchronization between script, layout, the constellation and the compositor. In particular, if a layout is in progress when an iframe's browsing context is discarded, this can trigger panic.

This PR fixes this in two ways:

1. Making the pipeline and browsing context ids optional in layout's representation of an iframe.
2. Shutting down layout before discarding a browsing context, rather than after.

This is belt and braces.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17482 and #18477
- [X] These changes do not require tests because the PR is fixing a panic caused by a race condition

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18514)
<!-- Reviewable:end -->

6 files changed, 58 insertions, 30 deletions

diff --git a/components/layout/display_list_builder.rs b/components/layout/display_list_builder.rs
index 057966c1000..08b19d46a87 100644
--- a/components/layout/display_list_builder.rs
+++ b/components/layout/display_list_builder.rs
@@ -2022,6 +2022,15 @@ impl FragmentDisplayListBuilding for Fragment {
             }
             SpecificFragmentInfo::Iframe(ref fragment_info) => {
                 if !stacking_relative_content_box.is_empty() {
+                    let browsing_context_id = match fragment_info.browsing_context_id {
+                        Some(browsing_context_id) => browsing_context_id,
+                        None => return warn!("No browsing context id for iframe."),
+                    };
+                    let pipeline_id = match fragment_info.pipeline_id {
+                        Some(pipeline_id) => pipeline_id,
+                        None => return warn!("No pipeline id for iframe {}.", browsing_context_id),
+                    };
+
                     let base = state.create_base_display_item(
                         &stacking_relative_content_box,
                         build_local_clip(&self.style),
@@ -2030,12 +2039,12 @@ impl FragmentDisplayListBuilding for Fragment {
                         DisplayListSection::Content);
                     let item = DisplayItem::Iframe(box IframeDisplayItem {
                         base: base,
-                        iframe: fragment_info.pipeline_id,
+                        iframe: pipeline_id,
                     });
 
                     let size = Size2D::new(item.bounds().size.width.to_f32_px(),
                                            item.bounds().size.height.to_f32_px());
-                    state.iframe_sizes.push((fragment_info.browsing_context_id,
+                    state.iframe_sizes.push((browsing_context_id,
                                              TypedSize2D::from_untyped(&size)));
 
                     state.add_display_item(item);
diff --git a/components/layout/fragment.rs b/components/layout/fragment.rs
index 5dcfea6da53..c7ef0a74e14 100644
--- a/components/layout/fragment.rs
+++ b/components/layout/fragment.rs
@@ -488,10 +488,10 @@ impl ImageFragmentInfo {
 /// size of this iframe can be communicated via the constellation to the iframe's own layout thread.
 #[derive(Clone)]
 pub struct IframeFragmentInfo {
-    /// The frame ID of this iframe.
-    pub browsing_context_id: BrowsingContextId,
-    /// The pipelineID of this iframe.
-    pub pipeline_id: PipelineId,
+    /// The frame ID of this iframe. None if there is no nested browsing context.
+    pub browsing_context_id: Option<BrowsingContextId>,
+    /// The pipelineID of this iframe. None if there is no nested browsing context.
+    pub pipeline_id: Option<PipelineId>,
 }
 
 impl IframeFragmentInfo {
diff --git a/components/layout_thread/dom_wrapper.rs b/components/layout_thread/dom_wrapper.rs
index 962fe09a726..cd8e5173ffe 100644
--- a/components/layout_thread/dom_wrapper.rs
+++ b/components/layout_thread/dom_wrapper.rs
@@ -983,12 +983,14 @@ impl<'ln> ThreadSafeLayoutNode for ServoThreadSafeLayoutNode<'ln> {
         this.svg_data()
     }
 
-    fn iframe_browsing_context_id(&self) -> BrowsingContextId {
+    // Can return None if the iframe has no nested browsing context
+    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId> {
         let this = unsafe { self.get_jsmanaged() };
         this.iframe_browsing_context_id()
     }
 
-    fn iframe_pipeline_id(&self) -> PipelineId {
+    // Can return None if the iframe has no nested browsing context
+    fn iframe_pipeline_id(&self) -> Option<PipelineId> {
         let this = unsafe { self.get_jsmanaged() };
         this.iframe_pipeline_id()
     }
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 6b16cc242db..77e499a6f5a 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -1024,8 +1024,8 @@ pub trait LayoutNodeHelpers {
     fn image_url(&self) -> Option<ServoUrl>;
     fn canvas_data(&self) -> Option<HTMLCanvasData>;
     fn svg_data(&self) -> Option<SVGSVGData>;
-    fn iframe_browsing_context_id(&self) -> BrowsingContextId;
-    fn iframe_pipeline_id(&self) -> PipelineId;
+    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId>;
+    fn iframe_pipeline_id(&self) -> Option<PipelineId>;
     fn opaque(&self) -> OpaqueNode;
 }
 
@@ -1175,16 +1175,16 @@ impl LayoutNodeHelpers for LayoutJS<Node> {
             .map(|svg| svg.data())
     }
 
-    fn iframe_browsing_context_id(&self) -> BrowsingContextId {
+    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId> {
         let iframe_element = self.downcast::<HTMLIFrameElement>()
             .expect("not an iframe element!");
-        iframe_element.browsing_context_id().unwrap()
+        iframe_element.browsing_context_id()
     }
 
-    fn iframe_pipeline_id(&self) -> PipelineId {
+    fn iframe_pipeline_id(&self) -> Option<PipelineId> {
         let iframe_element = self.downcast::<HTMLIFrameElement>()
             .expect("not an iframe element!");
-        iframe_element.pipeline_id().unwrap()
+        iframe_element.pipeline_id()
     }
 
     #[allow(unsafe_code)]
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 2fa0071ed65..b9bcf5be7c8 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1750,10 +1750,35 @@ impl ScriptThread {
             load.pipeline_id == id
         });
 
+        let document = self.documents.borrow_mut().remove(id);
+
+        // We should never have a pipeline that's still an incomplete load,
+        // but also has a Document.
+        debug_assert!(idx.is_none() || document.is_none());
+
+        // Remove any incomplete load.
         let chan = if let Some(idx) = idx {
             let load = self.incomplete_loads.borrow_mut().remove(idx);
             load.layout_chan.clone()
-        } else if let Some(document) = self.documents.borrow_mut().remove(id) {
+        } else if let Some(ref document) = document {
+            document.window().layout_chan().clone()
+        } else {
+            return warn!("Exiting nonexistant pipeline {}.", id);
+        };
+
+        // We shut down layout before removing the document,
+        // since layout might still be in the middle of laying it out.
+        debug!("preparing to shut down layout for page {}", id);
+        let (response_chan, response_port) = channel();
+        chan.send(message::Msg::PrepareToExit(response_chan)).ok();
+        let _ = response_port.recv();
+
+        debug!("shutting down layout for page {}", id);
+        chan.send(message::Msg::ExitNow).ok();
+        self.script_sender.send((id, ScriptMsg::PipelineExited)).ok();
+
+        // Now that layout is shut down, it's OK to remove the document.
+        if let Some(document) = document {
             // We don't want to dispatch `mouseout` event pointing to non-existing element
             if let Some(target) = self.topmost_mouse_over_target.get() {
                 if target.upcast::<Node>().owner_doc() == document {
@@ -1761,22 +1786,14 @@ impl ScriptThread {
                 }
             }
 
+            // We discard the browsing context after requesting layout shut down,
+            // to avoid running layout on detached iframes.
             let window = document.window();
             if discard_bc == DiscardBrowsingContext::Yes {
                 window.window_proxy().discard_browsing_context();
             }
             window.clear_js_runtime();
-            window.layout_chan().clone()
-        } else {
-            return warn!("Exiting nonexistant pipeline {}.", id);
-        };
-
-        let (response_chan, response_port) = channel();
-        chan.send(message::Msg::PrepareToExit(response_chan)).ok();
-        debug!("shutting down layout for page {}", id);
-        let _ = response_port.recv();
-        chan.send(message::Msg::ExitNow).ok();
-        self.script_sender.send((id, ScriptMsg::PipelineExited)).ok();
+        }
 
         debug!("Exited pipeline {}.", id);
     }
diff --git a/components/script_layout_interface/wrapper_traits.rs b/components/script_layout_interface/wrapper_traits.rs
index c58b6292ef0..d022360305b 100644
--- a/components/script_layout_interface/wrapper_traits.rs
+++ b/components/script_layout_interface/wrapper_traits.rs
@@ -275,12 +275,12 @@ pub trait ThreadSafeLayoutNode: Clone + Copy + Debug + GetLayoutData + NodeInfo
     fn svg_data(&self) -> Option<SVGSVGData>;
 
     /// If this node is an iframe element, returns its browsing context ID. If this node is
-    /// not an iframe element, fails.
-    fn iframe_browsing_context_id(&self) -> BrowsingContextId;
+    /// not an iframe element, fails. Returns None if there is no nested browsing context.
+    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId>;
 
     /// If this node is an iframe element, returns its pipeline ID. If this node is
-    /// not an iframe element, fails.
-    fn iframe_pipeline_id(&self) -> PipelineId;
+    /// not an iframe element, fails. Returns None if there is no nested browsing context.
+    fn iframe_pipeline_id(&self) -> Option<PipelineId>;
 
     fn get_colspan(&self) -> u32;