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

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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

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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)]

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);
     }