script: Unconditionally send exit message during pipeline shutdown (#36808)

If a `WebView` is dropped immediately after creating it, the exit
pipeline message can arrive to the `ScriptThread` before the `Document`
is created for the pipeline. If this happens, we should still send a
message to the `Constellation` informing it that the pipeline is closed,
otherwise it will never know that this has happened properly.

Testing: This change includes a new unit test.
Fixes: #36807.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/script_thread.rs

@@ -2813,8 +2813,6 @@ impl ScriptThread {
     ) {
         debug!("{id}: Starting pipeline exit.");
 
-        self.closed_pipelines.borrow_mut().insert(id);
-
         // Abort the parser, if any,
         // to prevent any further incoming networking messages from being handled.
         let document = self.documents.borrow_mut().remove(id);
@@ -2835,12 +2833,6 @@ impl ScriptThread {
             debug!("{id}: Shutting down layout");
             document.window().layout_mut().exit_now();
 
-            debug!("{id}: Sending PipelineExited message to constellation");
-            self.senders
-                .pipeline_to_constellation_sender
-                .send((id, ScriptToConstellationMessage::PipelineExited))
-                .ok();
-
             // Clear any active animations and unroot all of the associated DOM objects.
             debug!("{id}: Clearing animations");
             document.animations().clear();
@@ -2863,6 +2855,15 @@ impl ScriptThread {
             window.clear_js_runtime();
         }
 
+        // Prevent any further work for this Pipeline.
+        self.closed_pipelines.borrow_mut().insert(id);
+
+        debug!("{id}: Sending PipelineExited message to constellation");
+        self.senders
+            .pipeline_to_constellation_sender
+            .send((id, ScriptToConstellationMessage::PipelineExited))
+            .ok();
+
         debug!("{id}: Finished pipeline exit");
     }
 

components/servo/tests/webview.rs

@@ -44,6 +44,18 @@ fn test_create_webview(servo_test: &ServoTest) -> Result<(), anyhow::Error> {
     Ok(())
 }
 
-fn main() {
+fn test_create_webview_and_immediately_drop_webview_before_shutdown(
-    run_api_tests!(test_create_webview);
+    servo_test: &ServoTest,
+) -> Result<(), anyhow::Error> {
+    WebViewBuilder::new(servo_test.servo()).build();
+    Ok(())
+}
+
+fn main() {
+    run_api_tests!(
+        test_create_webview,
+        // This test needs to be last, as it tests creating and dropping
+        // a WebView right before shutdown.
+        test_create_webview_and_immediately_drop_webview_before_shutdown
+    );
 }