servo: Merge #16861 - Use microtasks to await a stable state (from gterzian:use_microtask_to_await_stable_state); r=jdm

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@jdm @KiChjang First pass at using microtasks to await a stable state. I ran into all sorts of problems to get it to compile, I think it's mainly related to the fact that the microtasks are stored in a `Vec`, which meant the `Runnalbe.handler(self: Box<Self>)` couldn't be called while iterating over the Vec... It compiles now although I haven't run any tests. I'm assuming I'm missing something fundamental and was hoping my changes would highlight the problems I run into, and you had a better idea of how to implement this... Perhaps we shouldn't pass a `Runnable` to `await_stable_state` at all?

---
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15375 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: f05491166f21879f74758b2f03bbc4c4a4c31eb8

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : efc9208e1f0fcd946247ef753d8b7c04105e5b85

servo/components/script/dom/htmlmediaelement.rs

@@ -31,6 +31,7 @@ use dom_struct::dom_struct;
 use html5ever::{LocalName, Prefix};
 use ipc_channel::ipc;
 use ipc_channel::router::ROUTER;
+use microtask::{Microtask, MicrotaskRunnable};
 use net_traits::{FetchResponseListener, FetchMetadata, Metadata, NetworkError};
 use net_traits::request::{CredentialsMode, Destination, RequestInit, Type as RequestType};
 use network_listener::{NetworkListener, PreInvoke};
@@ -429,7 +430,11 @@ impl HTMLMediaElement {
 
         // Step 4
         let doc = document_from_node(self);
-        ScriptThread::await_stable_state(ResourceSelectionTask::new(self, doc.base_url()));
+        let task = MediaElementMicrotask::ResourceSelectionTask {
+            elem: Root::from_ref(self),
+            base_url: doc.base_url()
+        };
+        ScriptThread::await_stable_state(Microtask::MediaElement(task));
     }
 
     // https://html.spec.whatwg.org/multipage/#concept-media-load-algorithm
@@ -781,7 +786,36 @@ impl VirtualMethods for HTMLMediaElement {
         self.super_type().unwrap().unbind_from_tree(context);
 
         if context.tree_in_doc {
-            ScriptThread::await_stable_state(PauseIfNotInDocumentTask::new(self));
+            let task = MediaElementMicrotask::PauseIfNotInDocumentTask {
+                elem: Root::from_ref(self)
+            };
+            ScriptThread::await_stable_state(Microtask::MediaElement(task));
+        }
+    }
+}
+
+#[derive(JSTraceable, HeapSizeOf)]
+pub enum MediaElementMicrotask {
+    ResourceSelectionTask {
+        elem: Root<HTMLMediaElement>,
+        base_url: ServoUrl
+    },
+    PauseIfNotInDocumentTask {
+        elem: Root<HTMLMediaElement>,
+    }
+}
+
+impl MicrotaskRunnable for MediaElementMicrotask {
+    fn handler(&self) {
+        match self {
+            &MediaElementMicrotask::ResourceSelectionTask { ref elem, ref base_url } => {
+                elem.resource_selection_algorithm_sync(base_url.clone());
+            },
+            &MediaElementMicrotask::PauseIfNotInDocumentTask { ref elem } => {
+                if !elem.upcast::<Node>().is_in_doc() {
+                    elem.internal_pause_steps();
+                }
+            },
         }
     }
 }
@@ -809,28 +843,6 @@ impl Runnable for FireSimpleEventTask {
     }
 }
 
-struct ResourceSelectionTask {
-    elem: Trusted<HTMLMediaElement>,
-    base_url: ServoUrl,
-}
-
-impl ResourceSelectionTask {
-    fn new(elem: &HTMLMediaElement, url: ServoUrl) -> ResourceSelectionTask {
-        ResourceSelectionTask {
-            elem: Trusted::new(elem),
-            base_url: url,
-        }
-    }
-}
-
-impl Runnable for ResourceSelectionTask {
-    fn name(&self) -> &'static str { "ResourceSelectionTask" }
-
-    fn handler(self: Box<ResourceSelectionTask>) {
-        self.elem.root().resource_selection_algorithm_sync(self.base_url);
-    }
-}
-
 struct DedicatedMediaSourceFailureTask {
     elem: Trusted<HTMLMediaElement>,
 }
@@ -851,29 +863,6 @@ impl Runnable for DedicatedMediaSourceFailureTask {
     }
 }
 
-struct PauseIfNotInDocumentTask {
-    elem: Trusted<HTMLMediaElement>,
-}
-
-impl PauseIfNotInDocumentTask {
-    fn new(elem: &HTMLMediaElement) -> PauseIfNotInDocumentTask {
-        PauseIfNotInDocumentTask {
-            elem: Trusted::new(elem),
-        }
-    }
-}
-
-impl Runnable for PauseIfNotInDocumentTask {
-    fn name(&self) -> &'static str { "PauseIfNotInDocumentTask" }
-
-    fn handler(self: Box<PauseIfNotInDocumentTask>) {
-        let elem = self.elem.root();
-        if !elem.upcast::<Node>().is_in_doc() {
-            elem.internal_pause_steps();
-        }
-    }
-}
-
 enum ResourceSelectionMode {
     Object,
     Attribute(String),

servo/components/script/microtask.rs

@@ -11,6 +11,7 @@ use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::PromiseBinding::PromiseJobCallback;
 use dom::bindings::js::Root;
 use dom::globalscope::GlobalScope;
+use dom::htmlmediaelement::MediaElementMicrotask;
 use dom::mutationobserver::MutationObserver;
 use msg::constellation_msg::PipelineId;
 use std::cell::Cell;
@@ -29,9 +30,14 @@ pub struct MicrotaskQueue {
 #[derive(JSTraceable, HeapSizeOf)]
 pub enum Microtask {
     Promise(EnqueuedPromiseCallback),
+    MediaElement(MediaElementMicrotask),
     NotifyMutationObservers,
 }
 
+pub trait MicrotaskRunnable {
+    fn handler(&self) {}
+}
+
 /// A promise callback scheduled to run during the next microtask checkpoint (#4283).
 #[derive(JSTraceable, HeapSizeOf)]
 pub struct EnqueuedPromiseCallback {
@@ -72,6 +78,9 @@ impl MicrotaskQueue {
                         if let Some(target) = target_provider(job.pipeline) {
                             let _ = job.callback.Call_(&*target, ExceptionHandling::Report);
                         }
+                    },
+                    Microtask::MediaElement(ref task) => {
+                        task.handler();
                     }
                     Microtask::NotifyMutationObservers => {
                         MutationObserver::notify_mutation_observers();

servo/components/script/script_thread.rs

@@ -670,14 +670,11 @@ impl ScriptThread {
     }
 
     // https://html.spec.whatwg.org/multipage/#await-a-stable-state
-    pub fn await_stable_state<T: Runnable + Send + 'static>(task: T) {
-        //TODO use microtasks when they exist
+    pub fn await_stable_state(task: Microtask) {
         SCRIPT_THREAD_ROOT.with(|root| {
             if let Some(script_thread) = root.get() {
                 let script_thread = unsafe { &*script_thread };
-                let _ = script_thread.chan.send(CommonScriptMsg::RunnableMsg(
-                    ScriptThreadEventCategory::DomEvent,
-                    box task));
+                script_thread.microtask_queue.enqueue(task);
             }
         });
     }