Auto merge of #20740 - fabricedesre:mutation-observer-disconnect, r=jdm

Implement MutationObserver.disconnect()

<!-- Please describe your changes on the following line: -->
This implements https://dom.spec.whatwg.org/#dom-mutationobserver-disconnect
I added a `node_list` to the `MutationObserver` struct to keep track of the nodes involved in an observer because otherwise I could not find a way to unregister the observers on the node themselves without traversing the whole document tree. Let me know if there's something I missed.

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

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

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3 files changed, 25 insertions, 2 deletions

diff --git a/components/script/dom/mutationobserver.rs b/components/script/dom/mutationobserver.rs
index 97b4ca36d27..d0dc201f12e 100644
--- a/components/script/dom/mutationobserver.rs
+++ b/components/script/dom/mutationobserver.rs
@@ -27,6 +27,7 @@ pub struct MutationObserver {
     #[ignore_malloc_size_of = "can't measure Rc values"]
     callback: Rc<MutationCallback>,
     record_queue: DomRefCell<Vec<DomRoot<MutationRecord>>>,
+    node_list: DomRefCell<Vec<DomRoot<Node>>>,
 }
 
 pub enum Mutation<'a> {
@@ -38,7 +39,7 @@ pub enum Mutation<'a> {
 
 #[derive(JSTraceable, MallocSizeOf)]
 pub struct RegisteredObserver {
-    observer: DomRoot<MutationObserver>,
+    pub observer: DomRoot<MutationObserver>,
     options: ObserverOptions,
 }
 
@@ -64,6 +65,7 @@ impl MutationObserver {
             reflector_: Reflector::new(),
             callback: callback,
             record_queue: DomRefCell::new(vec![]),
+            node_list: DomRefCell::new(vec![]),
         }
     }
 
@@ -286,6 +288,8 @@ impl MutationObserverMethods for MutationObserver {
                     child_list
                 },
             });
+
+            self.node_list.borrow_mut().push(DomRoot::from_ref(target));
         }
 
         Ok(())
@@ -297,4 +301,16 @@ impl MutationObserverMethods for MutationObserver {
         self.record_queue.borrow_mut().clear();
         records
     }
+
+    /// https://dom.spec.whatwg.org/#dom-mutationobserver-disconnect
+    fn Disconnect(&self) {
+        // Step 1
+        let mut nodes = self.node_list.borrow_mut();
+        for node in nodes.drain(..) {
+            node.remove_mutation_observer(self);
+        }
+
+        // Step 2
+        self.record_queue.borrow_mut().clear();
+    }
 }
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 64b808c3859..edc1b6d8366 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -392,6 +392,13 @@ impl Node {
          self.mutation_observers.borrow_mut()
     }
 
+    /// Removes the mutation observer for a given node.
+    pub fn remove_mutation_observer(&self, observer: &MutationObserver) {
+        self.mutation_observers.borrow_mut().retain(|reg_obs| {
+            &*reg_obs.observer != observer
+        })
+    }
+
     /// Dumps the subtree rooted at this node, for debugging.
     pub fn dump(&self) {
         self.dump_indent(0);
diff --git a/components/script/dom/webidls/MutationObserver.webidl b/components/script/dom/webidls/MutationObserver.webidl
index 015686c47d9..426e995fc63 100644
--- a/components/script/dom/webidls/MutationObserver.webidl
+++ b/components/script/dom/webidls/MutationObserver.webidl
@@ -11,7 +11,7 @@
 interface MutationObserver {
     [Throws]
     void observe(Node target, optional MutationObserverInit options);
-    //void disconnect();
+    void disconnect();
     sequence<MutationRecord> takeRecords();
 };