1 files changed, 177 insertions, 0 deletions
diff --git a/toolkit/modules/ContentDOMReference.sys.mjs b/toolkit/modules/ContentDOMReference.sys.mjs
new file mode 100644
index 0000000000..ef4896ff67
--- /dev/null
+++ b/toolkit/modules/ContentDOMReference.sys.mjs
@@ -0,0 +1,177 @@
+/* vim: set ts=2 sw=2 sts=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This module holds weak references to DOM elements that exist within the
+ * current content process, and converts them to a unique identifier that can be
+ * passed between processes. The identifer, if received by the same content process
+ * that issued it, can then be converted back into the DOM element (presuming the
+ * element hasn't had all of its other references dropped).
+ *
+ * The hope is that this module can eliminate the need for passing CPOW references
+ * between processes during runtime.
+ */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "finalizationService",
+  "@mozilla.org/toolkit/finalizationwitness;1",
+  "nsIFinalizationWitnessService"
+);
+
+/**
+ * @typedef {number} ElementID
+ * @typedef {Object} ElementIdentifier
+ */
+
+const FINALIZATION_TOPIC = "content-dom-reference-finalized";
+
+// A WeakMap which ties finalization witness objects to the lifetime of the DOM
+// nodes they're meant to witness. When the DOM node in the map key is
+// finalized, the WeakMap stops holding the finalization witness in its value
+// alive, which alerts our observer that the element has been destroyed.
+const finalizerRoots = new WeakMap();
+
+/**
+ * An identifier generated by ContentDOMReference is a unique pair of BrowsingContext
+ * ID and a numeric ID. gRegistry maps BrowsingContext's to an object with the following
+ * properties:
+ *
+ *   IDToElement:
+ *     A Map of IDs to WeakReference's to the elements they refer to.
+ *
+ *   elementToID:
+ *     A WeakMap from a DOM element to an ID that refers to it.
+ */
+var gRegistry = new WeakMap();
+
+export var ContentDOMReference = {
+  _init() {
+    Services.obs.addObserver(this, FINALIZATION_TOPIC);
+  },
+
+  observe(subject, topic, data) {
+    if (topic !== FINALIZATION_TOPIC) {
+      throw new Error("Unexpected observer topic");
+    }
+
+    let identifier = JSON.parse(data);
+    this._revoke(identifier);
+  },
+
+  /**
+   * Generate and return an identifier for a given DOM element.
+   *
+   * @param {Element} element The DOM element to generate the identifier for.
+   * @return {ElementIdentifier} The identifier for the DOM element that can be passed between
+   * processes as a message.
+   */
+  get(element) {
+    if (!element) {
+      throw new Error(
+        "Can't create a ContentDOMReference identifier for " +
+          "non-existant nodes."
+      );
+    }
+
+    let browsingContext = BrowsingContext.getFromWindow(element.ownerGlobal);
+    let mappings = gRegistry.get(browsingContext);
+    if (!mappings) {
+      mappings = {
+        IDToElement: new Map(),
+        elementToID: new WeakMap(),
+      };
+      gRegistry.set(browsingContext, mappings);
+    }
+
+    let id = mappings.elementToID.get(element);
+    if (id) {
+      // We already had this element registered, so return the pre-existing ID.
+      return { browsingContextId: browsingContext.id, id };
+    }
+
+    // We must be registering a new element at this point.
+    id = Math.random();
+    mappings.elementToID.set(element, id);
+    mappings.IDToElement.set(id, Cu.getWeakReference(element));
+
+    let identifier = { browsingContextId: browsingContext.id, id };
+
+    finalizerRoots.set(
+      element,
+      lazy.finalizationService.make(
+        FINALIZATION_TOPIC,
+        JSON.stringify(identifier)
+      )
+    );
+
+    return identifier;
+  },
+
+  /**
+   * Resolves an identifier back into the DOM Element that it was generated from.
+   *
+   * @param {ElementIdentifier} The identifier generated via ContentDOMReference.get for a
+   * DOM element.
+   * @return {Element} The DOM element that the identifier was generated for, or
+   * null if the element does not still exist.
+   */
+  resolve(identifier) {
+    let browsingContext = BrowsingContext.get(identifier.browsingContextId);
+    let { id } = identifier;
+    return this._resolveIDToElement(browsingContext, id);
+  },
+
+  /**
+   * Removes an identifier from the registry so that subsequent attempts
+   * to resolve it will result in null. This is done automatically when the
+   * target node is GCed.
+   *
+   * @param {ElementIdentifier} The identifier to revoke, issued by ContentDOMReference.get for
+   * a DOM element.
+   */
+  _revoke(identifier) {
+    let browsingContext = BrowsingContext.get(identifier.browsingContextId);
+    let { id } = identifier;
+
+    let mappings = gRegistry.get(browsingContext);
+    if (!mappings) {
+      return;
+    }
+
+    mappings.IDToElement.delete(id);
+  },
+
+  /**
+   * Private helper function that resolves a BrowsingContext and ID (the
+   * pair that makes up an identifier) to a DOM element.
+   *
+   * @param {BrowsingContext} browsingContext The BrowsingContext that was hosting
+   * the DOM element at the time that the identifier was generated.
+   * @param {ElementID} id The ID generated for the DOM element.
+   *
+   * @return {Element} The DOM element that the identifier was generated for, or
+   * null if the element does not still exist.
+   */
+  _resolveIDToElement(browsingContext, id) {
+    let mappings = gRegistry.get(browsingContext);
+    if (!mappings) {
+      return null;
+    }
+
+    let weakReference = mappings.IDToElement.get(id);
+    if (!weakReference) {
+      return null;
+    }
+
+    return weakReference.get();
+  },
+};
+
+ContentDOMReference._init();