Bug 1597999 - Implements ChromeUtils.getObjectNodeId. r=jimb

This helps retrieving the NodeId for any JS Object you pass in.
This allows identifying a particular object when using HeapSnapshot API.
`HeapSnapshot.computeDominatorTree()` returns a tree of object,
but there is no way to identify a given JS object in it.
Getting the node id of a given JS object helps you browse this tree
from a given object you know is leaking.

Differential Revision: https://phabricator.services.mozilla.com/D53971

--HG--
extra : moz-landing-system : lando

devtools/shared/heapsnapshot/HeapSnapshot.cpp

@@ -1504,6 +1504,15 @@ void ChromeUtils::SaveHeapSnapshotShared(
                         edgeCount);
 }
 
+/* static */
+uint64_t ChromeUtils::GetObjectNodeId(GlobalObject& global,
+                                      JS::HandleObject val) {
+  JS::RootedObject obj(global.Context(), val);
+
+  JS::ubi::Node node(obj);
+  return node.identifier();
+}
+
 /* static */
 void ChromeUtils::SaveHeapSnapshot(GlobalObject& global,
                                    const HeapSnapshotBoundaries& boundaries,

devtools/shared/heapsnapshot/tests/unit/test_HeapSnapshot_getObjectNodeId_01.js

@@ -0,0 +1,57 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+"use strict";
+
+// Test ChromeUtils.getObjectNodeId()
+
+function run_test() {
+  // Create a test object, which we want to analyse
+  const testObject = {
+    foo: {
+      bar: {},
+    },
+  };
+
+  const path = ChromeUtils.saveHeapSnapshot({ runtime: true });
+  const snapshot = ChromeUtils.readHeapSnapshot(path);
+
+  // Get the NodeId for our test object
+  const objectNodeIdRoot = ChromeUtils.getObjectNodeId(testObject);
+  const objectNodeIdFoo = ChromeUtils.getObjectNodeId(testObject.foo);
+  const objectNodeIdBar = ChromeUtils.getObjectNodeId(testObject.foo.bar);
+
+  // Also try to ensure that this is the right object via its retained path
+  const shortestPaths = snapshot.computeShortestPaths(
+    objectNodeIdRoot,
+    [objectNodeIdBar],
+    50
+  );
+  ok(shortestPaths);
+  ok(shortestPaths instanceof Map);
+  ok(
+    shortestPaths.size == 1,
+    "We get only one path between the root object and bar object"
+  );
+
+  const paths = shortestPaths.get(objectNodeIdBar);
+  ok(paths.length == 1, "There is only one path between root and bar");
+  ok(
+    paths[0].length == 2,
+    "The shortest path is made of two edges: foo and bar"
+  );
+
+  const [path1, path2] = paths[0];
+  ok(
+    path1.predecessor == objectNodeIdRoot,
+    "The first edge goes from the root object"
+  );
+  ok(path1.edge == "foo", "The first edge is the foo attribute");
+
+  ok(
+    path2.predecessor == objectNodeIdFoo,
+    "The second edge goes from the foo object"
+  );
+  ok(path2.edge == "bar", "The first edge is the bar attribute");
+
+  do_test_finished();
+}

devtools/shared/heapsnapshot/tests/unit/xpcshell.ini

@@ -76,6 +76,7 @@ support-files =
 [test_HeapSnapshot_describeNode_01.js]
 [test_HeapSnapshot_computeShortestPaths_01.js]
 [test_HeapSnapshot_computeShortestPaths_02.js]
+[test_HeapSnapshot_getObjectNodeId_01.js]
 [test_HeapSnapshot_takeCensus_01.js]
 [test_HeapSnapshot_takeCensus_02.js]
 [test_HeapSnapshot_takeCensus_03.js]

dom/base/ChromeUtils.h

@@ -38,6 +38,9 @@ class ChromeUtils {
                                      ErrorResult& rv);
 
  public:
+  // Implemented in devtools/shared/heapsnapshot/HeapSnapshot.cpp
+  static uint64_t GetObjectNodeId(GlobalObject& global, JS::HandleObject aVal);
+
   // Implemented in devtools/shared/heapsnapshot/HeapSnapshot.cpp
   static void SaveHeapSnapshot(GlobalObject& global,
                                const HeapSnapshotBoundaries& boundaries,

dom/chrome-webidl/ChromeUtils.webidl

@@ -27,6 +27,12 @@ interface MozQueryInterface {
  */
 [ChromeOnly, Exposed=(Window,Worker)]
 namespace ChromeUtils {
+  /**
+   * Get the |NodeId| for the given JS Object.
+   * |NodeId| is the identifier of |JS::ubi::Node|.
+   */
+  NodeId getObjectNodeId(object obj);
+
   /**
    * Serialize a snapshot of the heap graph, as seen by |JS::ubi::Node| and
    * restricted by |boundaries|, and write it to the provided file path.