src: do not track weak BaseObjects as childrens of Realms

Heap dumps are used for analyzing the relationship between
objects that keep each other alive, so that retainers of
memory can be detected and memory leaks fixed.

6e60ab7 broke this for a wide range of `BaseObject`
instances. Marking all `BaseObject`s, including weak ones,
as children of `Realm` and `Environment` instances gives
the incorrect impression that they are held alive by those
objects, effectively hiding the "real" set of strong roots
that keep other objects alive through GC.

An upcoming change in V8 will allow us to represent this
relationship between `Realm` and `Environment` and its
`BaseObject` and `CppgcMixin` instances properly.

Refs: v8/v8@e37cadf
Refs: #57417
Signed-off-by: Anna Henningsen <anna@addaleax.net>
PR-URL: #63842
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

Author: addaleax

src/base_object.cc

@@ -174,9 +174,15 @@ void BaseObjectList::Cleanup() {
   }
 }
 
-void BaseObjectList::MemoryInfo(node::MemoryTracker* tracker) const {
+void BaseObjectList::MemoryInfo(MemoryTracker* tracker) const {
   for (auto bo : *this) {
-    if (bo->IsDoneInitializing()) tracker->Track(bo);
+    if (bo->IsDoneInitializing()) {
+      // TODO(addaleax): Add weak edges instead of no edges once
+      // https://github.com/v8/v8/commit/e37cadf1143a8c5bbe44c0408186b5a26cc23863
+      // is available for us
+      tracker->Track(
+          bo, bo->persistent().IsWeak() ? MemoryTracker::kWeakEdge : nullptr);
+    }
   }
 }
 

src/cppgc_helpers.cc

@@ -16,7 +16,10 @@ void CppgcWrapperList::MemoryInfo(MemoryTracker* tracker) const {
   for (auto node : *this) {
     CppgcMixin* ptr = node->persistent.Get();
     if (ptr != nullptr) {
-      tracker->Track(ptr);
+      // TODO(addaleax): Add weak edges instead of no edges once
+      // https://github.com/v8/v8/commit/e37cadf1143a8c5bbe44c0408186b5a26cc23863
+      // is available for us
+      tracker->Track(ptr, MemoryTracker::kWeakEdge);
     }
   }
 }

test/common/heap.js

@@ -336,11 +336,25 @@ function validateByRetainingPath(...args) {
   return validateByRetainingPathFromNodes(nodes, ...args);
 }
 
+function getRetainingNodes(startingNode, filter) {
+  const seen = new Set();
+  function listNodes(node) {
+    if (!filter(node) || seen.has(node)) return;
+    seen.add(node);
+    for (const edge of node.incomingEdges) {
+      listNodes(edge.from);
+    }
+  }
+  listNodes(startingNode);
+  return [...seen];
+}
+
 module.exports = {
   recordState,
   validateSnapshotNodes,
   validateByRetainingPath,
   validateByRetainingPathFromNodes,
   getHeapSnapshotOptionTests,
   createJSHeapSnapshot,
+  getRetainingNodes,
 };

test/pummel/test-heapdump-zlib.js

@@ -3,7 +3,7 @@
 
 const common = require('../common');
 const assert = require('assert');
-const { validateByRetainingPath, validateByRetainingPathFromNodes } = require('../common/heap');
+const { validateByRetainingPath, validateByRetainingPathFromNodes, getRetainingNodes } = require('../common/heap');
 const zlib = require('zlib');
 
 // Before zlib stream is created, no ZlibStream should be created.
@@ -27,6 +27,18 @@ const gzip = zlib.createGzip();
   assert.strictEqual(withMemory.length, 0);
 }
 
+{
+  // Assert that the `ZlibStream` has no unexpected connections
+  // to other `Node / ...` nodes.
+  const contexts = validateByRetainingPath('Node / ZlibContext', []);
+  assert.deepStrictEqual(
+    getRetainingNodes(contexts[0], (node) => node.name?.startsWith('Node /'))
+      .map((node) => node.name).sort(), [
+      'Node / ZlibContext',
+      'Node / ZlibStream',
+    ]);
+}
+
 // After zlib stream is written, zlib_memory should be created.
 gzip.write('hello world', common.mustCall(() => {
   validateByRetainingPath('Node / ZlibStream', [