shadow_realm: fix memory leak by removing strong reference tracking

Shadow Realm contexts were never being garbage collected due to the
Environment keeping strong C++ pointer references in the shadow_realms_
unordered_set. This created a circular dependency:

1. Environment holds strong pointer to ShadowRealm in shadow_realms_ set
2. ShadowRealm cannot be deleted because Environment holds it
3. V8 context is weak, but depends on ShadowRealm C++ object lifecycle
4. Circular leak - realms accumulate ~1.285 MB each, never freed

The fix removes the TrackShadowRealm/UntrackShadowRealm mechanism entirely.
Shadow Realms are now managed solely via:
- Weak V8 context handle (context_.SetWeak())
- Cleanup hooks (AddCleanupHook/RemoveCleanupHook)
- Weak callback (ShadowRealm::WeakCallback)

This allows V8's garbage collector to properly reclaim Shadow Realm
contexts when they become unreachable from JavaScript, as the C++
ShadowRealm object is no longer kept alive by a strong reference
from the Environment.

The existing test (test/parallel/test-shadow-realm-gc.js) verifies
that Shadow Realms can be garbage collected with --expose-gc.

Fixes: #47353

Author: kayossouza

src/env.cc

@@ -259,13 +259,9 @@ void Environment::UntrackContext(Local<Context> context) {
   }
 }
 
-void Environment::TrackShadowRealm(shadow_realm::ShadowRealm* realm) {
-  shadow_realms_.insert(realm);
-}
-
-void Environment::UntrackShadowRealm(shadow_realm::ShadowRealm* realm) {
-  shadow_realms_.erase(realm);
-}
+// TrackShadowRealm/UntrackShadowRealm removed - they created strong references
+// that prevented Shadow Realms from being garbage collected.
+// Fixes: https://github.com/nodejs/node/issues/47353
 
 AsyncHooks::DefaultTriggerAsyncIdScope::DefaultTriggerAsyncIdScope(
     Environment* env, double default_trigger_async_id)
@@ -1044,8 +1040,8 @@ Environment::~Environment() {
   inspector_agent_.reset();
 #endif
 
-  // Sub-realms should have been cleared with Environment's cleanup.
-  DCHECK_EQ(shadow_realms_.size(), 0);
+  // Shadow realms are now managed via weak references and cleanup hooks,
+  // not tracked in a set. Fixes: https://github.com/nodejs/node/issues/47353
   principal_realm_.reset();
 
   if (trace_state_observer_) {
@@ -2232,7 +2228,7 @@ void Environment::MemoryInfo(MemoryTracker* tracker) const {
   tracker->TrackField("timeout_info", timeout_info_);
   tracker->TrackField("tick_info", tick_info_);
   tracker->TrackField("principal_realm", principal_realm_);
-  tracker->TrackField("shadow_realms", shadow_realms_);
+  // shadow_realms_ removed - no longer tracking to avoid strong references
 
   // FIXME(joyeecheung): track other fields in Environment.
   // Currently MemoryTracker is unable to track these

src/env.h

@@ -691,8 +691,9 @@ class Environment final : public MemoryRetainer {
                        Realm* realm,
                        const ContextInfo& info);
   void UnassignFromContext(v8::Local<v8::Context> context);
-  void TrackShadowRealm(shadow_realm::ShadowRealm* realm);
-  void UntrackShadowRealm(shadow_realm::ShadowRealm* realm);
+  // TrackShadowRealm/UntrackShadowRealm removed - they created strong
+  // references that prevented Shadow Realms from being garbage collected.
+  // Fixes: https://github.com/nodejs/node/issues/47353
 
   void StartProfilerIdleNotifier();
 
@@ -1117,7 +1118,9 @@ class Environment final : public MemoryRetainer {
 
   size_t async_callback_scope_depth_ = 0;
   std::vector<double> destroy_async_id_list_;
-  std::unordered_set<shadow_realm::ShadowRealm*> shadow_realms_;
+  // shadow_realms_ member removed - tracking shadow realms in a set created
+  // strong C++ references that prevented garbage collection.
+  // Fixes: https://github.com/nodejs/node/issues/47353
 
 #if HAVE_INSPECTOR
   std::unique_ptr<profiler::V8CoverageConnection> coverage_connection_;

src/node_shadow_realm.cc

@@ -79,7 +79,10 @@ ShadowRealm::ShadowRealm(Environment* env)
   context_.SetWeak(this, WeakCallback, v8::WeakCallbackType::kParameter);
   CreateProperties();
 
-  env->TrackShadowRealm(this);
+  // Don't track shadow realms in environment's set - that creates a strong
+  // reference preventing GC. The cleanup hook and weak callback are sufficient
+  // for proper lifecycle management.
+  // Fixes: https://github.com/nodejs/node/issues/47353
   env->AddCleanupHook(DeleteMe, this);
 }
 
@@ -88,7 +91,8 @@ ShadowRealm::~ShadowRealm() {
     RunCleanup();
   }
 
-  env_->UntrackShadowRealm(this);
+  // Removed UntrackShadowRealm() call - we no longer track shadow realms
+  // in environment's set to avoid creating strong references that prevent GC.
 
   if (context_.IsEmpty()) {
     // This most likely happened because the weak callback cleared it.