vm: fix leak in vm.compileFunction when importModuleDynamically is used

Previously in the implementation there was a cycle that V8 could not
detect:

Strong global reference to CompiledFnEntry (JS wrapper)
    -> strong reference to callback setting (through the
       callbackMap key-value pair)
    -> importModuleDynamically (wrapper in internalCompileFunction())
    -> Strong reference to the compiled function (through closure in
       internalCompileFunction())

The CompiledFnEntry only gets GC'ed when the compiled function is GC'ed.
Since the compiled function is always reachable as described above,
there is a leak.

We only needed the first strong global reference because we didn't want
the function to outlive the CompiledFnEntry. In this case it can be
solved by using a private symbol instead of going with the global
reference + destruction in the weak callback, which V8's GC is not
going to understand.

Author: joyeecheung

src/env_properties.h

@@ -20,6 +20,7 @@
 #define PER_ISOLATE_PRIVATE_SYMBOL_PROPERTIES(V)                               \
   V(arrow_message_private_symbol, "node:arrowMessage")                         \
   V(contextify_context_private_symbol, "node:contextify:context")              \
+  V(compiled_function_entry, "node:compiled_function_entry")                   \
   V(decorated_private_symbol, "node:decorated")                                \
   V(napi_type_tag, "node:napi:type_tag")                                       \
   V(napi_wrapper, "node:napi:wrapper")                                         \

src/node_contextify.cc

@@ -77,7 +77,6 @@ using v8::Uint32;
 using v8::UnboundScript;
 using v8::Value;
 using v8::WeakCallbackInfo;
-using v8::WeakCallbackType;
 
 // The vm module executes code in a sandboxed environment with a different
 // global object than the rest of the code. This is achieved by applying
@@ -1263,8 +1262,7 @@ void ContextifyContext::CompileFunction(
            context).ToLocal(&cache_key)) {
     return;
   }
-  CompiledFnEntry* entry = new CompiledFnEntry(env, cache_key, id, fn);
-  env->id_to_function_map.emplace(id, entry);
+  new CompiledFnEntry(env, cache_key, id, fn);
 
   Local<Object> result = Object::New(isolate);
   if (result->Set(parsing_context, env->function_string(), fn).IsNothing())
@@ -1296,23 +1294,18 @@ void ContextifyContext::CompileFunction(
   args.GetReturnValue().Set(result);
 }
 
-void CompiledFnEntry::WeakCallback(
-    const WeakCallbackInfo<CompiledFnEntry>& data) {
-  CompiledFnEntry* entry = data.GetParameter();
-  delete entry;
-}
-
 CompiledFnEntry::CompiledFnEntry(Environment* env,
                                  Local<Object> object,
                                  uint32_t id,
                                  Local<Function> fn)
-    : BaseObject(env, object), id_(id), fn_(env->isolate(), fn) {
-  fn_.SetWeak(this, WeakCallback, v8::WeakCallbackType::kParameter);
+    : BaseObject(env, object), id_(id) {
+  MakeWeak();
+  fn->SetPrivate(env->context(), env->compiled_function_entry(), object);
+  env->id_to_function_map.emplace(id, this);
 }
 
 CompiledFnEntry::~CompiledFnEntry() {
   env()->id_to_function_map.erase(id_);
-  fn_.ClearWeak();
 }
 
 static void StartSigintWatchdog(const FunctionCallbackInfo<Value>& args) {

src/node_contextify.h

@@ -194,9 +194,6 @@ class CompiledFnEntry final : public BaseObject {
 
  private:
   uint32_t id_;
-  v8::Global<v8::Function> fn_;
-
-  static void WeakCallback(const v8::WeakCallbackInfo<CompiledFnEntry>& data);
 };
 
 v8::Maybe<bool> StoreCodeCacheResult(

test/pummel/test-vm-compile-function-leak.js

@@ -0,0 +1,14 @@
+'use strict';
+
+// Flags: --max-old-space-size=10
+
+require('../common');
+const vm = require('vm');
+
+const code = `console.log("${'hello world '.repeat(1e5)}");`;
+
+for (let i = 0; i < 10000; i++) {
+  vm.compileFunction(code, [], {
+    importModuleDynamically: () => {},
+  });
+}