lib: implement WeakReference on top of JS WeakRef

The C++ implementation can now be done entirely in JS using WeakRef.
Re-implement it in JS instead to simplify the code.

Author: joyeecheung

lib/diagnostics_channel.js

@@ -28,7 +28,7 @@ const {
 
 const { triggerUncaughtException } = internalBinding('errors');
 
-const { WeakReference } = internalBinding('util');
+const { WeakReference } = require('internal/util');
 
 // Can't delete when weakref count reaches 0 as it could increment again.
 // Only GC can be used as a valid time to clean up the channels map.

lib/domain.js

@@ -54,7 +54,7 @@ const { useDomainTrampoline } = require('internal/async_hooks');
 
 // TODO(addaleax): Use a non-internal solution for this.
 const kWeak = Symbol('kWeak');
-const { WeakReference } = internalBinding('util');
+const { WeakReference } = require('internal/util');
 
 // Overwrite process.domain with a getter/setter that will allow for more
 // effective optimizations

lib/internal/util.js

@@ -33,6 +33,7 @@ const {
   SafeMap,
   SafeSet,
   SafeWeakMap,
+  SafeWeakRef,
   StringPrototypeReplace,
   StringPrototypeToLowerCase,
   StringPrototypeToUpperCase,
@@ -797,6 +798,37 @@ function guessHandleType(fd) {
   return handleTypes[type];
 }
 
+class WeakReference {
+  constructor(object) {
+    this.weak = new SafeWeakRef(object);
+    this.strong = null;
+    this.refCount = 0;
+  }
+
+  incRef() {
+    this.refCount++;
+    if (this.refCount === 1) {
+      const derefed = this.weak.deref();
+      if (derefed !== undefined) {
+        this.strong = derefed;
+      }
+    }
+    return this.refCount;
+  }
+
+  decRef() {
+    this.refCount--;
+    if (this.refCount === 0) {
+      this.strong = null;
+    }
+    return this.refCount;
+  }
+
+  get() {
+    return this.weak.deref();
+  }
+}
+
 module.exports = {
   getLazy,
   assertCrypto,
@@ -855,4 +887,5 @@ module.exports = {
   kEnumerableProperty,
   setOwnProperty,
   pendingDeprecate,
+  WeakReference,
 };

test/fixtures/snapshot/weak-reference-gc.js

@@ -1,7 +1,6 @@
 'use strict';
 
-const { internalBinding } = require('internal/test/binding');
-const { WeakReference } = internalBinding('util');
+const { WeakReference } = require('internal/util');
 const {
   setDeserializeMainFunction
 } = require('v8').startupSnapshot

test/fixtures/snapshot/weak-reference.js

@@ -1,7 +1,6 @@
 'use strict';
 
-const { internalBinding } = require('internal/test/binding');
-const { WeakReference } = internalBinding('util');
+const { WeakReference } = require('internal/util');
 const {
   setDeserializeMainFunction
 } = require('v8').startupSnapshot

test/parallel/test-internal-util-weakreference.js

@@ -2,8 +2,7 @@
 'use strict';
 const common = require('../common');
 const assert = require('assert');
-const { internalBinding } = require('internal/test/binding');
-const { WeakReference } = internalBinding('util');
+const { WeakReference } = require('internal/util');
 
 let obj = { hello: 'world' };
 const ref = new WeakReference(obj);