Issue 258 - [Typed objects] Implement a garbage collection mechanism

lib/typedObjects/objectTypes.js

Index: lib/typedObjects/objectTypes.js
===================================================================
--- a/lib/typedObjects/objectTypes.js
+++ b/lib/typedObjects/objectTypes.js
@@ -14,16 +14,21 @@
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 "use strict";
 
 let {fixedPropertyDescriptor, getViewsForType, defineProperties} = require("typedObjects/utils");
 let {Reference, TypedReference} = require("typedObjects/references");
+let {uint8, uint32} = require("typedObjects/primitiveTypes");
+
+const STATE_UNINITIALIZED = 0;
+const STATE_CREATED = 1;
+const STATE_RELEASING = 2;
 
 /**
  * List of registered types (typeId is the index in that array).
  */
 let types = [];
 
 function fromReference(reference)
 {
@@ -78,21 +83,54 @@ function create()
     }
   }
 
   let result = Object.create(this.proto, {
     typeId: fixedPropertyDescriptor(this.typeId),
     bufferIndex: fixedPropertyDescriptor(bufferIndex),
     byteOffset: fixedPropertyDescriptor(byteOffset)
   });
+
+  result._state = STATE_UNINITIALIZED;
+  for (let [prop, value] of this.cleanupValues)
+    result[prop] = value;
+  result._state = STATE_CREATED;
+  result._refCount = 1;
+
   if (this.constructor)
     this.constructor.apply(result, arguments);
   return result;
 }
 
+function free(obj)
+{
+  try
+  {
+    if (this.destructor)
+    {
+      this.destructor.call(obj);
+      if (obj._refCount | 0)
+        throw new Error("Reference count is no longer zero after calling the destructor");
+    }
+  }
+  finally
+  {
+    for (let [prop, value] of this.cleanupValues)
+      obj[prop] = value;
+
+    // Mark object as first free spot
+    let oldFreeBufferIndex = this.firstFree.bufferIndex;
+    let oldFreeByteOffset = this.firstFree.byteOffset;
+    this.firstFree.bufferIndex = obj.bufferIndex;
+    this.firstFree.byteOffset = obj.byteOffset;
+    this.firstFree.targetBufferIndex = oldFreeBufferIndex;
+    this.firstFree.targetByteOffset = oldFreeByteOffset;
+  }
+}
+
 function createGetter(offset)
 {
   offset = offset | 0;
 
   let views = Array.prototype.slice.call(arguments, 1);
   let reference = new Reference(types, views);
   return function()
   {
@@ -111,21 +149,30 @@ function createSetter(typeId, offset)
   let reference = new Reference(types, views);
   return function(value)
   {
     if (value && !isinstance(typeId, value))
       throw new Error("Incompatible type");
 
     reference.bufferIndex = this.bufferIndex | 0;
     reference.byteOffset = (this.byteOffset | 0) + offset;
+
+    if ((this._state | 0) > STATE_UNINITIALIZED)
+    {
+      let oldValue = fromReference(reference);
+      if (oldValue)
+        oldValue.release();
+    }
+
     if (value)
     {
       reference.typeId = value.typeId;
       reference.targetBufferIndex = value.bufferIndex;
       reference.targetByteOffset = value.byteOffset;
+      value.retain();
     }
     else
       reference.typeId = -1;
   };
 }
 
 /**
  * Overridden methods get the respective method of the superclass as the first
@@ -193,58 +240,72 @@ function extend(parentTypeInfo, typeDesc
     else
       throw new Error("Unrecognized type " + type + " given for property " + name);
   }
 
   let proto = {};
   let buffers = [];
   let viewTypes = [];
   let views = [];
-  let byteLength = defineProperties(proto, properties, viewTypes, views, 0);
+  let cleanupValues = [];
+  let byteLength = defineProperties(proto, properties, viewTypes, views, 0, cleanupValues);
   Object.defineProperties(proto, methods);
 
   // Round up to be a multiple of the maximal property size
   byteLength = ((byteLength - 1) | (maxReferenceLength - 1)) + 1;
 
   // We need to be able to store a typed reference in the object's buffer
   byteLength = Math.max(byteLength, TypedReference.byteLength) | 0;
   let typedReferenceViews = getViewsForType(TypedReference, viewTypes, views);
 
-  // Take constructor from meta parameter, allow calling superclass constructor.
+  // Take constructor and destructor from meta parameters, allow calling
+  // superclass constructor/destructor.
   let constructor = parentTypeInfo && parentTypeInfo.constructor;
   if (meta.hasOwnProperty("constructor") && typeof meta.constructor == "function")
   {
     if (constructor)
       constructor = createSubclassMethod(meta.constructor, constructor);
     else
       constructor = meta.constructor;
   }
 
+  let destructor = parentTypeInfo && parentTypeInfo.destructor;
+  if (meta.hasOwnProperty("destructor") && typeof meta.destructor == "function")
+  {
+    if (destructor)
+      destructor = createSubclassMethod(meta.destructor, destructor);
+    else
+      destructor = meta.destructor;
+  }
+
   let typeId = types.length | 0;
   let typeInfo = {
     byteLength: byteLength,
     bufferSize: "bufferSize" in meta ? Math.max(meta.bufferSize | 0, 2) : 128,
     firstFree: new TypedReference(typeId, typedReferenceViews),
     proto: proto,
     properties: properties,
     methods: methods,
     buffers: buffers,
     viewTypes: viewTypes,
     views: views,
+    cleanupValues: cleanupValues,
     typeId: typeId,
     parentTypeInfo: parentTypeInfo,
-    constructor: constructor
+    constructor: constructor,
+    destructor: destructor
   };
 
   let result = create.bind(typeInfo);
   Object.defineProperties(result, {
     byteLength: fixedPropertyDescriptor(byteLength),
 
     referenceLength: fixedPropertyDescriptor(Reference.byteLength),
     viewTypes: fixedPropertyDescriptor(Reference.viewTypes),
+    cleanupValue: fixedPropertyDescriptor(null),
 
     typeId: fixedPropertyDescriptor(typeId),
     extend: fixedPropertyDescriptor(extend.bind(null, typeInfo)),
     isinstance: fixedPropertyDescriptor(isinstance.bind(null, typeId)),
 
     createGetter: fixedPropertyDescriptor(createGetter),
     createSetter: fixedPropertyDescriptor(createSetter.bind(null, typeId))
   });
@@ -264,17 +325,35 @@ function isinstance(typeId, obj)
     if ((typeInfo.typeId | 0) == typeId)
       return true;
     typeInfo = typeInfo.parentTypeInfo;
   }
   return false;
 }
 
 let ObjectBase = exports.ObjectBase = extend(null, {
+  _state: uint8,
+  _refCount: uint32,
+
   equals: function(obj)
   {
     if (!obj)
       return false;
     return this.typeId == obj.typeId && this.bufferIndex == obj.bufferIndex && this.byteOffset == obj.byteOffset;
+  },
+
+  retain: function()
+  {
+    this._refCount++;
+  },
+
+  release: function()
+  {
+    this._refCount--;
+    if (this._refCount == 0 && this._state < STATE_RELEASING)
+    {
+      this._state = STATE_RELEASING;
+      free.call(types[this.typeId | 0], this);
+    }
   }
 }, null);
 
 exports.ObjectType = ObjectBase.extend;