Introduce MaybeHandle to police exception checking in handlified code.

src/handles.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 16 matching lines @@
 
 #ifndef V8_HANDLES_H_
 #define V8_HANDLES_H_
 
 #include "allocation.h"
 #include "objects.h"
 
 namespace v8 {
 namespace internal {
 
+template<typename T>
+class MaybeHandle {

[Michael Starzinger, 2014/04/02 14:24:46]

Can we get a few comments above this class about why we have it and what
distinguishes it from an ordinary Handle?

+ public:
+  MaybeHandle() : location_(NULL) { }
+
+  // Constructor for handling automatic up casting.
+  // Ex. Handle<JSArray> can be passed when MaybeHandle<Object> is expected.
+  template <class S> MaybeHandle(Handle<S> handle) {
+#ifdef DEBUG
+    T* a = NULL;
+    S* b = NULL;
+    a = b;  // Fake assignment to enforce type checks.
+    USE(a);
+#endif
+    this->location_ = reinterpret_cast<T**>(handle.location_);
+  }
+
+  static MaybeHandle<T> Exception() {
+    return Handle<T>::null();
+  }
+
+  INLINE(Handle<T> ToHandleChecked()) {
+    CHECK(!is_null());
+    return Handle<T>(location_);
+  }
+
+  INLINE(bool ToHandle(Handle<T>* out)) {
+    if (is_null()) {
+      *out = Handle<T>::null();
+      return false;
+    } else {
+      *out = Handle<T>(location_);
+      return true;
+    }
+  }
+
+  INLINE(bool is_null()) const { return this->location_ == NULL; }
+
+ protected:
+  // To access location_ from derived Handle<T> classes, we need to explicitly
+  // use this->location_, since the compiler does not resolve it correctly.
+  T** location_;
+};
+
 // ----------------------------------------------------------------------------
 // A Handle provides a reference to an object that survives relocation by
 // the garbage collector.
 // Handles are only valid within a HandleScope.
 // When a handle is created for an object a cell is allocated in the heap.
 
 template<typename T>
-class Handle {
+class Handle : public MaybeHandle<T> {

[Toon Verwaest, 2014/04/02 14:40:24]

I find it pretty confusing that a "checked handle" is an "unchecked handle".

  public:
-  INLINE(explicit Handle(T** location)) { location_ = location; }
+  INLINE(explicit Handle(T** location)) { this->location_ = location; }
   INLINE(explicit Handle(T* obj));
   INLINE(Handle(T* obj, Isolate* isolate));
 
-  INLINE(Handle()) : location_(NULL) {}
+  INLINE(Handle()) {}

[Michael Starzinger, 2014/04/02 14:24:46]

As discussed offline: The ultimate goal of this endeavor is to get rid of this
constructor. I totally understand that this will take some time, but can we
leave a TODO here in that regard? Without reaching this goal, all the changes
leading up to it would increase complexity instead of reducing it.

 
   // Constructor for handling automatic up casting.
   // Ex. Handle<JSFunction> can be passed when Handle<Object> is expected.
   template <class S> Handle(Handle<S> handle) {
 #ifdef DEBUG
     T* a = NULL;
     S* b = NULL;
     a = b;  // Fake assignment to enforce type checks.
     USE(a);
 #endif
-    location_ = reinterpret_cast<T**>(handle.location_);
+    this->location_ = reinterpret_cast<T**>(handle.location_);
   }
 
   INLINE(T* operator->() const) { return operator*(); }
 
   // Check if this handle refers to the exact same object as the other handle.
   INLINE(bool is_identical_to(const Handle<T> other) const);
 
   // Provides the C++ dereference operator.
   INLINE(T* operator*() const);
 
   // Returns the address to where the raw pointer is stored.
   INLINE(T** location() const);
 
   template <class S> static Handle<T> cast(Handle<S> that) {
     T::cast(*reinterpret_cast<T**>(that.location_));
     return Handle<T>(reinterpret_cast<T**>(that.location_));
   }
 
   static Handle<T> null() { return Handle<T>(); }

[Michael Starzinger, 2014/04/02 14:24:46]

Likewise.

-  bool is_null() const { return location_ == NULL; }
 
   // Closes the given scope, but lets this handle escape. See
   // implementation in api.h.
   inline Handle<T> EscapeFrom(v8::EscapableHandleScope* scope);
 
 #ifdef DEBUG
   enum DereferenceCheckMode { INCLUDE_DEFERRED_CHECK, NO_DEFERRED_CHECK };
 
   bool IsDereferenceAllowed(DereferenceCheckMode mode) const;
 #endif  // DEBUG
 
  private:
-  T** location_;
-
   // Handles of different classes are allowed to access each other's location_.
   template<class S> friend class Handle;
+  template<class S> friend class MaybeHandle;
 };
 
 
 // Convenience wrapper.
 template<class T>
 inline Handle<T> handle(T* t, Isolate* isolate) {
   return Handle<T>(t, isolate);
 }
 
 
@@ skipping 211 matching lines @@
 
   void Initialize() {
     next = limit = NULL;
     level = 0;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_HANDLES_H_