src: fix multi-base class ObjectWrap::Unwrap<T>()

Fix pointer unwrapping when T is a class with more than one base class.

Before this commit, the wrapped void* pointer was cast directly to T*
without going through ObjectWrap* first, possibly leading to a class
instance pointer that points to the wrong vtable.

This change required some cleanup in various files; some classes
used private rather than public inheritance, others didn't derive
from ObjectWrap at all...

Fixes #6188.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/node_object_wrap.h b/src/node_object_wrap.h
index 5c6ab2865b..683123559f 100644
--- a/src/node_object_wrap.h
+++ b/src/node_object_wrap.h
@@ -56,7 +56,11 @@ class NODE_EXTERN ObjectWrap {
   static inline T* Unwrap(v8::Handle<v8::Object> handle) {
     assert(!handle.IsEmpty());
     assert(handle->InternalFieldCount() > 0);
-    return static_cast<T*>(handle->GetAlignedPointerFromInternalField(0));
+    // Cast to ObjectWrap before casting to T.  A direct cast from void
+    // to T won't work right when T has more than one base class.
+    void* ptr = handle->GetAlignedPointerFromInternalField(0);
+    ObjectWrap* wrap = static_cast<ObjectWrap*>(ptr);
+    return static_cast<T*>(wrap);
   }