vm: cleanup module memory leakage

There are some paths here that led to dangling contexts. By being smarter with
handle management we can get rid of all the cleanup code and fix those issues.

This is a backport of commit 7063575.

src/node_script.cc

@@ -348,12 +348,18 @@ Handle<Value> WrappedScript::EvalMachine(const Arguments& args) {
     display_error = true;
   }
 
-  Persistent<Context> context;
+  Handle<Context> context = Context::GetCurrent();
 
   Local<Array> keys;
   if (context_flag == newContext) {
     // Create the new context
-    context = Context::New();
+    // Context::New returns a Persistent<Context>, but we only need it for this
+    // function. Here we grab a temporary handle to the new context, assign it
+    // to a local handle, and then dispose the persistent handle. This ensures
+    // that when this function exits the context will be disposed.
+    Persistent<Context> tmp = Context::New();
+    context = Local<Context>::New(tmp);
+    tmp.Dispose();
 
   } else if (context_flag == userContext) {
     // Use the passed in context
@@ -362,11 +368,10 @@ Handle<Value> WrappedScript::EvalMachine(const Arguments& args) {
     context = nContext->GetV8Context();
   }
 
+  Context::Scope context_scope(context);
+
   // New and user context share code. DRY it up.
   if (context_flag == userContext || context_flag == newContext) {
-    // Enter the context
-    context->Enter();
-
     // Copy everything from the passed in sandbox (either the persistent
     // context for runInContext(), or the sandbox arg to runInNewContext()).
     CloneObject(args.This(), sandbox, context->Global()->GetPrototype());
@@ -408,11 +413,6 @@ Handle<Value> WrappedScript::EvalMachine(const Arguments& args) {
   if (output_flag == returnResult) {
     result = script->Run();
     if (result.IsEmpty()) {
-      if (context_flag == newContext) {
-        context->DetachGlobal();
-        context->Exit();
-        context.Dispose();
-      }
       return try_catch.ReThrow();
     }
   } else {
@@ -430,16 +430,6 @@ Handle<Value> WrappedScript::EvalMachine(const Arguments& args) {
     CloneObject(args.This(), context->Global()->GetPrototype(), sandbox);
   }
 
-  if (context_flag == newContext) {
-    // Clean up, clean up, everybody everywhere!
-    context->DetachGlobal();
-    context->Exit();
-    context.Dispose();
-  } else if (context_flag == userContext) {
-    // Exit the passed in context.
-    context->Exit();
-  }
-
   return result == args.This() ? result : scope.Close(result);
 }