src: fix backtrace for V8 6.8

V8 6.8 replaces Closure inside Context with ScopeInfo. Those are minimal
changes to adopt llnode to the new behavior.

Ref: https://chromium-review.googlesource.com/#/c/785151/
Fixes: #193

src/llv8-constants.cc

@@ -347,8 +347,9 @@ void ScopeInfo::Load() {
 
 
 void Context::Load() {
-  kClosureIndex =
-      LoadConstant("class_Context__closure_index__int", "context_idx_closure");
+  kClosureIndex = LoadConstant("class_Context__closure_index__int",
+                               "context_idx_closure", -1);
+  kScopeInfoIndex = LoadConstant("context_idx_scope_info", -1);
   kPreviousIndex =
       LoadConstant("class_Context__previous_index__int", "context_idx_prev");
   kMinContextSlots = LoadConstant("class_Context__min_context_slots__int",

src/llv8-constants.h

@@ -219,6 +219,7 @@ class Context : public Module {
   MODULE_DEFAULT_METHODS(Context);
 
   int64_t kClosureIndex;
+  int64_t kScopeInfoIndex;
   int64_t kGlobalObjectIndex;
   int64_t kPreviousIndex;
   int64_t kMinContextSlots;

src/llv8.cc

@@ -1089,6 +1089,18 @@ std::string FixedArray::InspectContents(int length, Error& err) {
   return res;
 }
 
+HeapObject Context::GetScopeInfo(Error& err) {
+  if (v8()->context()->kScopeInfoIndex != -1) {
+    return FixedArray::Get<HeapObject>(v8()->context()->kScopeInfoIndex, err);
+  }
+  JSFunction closure = Closure(err);
+  if (err.Fail()) return HeapObject();
+
+  SharedFunctionInfo info = closure.Info(err);
+  if (err.Fail()) return HeapObject();
+
+  return info.GetScopeInfo(err);
+}
 
 std::string Context::Inspect(Error& err) {
   std::string res;
@@ -1100,13 +1112,7 @@ std::string Context::Inspect(Error& err) {
   Value previous = Previous(err);
   if (err.Fail()) return std::string();
 
-  JSFunction closure = Closure(err);
-  if (err.Fail()) return std::string();
-
-  SharedFunctionInfo info = closure.Info(err);
-  if (err.Fail()) return std::string();
-
-  HeapObject scope_obj = info.GetScopeInfo(err);
+  HeapObject scope_obj = GetScopeInfo(err);
   if (err.Fail()) return std::string();
 
   ScopeInfo scope(scope_obj);
@@ -1128,7 +1134,11 @@ std::string Context::Inspect(Error& err) {
   }
 
   if (!res.empty()) res += "\n";
-  {
+
+  if (v8()->context()->kClosureIndex != -1) {
+    JSFunction closure;
+    closure = Closure(err);
+    if (err.Fail()) return std::string();
     char tmp[128];
     snprintf(tmp, sizeof(tmp), "    (closure)=0x%016" PRIx64 " {",
              closure.raw());

src/llv8.h

@@ -401,11 +401,14 @@ class Context : public FixedArray {
  public:
   V8_VALUE_DEFAULT_METHODS(Context, FixedArray)
 
-  inline JSFunction Closure(Error& err);
+  inline HeapObject GetScopeInfo(Error& err);
   inline Value Previous(Error& err);
   inline Value ContextSlot(int index, Error& err);
 
   std::string Inspect(Error& err);
+
+ private:
+  inline JSFunction Closure(Error& err);
 };
 
 class ScopeInfo : public FixedArray {