disallow circular struct references (#8851)

* detect and fail for circular struct dependencies

* pr comments

* pr comment

include/flatbuffers/idl.h

@@ -395,13 +395,20 @@ struct FieldDef : public Definition {
 };
 
 struct StructDef : public Definition {
+  enum class CycleStatus {
+    NotChecked,
+    InProgress,
+    Checked,
+  };
+
   StructDef()
       : fixed(false),
         predecl(true),
         sortbysize(true),
         has_key(false),
         minalign(1),
-        bytesize(0) {}
+        bytesize(0),
+        cycle_status{CycleStatus::NotChecked} {}
 
   void PadLastField(size_t min_align) {
     auto padding = PaddingBytes(bytesize, min_align);
@@ -423,6 +430,8 @@ struct StructDef : public Definition {
   size_t minalign;  // What the whole object needs to be aligned to.
   size_t bytesize;  // Size if fixed.
 
+  CycleStatus cycle_status;  // used for determining if we have circular references
+
   flatbuffers::unique_ptr<std::string> original_location;
   std::vector<voffset_t> reserved_ids;
 };
@@ -1101,6 +1110,8 @@ class Parser : public ParserState {
   // others includes.
   std::vector<IncludedFile> GetIncludedFiles() const;
 
+  bool HasCircularStructDependency();
+
  private:
   class ParseDepthGuard;