[idl_parser] Improve symbols lookup thru parent namespaces (#6407)

Added the new method LookupTableByName for searching symbols in parent namespaces.
This method speedup (x50 or greater) symbol resolution (enum or struct) in parent namespaces.
The speedup was measured without `table.empty()` guard condition.

This method should suppress fuzzer timeout issue without artificial limits for nested namespaces (https://oss-fuzz.com/testcase-detail/6244168439169024).

Additionally, this commit speedup (x5) the GetFullyQualifiedName method by removing unnecessary temporary std::string object.

include/flatbuffers/idl.h

@@ -856,6 +856,7 @@ class Parser : public ParserState {
                        flexbuffers::Builder *builder);
 
   StructDef *LookupStruct(const std::string &id) const;
+  StructDef *LookupStructThruParentNamespaces(const std::string &id) const;
 
   std::string UnqualifiedName(const std::string &fullQualifiedName);
 

src/idl_parser.cpp

@@ -227,9 +227,10 @@ std::string Namespace::GetFullyQualifiedName(const std::string &name,
   if (components.empty() || !max_components) { return name; }
   std::string stream_str;
   for (size_t i = 0; i < std::min(components.size(), max_components); i++) {
-    if (i) { stream_str += '.'; }
+    stream_str += components[i];
-    stream_str += std::string(components[i]);
+    stream_str += '.';
   }
+  if (!stream_str.empty()) stream_str.pop_back();
   if (name.length()) {
     stream_str += '.';
     stream_str += name;
@@ -237,6 +238,29 @@ std::string Namespace::GetFullyQualifiedName(const std::string &name,
   return stream_str;
 }
 
+template<typename T>
+T *LookupTableByName(const SymbolTable<T> &table, const std::string &name,
+                     const Namespace &current_namespace, size_t skip_top) {
+  const auto &components = current_namespace.components;
+  if (table.dict.empty()) return nullptr;
+  if (components.size() < skip_top) return nullptr;
+  const auto N = components.size() - skip_top;
+  std::string full_name;
+  for (size_t i = 0; i < N; i++) {
+    full_name += components[i];
+    full_name += '.';
+  }
+  for (size_t i = N; i > 0; i--) {
+    full_name += name;
+    auto obj = table.Lookup(full_name);
+    if (obj) return obj;
+    auto len = full_name.size() - components[i - 1].size() - 1 - name.size();
+    full_name.resize(len);
+  }
+  FLATBUFFERS_ASSERT(full_name.empty());
+  return table.Lookup(name);  // lookup in global namespace
+}
+
 // Declare tokens we'll use. Single character tokens are represented by their
 // ascii character code (e.g. '{'), others above 256.
 // clang-format off
@@ -576,13 +600,7 @@ CheckedError Parser::ParseNamespacing(std::string *id, std::string *last) {
 
 EnumDef *Parser::LookupEnum(const std::string &id) {
   // Search thru parent namespaces.
-  for (int components = static_cast<int>(current_namespace_->components.size());
+  return LookupTableByName(enums_, id, *current_namespace_, 0);
-       components >= 0; components--) {
-    auto ed = enums_.Lookup(
-        current_namespace_->GetFullyQualifiedName(id, components));
-    if (ed) return ed;
-  }
-  return nullptr;
 }
 
 StructDef *Parser::LookupStruct(const std::string &id) const {
@@ -591,6 +609,13 @@ StructDef *Parser::LookupStruct(const std::string &id) const {
   return sd;
 }
 
+StructDef *Parser::LookupStructThruParentNamespaces(
+    const std::string &id) const {
+  auto sd = LookupTableByName(structs_, id, *current_namespace_, 1);
+  if (sd) sd->refcount++;
+  return sd;
+}
+
 CheckedError Parser::ParseTypeIdent(Type &type) {
   std::string id = attribute_;
   EXPECT(kTokenIdentifier);
@@ -1980,13 +2005,8 @@ StructDef *Parser::LookupCreateStruct(const std::string &name,
     }
     return struct_def;
   }
-  if (!definition) {
+  if (!definition && !struct_def) {
-    // Search thru parent namespaces.
+    struct_def = LookupStructThruParentNamespaces(name);
-    for (size_t components = current_namespace_->components.size();
-         components && !struct_def; components--) {
-      struct_def = LookupStruct(
-          current_namespace_->GetFullyQualifiedName(name, components - 1));
-    }
   }
   if (!struct_def && create_if_new) {
     struct_def = new StructDef();