Fix #633: Bound ChaiScript call stack to prevent native stack overflow

Recursive user-defined operators (e.g. a `string::/=` whose body calls
itself through string interpolation) drove the AST evaluator into
unbounded native recursion and crashed the host process with SIGSEGV.
The dispatcher now refuses to enter a new function frame once
`Stack_Holder::call_depth` reaches `chaiscript::max_call_depth`
(default 256, overridable via the `CHAISCRIPT_MAX_CALL_DEPTH` macro)
and throws the new `chaiscript::exception::stack_overflow_error`
instead, letting both ChaiScript-level `try`/`catch` and C++ hosts
recover from runaway recursion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

include/chaiscript/chaiscript_defines.hpp

@@ -75,6 +75,14 @@ static_assert(_MSC_FULL_VER >= 190024210, "Visual C++ 2015 Update 3 or later req
 #define CHAISCRIPT_DEBUG false
 #endif
 
+// Upper bound on the depth of nested ChaiScript function calls. Hitting it
+// causes the dispatcher to throw chaiscript::exception::stack_overflow_error
+// instead of letting the native call stack overflow. Defining the macro on
+// the command line overrides the default.
+#ifndef CHAISCRIPT_MAX_CALL_DEPTH
+#define CHAISCRIPT_MAX_CALL_DEPTH 256
+#endif
+
 #include <cmath>
 #include <memory>
 #include <string>
@@ -88,6 +96,9 @@ namespace chaiscript {
   constexpr static const char *compiler_name = CHAISCRIPT_COMPILER_NAME;
   constexpr static const bool debug_build = CHAISCRIPT_DEBUG;
 
+  constexpr static const int max_call_depth = CHAISCRIPT_MAX_CALL_DEPTH;
+  static_assert(max_call_depth > 0, "CHAISCRIPT_MAX_CALL_DEPTH must be a positive integer");
+
   template<typename B, typename D, typename... Arg>
   inline std::shared_ptr<B> make_shared(Arg &&...arg) {
 #ifdef CHAISCRIPT_USE_STD_MAKE_SHARED

include/chaiscript/dispatchkit/dispatchkit.hpp

@@ -121,6 +121,19 @@ namespace chaiscript {
       global_non_const(const global_non_const &) = default;
       ~global_non_const() noexcept override = default;
     };
+
+    /// Exception thrown when the ChaiScript call stack exceeds
+    /// chaiscript::max_call_depth, signalling runaway recursion before the
+    /// native stack can overflow.
+    class stack_overflow_error : public std::runtime_error {
+    public:
+      stack_overflow_error() noexcept
+          : std::runtime_error("Maximum call stack depth exceeded") {
+      }
+
+      stack_overflow_error(const stack_overflow_error &) = default;
+      ~stack_overflow_error() noexcept override = default;
+    };
   } // namespace exception
 
   /// \brief Holds a collection of ChaiScript settings which can be applied to the ChaiScript runtime.
@@ -1010,6 +1023,10 @@ namespace chaiscript {
       void save_function_params(const Function_Params &t_params) { save_function_params(*m_stack_holder, t_params); }
 
       void new_function_call(Stack_Holder &t_s, Type_Conversions::Conversion_Saves &t_saves) {
+        if (t_s.call_depth >= max_call_depth) {
+          throw chaiscript::exception::stack_overflow_error{};
+        }
+
         if (t_s.call_depth == 0) {
           m_conversions.enable_conversion_saves(t_saves, true);
         }

unittests/recursion_depth_protection.chai

@@ -0,0 +1,39 @@
+// Regression test for issue #633: Stack-overflow due to infinite recursion
+// in user-defined operator (string interpolation).
+//
+// Before the fix the recursive `/=` invocation triggered unbounded native
+// recursion in the evaluator and crashed the host process with a SIGSEGV.
+// The engine now bounds call_depth and throws a catchable exception
+// instead of letting the native stack overflow.
+
+def string::`/=`(double d) {
+  this = "${this/= 2}/=${d}";
+  return this;
+}
+
+var s = "o World"
+var caught = false
+var message = ""
+
+try {
+  s /= 2
+  // unreachable: the recursive operator must abort with an exception
+  assert_true(false)
+} catch (e) {
+  caught = true
+  message = e.what()
+}
+
+assert_true(caught)
+
+// The reported error must mention the call-stack overflow so users can
+// distinguish it from an arbitrary script-level error.
+assert_true(find(message, "call stack") != -1)
+
+// A bounded recursion that stays below the limit must keep working.
+def count_down(n) {
+  if (n <= 0) { return 0 }
+  return count_down(n - 1) + 1
+}
+
+assert_equal(50, count_down(50))