Fix stack overflow (#807)

Fix stack overflow in HandleNode by explicitly limiting the depth of recursion.

include/yaml-cpp/depthguard.h

@@ -0,0 +1,77 @@
+#ifndef DEPTH_GUARD_H_00000000000000000000000000000000000000000000000000000000
+#define DEPTH_GUARD_H_00000000000000000000000000000000000000000000000000000000
+
+#if defined(_MSC_VER) ||                                            \
+    (defined(__GNUC__) && (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || \
+     (__GNUC__ >= 4))  // GCC supports "pragma once" correctly since 3.4
+#pragma once
+#endif
+
+#include "exceptions.h"
+
+namespace YAML {
+
+/**
+ * @brief The DeepRecursion class
+ *  An exception class which is thrown by DepthGuard. Ideally it should be
+ * a member of DepthGuard. However, DepthGuard is a templated class which means
+ * that any catch points would then need to know the template parameters. It is
+ * simpler for clients to not have to know at the catch point what was the
+ * maximum depth.
+ */
+class DeepRecursion : public ParserException {
+public:
+  virtual ~DeepRecursion() = default;
+
+  DeepRecursion(int depth, const Mark& mark_, const std::string& msg_);
+
+  // Returns the recursion depth when the exception was thrown
+  int depth() const {
+    return m_depth;
+  }
+
+private:
+  int m_depth = 0;
+};
+
+/**
+ * @brief The DepthGuard class
+ *  DepthGuard takes a reference to an integer. It increments the integer upon
+ * construction of DepthGuard and decrements the integer upon destruction.
+ *
+ * If the integer would be incremented past max_depth, then an exception is
+ * thrown. This is ideally geared toward guarding against deep recursion.
+ *
+ * @param max_depth
+ *  compile-time configurable maximum depth.
+ */
+template <int max_depth = 2000>
+class DepthGuard final {
+public:
+  DepthGuard(int & depth_, const Mark& mark_, const std::string& msg_) : m_depth(depth_) {
+    ++m_depth;
+    if ( max_depth <= m_depth ) {
+        throw DeepRecursion{m_depth, mark_, msg_};
+    }
+  }
+
+  DepthGuard(const DepthGuard & copy_ctor) = delete;
+  DepthGuard(DepthGuard && move_ctor) = delete;
+  DepthGuard & operator=(const DepthGuard & copy_assign) = delete;
+  DepthGuard & operator=(DepthGuard && move_assign) = delete;
+
+  ~DepthGuard() {
+    --m_depth;
+  }
+
+  int current_depth() const {
+    return m_depth;
+  }
+
+private:
+    int & m_depth;
+};
+
+} // namespace YAML
+
+#endif // DEPTH_GUARD_H_00000000000000000000000000000000000000000000000000000000