From 6ddac0130450b39b1e7c25d4bf66e46b73f7c6e4 Mon Sep 17 00:00:00 2001
From: Daniel Baston <dbaston@gmail.com>
Date: Fri, 6 Dec 2024 11:19:25 -0500
Subject: [PATCH] Avoid crash when converting dict with circular reference

Fixes https://github.com/pybind/pybind11_json/issues/73
---
 include/pybind11_json/pybind11_json.hpp | 17 ++++++++++++++---
 test/test_pybind11_json.cpp             | 14 ++++++++++++++
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/include/pybind11_json/pybind11_json.hpp b/include/pybind11_json/pybind11_json.hpp
index 2b7d9b2..c97c5aa 100644
--- a/include/pybind11_json/pybind11_json.hpp
+++ b/include/pybind11_json/pybind11_json.hpp
@@ -67,8 +67,12 @@ namespace pyjson
         }
     }
 
-    inline nl::json to_json(const py::handle& obj)
+    inline nl::json to_json_inner(const py::handle& obj, std::set<const PyObject*>& refs)
     {
+        if (auto [_, unique] = refs.insert(obj.ptr()); !unique) {
+            throw std::runtime_error("Circular reference detected");
+        }
+
         if (obj.ptr() == nullptr || obj.is_none())
         {
             return nullptr;
@@ -121,7 +125,7 @@ namespace pyjson
             auto out = nl::json::array();
             for (const py::handle value : obj)
             {
-                out.push_back(to_json(value));
+                out.push_back(to_json_inner(value, refs));
             }
             return out;
         }
@@ -130,12 +134,19 @@ namespace pyjson
             auto out = nl::json::object();
             for (const py::handle key : obj)
             {
-                out[py::str(key).cast<std::string>()] = to_json(obj[key]);
+                out[py::str(key).cast<std::string>()] = to_json_inner(obj[key], refs);
             }
             return out;
         }
         throw std::runtime_error("to_json not implemented for this type of object: " + py::repr(obj).cast<std::string>());
     }
+
+    inline nl::json to_json(const py::handle& obj)
+    {
+        std::set<const PyObject*> refs;
+        return to_json_inner(obj, refs);
+    }
+
 }
 
 // nlohmann_json serializers
diff --git a/test/test_pybind11_json.cpp b/test/test_pybind11_json.cpp
index 2d9cc82..b9ad588 100644
--- a/test/test_pybind11_json.cpp
+++ b/test/test_pybind11_json.cpp
@@ -481,3 +481,17 @@ TEST(pybind11_caster_fromjson, dict)
     ASSERT_EQ(j["number"].cast<int>(), 1234);
     ASSERT_EQ(j["hello"].cast<std::string>(), "world");
 }
+
+TEST(pybind11_caster_tojson, recursive_dict)
+{
+    py::scoped_interpreter guard;
+    py::module m = create_module("test");
+
+    m.def("to_json", &test_fromtojson);
+
+    // Simulate calling this binding from Python with a dictionary as argument
+    py::dict obj("number"_a=1234, "hello"_a="world");
+    obj["recur"] = obj;
+
+    ASSERT_ANY_THROW(m.attr("to_json")(obj));
+}