From dfcaebf91fa24f860175feb2e860b24b15cdb98a Mon Sep 17 00:00:00 2001
From: Emery Conrad <emery.conrad@chicagotrading.com>
Date: Thu, 22 Oct 2026 09:14:21 -0500
Subject: [PATCH] [cpyrt] Stop the using-directive search at a cycle

Mutual using-directives are valid C++ ([namespace.qual]/3). The attribute
lookup recursed through them until the stack overflowed. Search each
namespace at most once for each lookup.

Co-developed-with-the-help-of: Claude Code (Opus 5.5, human in the loop)
---
 src/cpyrt/CPPScope.cxx   | 31 ++++++++++++++++++++-----------
 test/test_advancedcpp.py | 20 ++++++++++++++++++++
 2 files changed, 40 insertions(+), 11 deletions(-)

diff --git a/src/cpyrt/CPPScope.cxx b/src/cpyrt/CPPScope.cxx
index dd5801ab..202ea2fe 100644
--- a/src/cpyrt/CPPScope.cxx
+++ b/src/cpyrt/CPPScope.cxx
@@ -505,18 +505,27 @@ static PyObject* meta_getattro(PyObject* pyclass, PyObject* pyname) {
       }
     }
 
-    // try all outstanding using namespaces in turn to find the attribute (will
-    // cache locally later; TODO: doing so may cause pathological cases)
-    for (auto pyref : *klass->fImp.fUsing) {
-      PyObject* pyuscope = cpyrt_GetWeakRef(pyref);
-      if (pyuscope) {
-        attr = PyObject_GetAttr(pyuscope, pyname);
-        if (!attr)
-          PyErr_Clear();
-        Py_DECREF(pyuscope);
+    // Search each namespace at most once per lookup ([namespace.qual]/3), so
+    // that using-directive cycles terminate.
+    thread_local std::set<std::pair<PyObject*, std::string>> searched;
+    thread_local int depth = 0;
+    if (searched.emplace(pyclass, name).second) {
+      ++depth;
+      // try all outstanding using namespaces in turn to find the attribute
+      // (will cache locally later; TODO: doing so may cause pathological cases)
+      for (auto pyref : *klass->fImp.fUsing) {
+        PyObject* pyuscope = cpyrt_GetWeakRef(pyref);
+        if (pyuscope) {
+          attr = PyObject_GetAttr(pyuscope, pyname);
+          if (!attr)
+            PyErr_Clear();
+          Py_DECREF(pyuscope);
+        }
+        if (attr)
+          break;
       }
-      if (attr)
-        break;
+      if (--depth == 0)
+        searched.clear();
     }
   }
 
diff --git a/test/test_advancedcpp.py b/test/test_advancedcpp.py
index 6bd7aa7b..b9a5cf4a 100644
--- a/test/test_advancedcpp.py
+++ b/test/test_advancedcpp.py
@@ -1098,3 +1098,23 @@ def test31_virtual_base_downcast(self):
         assert isinstance(obj3, ns.Visible3)
         assert obj3.value == 23
         assert obj3.z == 7
+
+    def test32_using_directive_cycle(self):
+        """Lookup through cyclic using directives searches each namespace once"""
+
+        import cppjit
+
+        cppjit.cppdef("""\
+        namespace udir_cycle_a {}
+        namespace udir_cycle_p { using namespace udir_cycle_a; int x = 1; }
+        namespace udir_cycle_a { using namespace udir_cycle_p; int y = 2; }""")
+
+        a = cppjit.gbl.udir_cycle_a
+        p = cppjit.gbl.udir_cycle_p
+        assert a.x == 1
+        assert p.y == 2
+
+        with raises(AttributeError):
+            a.nothere
+        with raises(AttributeError):
+            p.nothere