fix: Python 3.13t with GIL

.github/workflows/ci.yml

@@ -196,6 +196,38 @@ jobs:
         pytest tests/extra_setuptools
       if: "!(matrix.runs-on == 'windows-2022')"
 
+  manylinux:
+    name: Manylinux on 🐍 ${{ matrix.python }}
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python: ["3.13t", "3.13"]
+    timeout-minutes: 40
+    container: quay.io/pypa/musllinux_1_2_x86_64:latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Prepare venv
+        run: python${{ matrix.python }} -m venv .venv
+
+      - name: Install Python deps
+        run: .venv/bin/pip install -r tests/requirements.txt
+
+      - name: Configure C++11
+        run: >
+          cmake -S. -Bbuild
+          -DPYBIND11_WERROR=ON
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DPython_ROOT_DIR=.venv
+
+      - name: Build C++11
+        run: cmake --build build -j2
+
+      - name: Python tests C++11
+        run: cmake --build build --target pytest -j2
 
   deadsnakes:
     strategy:

include/pybind11/pytypes.h

@@ -183,7 +183,7 @@ class object_api : public pyobject_tag {
     str_attr_accessor doc() const;
 
     /// Return the object's current reference count
-    int ref_count() const { return static_cast<int>(Py_REFCNT(derived().ptr())); }
+    ssize_t ref_count() const { return Py_REFCNT(derived().ptr()); }
 
     // TODO PYBIND11_DEPRECATED(
     //     "Call py::type::handle_of(h) or py::type::of(h) instead of h.get_type()")

tests/test_class.py

@@ -377,7 +377,8 @@ class PyDog(m.Dog):
         refcount_3 = getrefcount(cls)
 
         assert refcount_1 == refcount_3
-        assert refcount_2 > refcount_1
+        # Free-threaded Python uses UINT32_MAX for immortal objects
+        assert (refcount_2 > refcount_1) or (refcount_2 == refcount_1 == 2**32 - 1)
 
 
 def test_reentrant_implicit_conversion_failure(msg):

tests/test_embed/CMakeLists.txt

@@ -7,6 +7,13 @@ if("${PYTHON_MODULE_EXTENSION}" MATCHES "pypy" OR "${Python_INTERPRETER_ID}" STR
   return()
 endif()
 
+if(TARGET Python::Module AND NOT TARGET Python::Python)
+  message(STATUS "Skipping embed test since no embed libs found")
+  add_custom_target(cpptest) # Dummy target since embedding is not supported.
+  set(_suppress_unused_variable_warning "${DOWNLOAD_CATCH}")
+  return()
+endif()
+
 find_package(Catch 2.13.9)
 
 if(CATCH_FOUND)

tests/test_pytypes.py

@@ -635,7 +635,8 @@ def test_memoryview_refcount(method):
     ref_before = sys.getrefcount(buf)
     view = method(buf)
     ref_after = sys.getrefcount(buf)
-    assert ref_before < ref_after
+    # Free threaded Python uses UINT32_MAX
+    assert ref_before < ref_after or ref_before == ref_after == 2**32 - 1
     assert list(view) == list(buf)