feat: scoped_critical_section (#5684)

* feat: scoped_critical_section

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* refactor: pull out to file

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* style: pre-commit fixes

* fix: GIL code in some compilers

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix: move to correct spot

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

CMakeLists.txt

@@ -205,6 +205,7 @@ set(PYBIND11_HEADERS
     include/pybind11/conduit/pybind11_conduit_v1.h
     include/pybind11/conduit/pybind11_platform_abi_id.h
     include/pybind11/conduit/wrap_include_python_h.h
+    include/pybind11/critical_section.h
     include/pybind11/options.h
     include/pybind11/eigen.h
     include/pybind11/eigen/common.h

docs/advanced/misc.rst

@@ -295,22 +295,29 @@ This module is sub-interpreter safe, for both ``shared_gil`` ("legacy") and
 function concurrently from different threads. This is safe because each sub-interpreter's GIL
 protects it's own Python objects from concurrent access.
 
-However, the module is no longer free-threading safe, for the same reason as before, because the
-calculation is not synchronized. We can synchronize it using a Python critical section.
+However, the module is no longer free-threading safe, for the same reason as
+before, because the calculation is not synchronized. We can synchronize it
+using a Python critical section. This will do nothing if not in free-threaded
+Python. You can have it lock one or two Python objects. You cannot nest it.
+(Note: In Python 3.13t, Python re-locks if you enter a critical section again,
+which happens in various places. This was optimized away in 3.14+. Use a
+``std::mutex`` instead if this is a problem).
 
 .. code-block:: cpp
-    :emphasize-lines: 1,5,10
+    :emphasize-lines: 1,4,8
+
+    #include <pybind11/critical_section.h>
+    // ...
 
     PYBIND11_MODULE(example, m, py::multiple_interpreters::per_interpreter_gil(), py::mod_gil_not_used()) {
         m.def("calc_next", []() {
             size_t old;
             py::dict g = py::globals();
-            Py_BEGIN_CRITICAL_SECTION(g);
+            py::scoped_critical_section guard(g);
             if (!g.contains("myseed"))
                 g["myseed"] = 0;
             old = g["myseed"];
             g["myseed"] = (old + 1) * 10;
-            Py_END_CRITICAL_SECTION();
             return old;
         });
     }

include/pybind11/critical_section.h

@@ -0,0 +1,50 @@
+// Copyright (c) 2016-2025 The Pybind Development Team.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#pragma once
+
+#include "pytypes.h"
+
+PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
+
+/// This does not do anything if there's a GIL. On free-threaded Python,
+/// it locks an object. This uses the CPython API, which has limits
+class scoped_critical_section {
+public:
+#ifdef Py_GIL_DISABLED
+    explicit scoped_critical_section(handle obj) : has2(false) {
+        PyCriticalSection_Begin(&section, obj.ptr());
+    }
+
+    scoped_critical_section(handle obj1, handle obj2) : has2(true) {
+        PyCriticalSection2_Begin(&section2, obj1.ptr(), obj2.ptr());
+    }
+
+    ~scoped_critical_section() {
+        if (has2) {
+            PyCriticalSection2_End(&section2);
+        } else {
+            PyCriticalSection_End(&section);
+        }
+    }
+#else
+    explicit scoped_critical_section(handle) {};
+    scoped_critical_section(handle, handle) {};
+    ~scoped_critical_section() = default;
+#endif
+
+    scoped_critical_section(const scoped_critical_section &) = delete;
+    scoped_critical_section &operator=(const scoped_critical_section &) = delete;
+
+private:
+#ifdef Py_GIL_DISABLED
+    bool has2;
+    union {
+        PyCriticalSection section;
+        PyCriticalSection2 section2;
+    };
+#endif
+};
+
+PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

tests/extra_python_package/test_files.py

@@ -44,6 +44,7 @@ main_headers = {
     "include/pybind11/chrono.h",
     "include/pybind11/common.h",
     "include/pybind11/complex.h",
+    "include/pybind11/critical_section.h",
     "include/pybind11/eigen.h",
     "include/pybind11/embed.h",
     "include/pybind11/eval.h",

tests/test_embed/test_interpreter.cpp

@@ -1,3 +1,4 @@
+#include <pybind11/critical_section.h>
 #include <pybind11/embed.h>
 
 // Silence MSVC C++17 deprecation warning from Catch regarding std::uncaught_exceptions (up to
@@ -365,15 +366,11 @@ TEST_CASE("Threads") {
 #ifdef Py_GIL_DISABLED
 #    if PY_VERSION_HEX < 0x030E0000
                 std::lock_guard<std::mutex> lock(mutex);
-                locals["count"] = locals["count"].cast<int>() + 1;
 #    else
-                Py_BEGIN_CRITICAL_SECTION(locals.ptr());
-                locals["count"] = locals["count"].cast<int>() + 1;
-                Py_END_CRITICAL_SECTION();
+                py::scoped_critical_section lock(locals);
 #    endif
-#else
-                locals["count"] = locals["count"].cast<int>() + 1;
 #endif
+                locals["count"] = locals["count"].cast<int>() + 1;
             });
         }