Add `PYBIND11_SIMPLE_GIL_MANAGEMENT` option (cmake, C++ define) (#4216)

* Add option to force the use of the PYPY GIL scoped acquire/release logic to support nested gil access, see https://github.com/pybind/pybind11/issues/1276 and https://github.com/pytorch/pytorch/issues/83101

* Apply suggestions from code review

* Update CMakeLists.txt

* docs: update upgrade guide

* Update docs/upgrade.rst

* All bells & whistles.

* Add Reminder to common.h, so that we will not forget to purge `!WITH_THREAD` branches when dropping Python 3.6

* New sentence instead of semicolon.

* Temporarily pull in snapshot of PR #4246

* Add `test_release_acquire`

* Add more unit tests for nested gil locking

* Add test_report_builtins_internals_keys

* Very minor enhancement: sort list only after filtering.

* Revert change in docs/upgrade.rst

* Add test_multi_acquire_release_cross_module, while also forcing unique PYBIND11_INTERNALS_VERSION for cross_module_gil_utils.cpp

* Hopefully fix apparently new ICC error.

```
2022-10-28T07:57:54.5187728Z -- The CXX compiler identification is Intel 2021.7.0.20220726
...
2022-10-28T07:58:53.6758994Z icpc: remark #10441: The Intel(R) C++ Compiler Classic (ICC) is deprecated and will be removed from product release in the second half of 2023. The Intel(R) oneAPI DPC++/C++ Compiler (ICX) is the recommended compiler moving forward. Please transition to use this compiler. Use '-diag-disable=10441' to disable this message.
2022-10-28T07:58:54.5801597Z In file included from /home/runner/work/pybind11/pybind11/include/pybind11/detail/../detail/type_caster_base.h(15),
2022-10-28T07:58:54.5803794Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/detail/../cast.h(15),
2022-10-28T07:58:54.5805740Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/detail/../attr.h(14),
2022-10-28T07:58:54.5809556Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/detail/class.h(12),
2022-10-28T07:58:54.5812154Z                  from /home/runner/work/pybind11/pybind11/include/pybind11/pybind11.h(13),
2022-10-28T07:58:54.5948523Z                  from /home/runner/work/pybind11/pybind11/tests/cross_module_gil_utils.cpp(13):
2022-10-28T07:58:54.5949009Z /home/runner/work/pybind11/pybind11/include/pybind11/detail/../detail/internals.h(177): error #2282: unrecognized GCC pragma
2022-10-28T07:58:54.5949374Z       PYBIND11_TLS_KEY_INIT(tstate)
2022-10-28T07:58:54.5949579Z       ^
2022-10-28T07:58:54.5949695Z
```

* clang-tidy fixes

* Workaround for PYPY WIN exitcode None

* Revert "Temporarily pull in snapshot of PR #4246"

This reverts commit 23ac16e859150f27fda25ca865cabcb4444e0770.

* Another workaround for PYPY WIN exitcode None

* Clean up how the tests are run "run in process" Part 1: uniformity

* Clean up how the tests are run "run in process" Part 2: use `@pytest.mark.parametrize` and clean up the naming.

* Skip some tests `#if defined(THREAD_SANITIZER)` (tested with TSAN using the Google-internal toolchain).

* Run all tests again but ignore ThreadSanitizer exitcode 66 (this is less likely to mask unrelated ThreadSanitizer issues in the future).

* bug fix: missing common.h include before using `PYBIND11_SIMPLE_GIL_MANAGEMENT`

For the tests in the github CI this does not matter, because
`PYBIND11_SIMPLE_GIL_MANAGEMENT` is always defined from the command line,
but when monkey-patching common.h locally, it matters.

* if process.exitcode is None: assert t_delta > 9.9

* More sophisiticated `_run_in_process()` implementation, clearly reporting `DEADLOCK`, additionally exercised via added `intentional_deadlock()`

* Wrap m.intentional_deadlock in a Python function, for `ForkingPickler` compatibility.

```
>       ForkingPickler(file, protocol).dump(obj)
E       TypeError: cannot pickle 'PyCapsule' object
```

Observed with all Windows builds including mingw but not PyPy, and macos-latest with Python 3.9, 3.10, 3.11 but not 3.6.

* Add link to potential solution for WOULD-BE-NICE-TO-HAVE feature.

* Add `SKIP_IF_DEADLOCK = True` option, to not pollute the CI results with expected `DEADLOCK` failures while we figure out what to do about them.

* Add COPY-PASTE-THIS: gdb ... command (to be used for debugging the detected deadlock)

* style: pre-commit fixes

* Do better than automatic pre-commit fixes.

* Add `PYBIND11_SIMPLE_GIL_MANAGEMENT` to `pytest_report_header()` (so that we can easily know when harvesting deadlock information from the CI logs).

Co-authored-by: Arnim Balzer <arnim@seechange.ai>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

.github/workflows/ci.yml

@@ -102,10 +102,12 @@ jobs:
       run: python -m pip install pytest-github-actions-annotate-failures
 
     # First build - C++11 mode and inplace
+    # More-or-less randomly adding -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON here.
     - name: Configure C++11 ${{ matrix.args }}
       run: >
         cmake -S . -B .
         -DPYBIND11_WERROR=ON
+        -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=11
@@ -129,10 +131,12 @@ jobs:
       run: git clean -fdx
 
     # Second build - C++17 mode and in a build directory
+    # More-or-less randomly adding -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF here.
     - name: Configure C++17
       run: >
         cmake -S . -B build2
         -DPYBIND11_WERROR=ON
+        -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=17

CMakeLists.txt

@@ -91,10 +91,16 @@ endif()
 option(PYBIND11_INSTALL "Install pybind11 header files?" ${PYBIND11_MASTER_PROJECT})
 option(PYBIND11_TEST "Build pybind11 test suite?" ${PYBIND11_MASTER_PROJECT})
 option(PYBIND11_NOPYTHON "Disable search for Python" OFF)
+option(PYBIND11_SIMPLE_GIL_MANAGEMENT
+       "Use simpler GIL management logic that does not support disassociation" OFF)
 set(PYBIND11_INTERNALS_VERSION
     ""
     CACHE STRING "Override the ABI version, may be used to enable the unstable ABI.")
 
+if(PYBIND11_SIMPLE_GIL_MANAGEMENT)
+  add_compile_definitions(PYBIND11_SIMPLE_GIL_MANAGEMENT)
+endif()
+
 cmake_dependent_option(
   USE_PYTHON_INCLUDE_DIR
   "Install pybind11 headers in Python include directory instead of default installation prefix"

include/pybind11/detail/common.h

@@ -206,6 +206,7 @@
 #endif
 
 #include <Python.h>
+// Reminder: WITH_THREAD is always defined if PY_VERSION_HEX >= 0x03070000
 #if PY_VERSION_HEX < 0x03060000
 #    error "PYTHON < 3.6 IS UNSUPPORTED. pybind11 v2.9 was the last to support Python 2 and 3.5."
 #endif
@@ -229,6 +230,10 @@
 #    undef copysign
 #endif
 
+#if defined(PYPY_VERSION) && !defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
+#    define PYBIND11_SIMPLE_GIL_MANAGEMENT
+#endif
+
 #if defined(_MSC_VER)
 #    if defined(PYBIND11_DEBUG_MARKER)
 #        define _DEBUG

include/pybind11/detail/internals.h

@@ -9,6 +9,12 @@
 
 #pragma once
 
+#include "common.h"
+
+#if defined(WITH_THREAD) && defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
+#    include "../gil.h"
+#endif
+
 #include "../pytypes.h"
 
 #include <exception>
@@ -49,7 +55,7 @@ inline PyObject *make_object_base_type(PyTypeObject *metaclass);
 // `Py_LIMITED_API` anyway.
 #    if PYBIND11_INTERNALS_VERSION > 4
 #        define PYBIND11_TLS_KEY_REF Py_tss_t &
-#        ifdef __GNUC__
+#        if defined(__GNUC__) && !defined(__INTEL_COMPILER)
 // Clang on macOS warns due to `Py_tss_NEEDS_INIT` not specifying an initializer
 // for every field.
 #            define PYBIND11_TLS_KEY_INIT(var)                                                    \
@@ -169,10 +175,12 @@ struct internals {
     PyTypeObject *default_metaclass;
     PyObject *instance_base;
 #if defined(WITH_THREAD)
+    // Unused if PYBIND11_SIMPLE_GIL_MANAGEMENT is defined:
     PYBIND11_TLS_KEY_INIT(tstate)
 #    if PYBIND11_INTERNALS_VERSION > 4
     PYBIND11_TLS_KEY_INIT(loader_life_support_tls_key)
 #    endif // PYBIND11_INTERNALS_VERSION > 4
+    // Unused if PYBIND11_SIMPLE_GIL_MANAGEMENT is defined:
     PyInterpreterState *istate = nullptr;
     ~internals() {
 #    if PYBIND11_INTERNALS_VERSION > 4
@@ -408,6 +416,10 @@ PYBIND11_NOINLINE internals &get_internals() {
         return **internals_pp;
     }
 
+#if defined(WITH_THREAD)
+#    if defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
+    gil_scoped_acquire gil;
+#    else
     // Ensure that the GIL is held since we will need to make Python calls.
     // Cannot use py::gil_scoped_acquire here since that constructor calls get_internals.
     struct gil_scoped_acquire_local {
@@ -417,6 +429,8 @@ PYBIND11_NOINLINE internals &get_internals() {
         ~gil_scoped_acquire_local() { PyGILState_Release(state); }
         const PyGILState_STATE state;
     } gil;
+#    endif
+#endif
     error_scope err_scope;
 
     PYBIND11_STR_TYPE id(PYBIND11_INTERNALS_ID);

include/pybind11/gil.h

@@ -10,7 +10,10 @@
 #pragma once
 
 #include "detail/common.h"
-#include "detail/internals.h"
+
+#if defined(WITH_THREAD) && !defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
+#    include "detail/internals.h"
+#endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
@@ -21,7 +24,9 @@ PyThreadState *get_thread_state_unchecked();
 
 PYBIND11_NAMESPACE_END(detail)
 
-#if defined(WITH_THREAD) && !defined(PYPY_VERSION)
+#if defined(WITH_THREAD)
+
+#    if !defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
 
 /* The functions below essentially reproduce the PyGILState_* API using a RAII
  * pattern, but there are a few important differences:
@@ -62,11 +67,11 @@ public:
 
         if (!tstate) {
             tstate = PyThreadState_New(internals.istate);
-#    if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+#        if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
             if (!tstate) {
                 pybind11_fail("scoped_acquire: could not create thread state!");
             }
-#    endif
+#        endif
             tstate->gilstate_counter = 0;
             PYBIND11_TLS_REPLACE_VALUE(internals.tstate, tstate);
         } else {
@@ -87,20 +92,20 @@ public:
 
     PYBIND11_NOINLINE void dec_ref() {
         --tstate->gilstate_counter;
-#    if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+#        if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
         if (detail::get_thread_state_unchecked() != tstate) {
             pybind11_fail("scoped_acquire::dec_ref(): thread state must be current!");
         }
         if (tstate->gilstate_counter < 0) {
             pybind11_fail("scoped_acquire::dec_ref(): reference count underflow!");
         }
-#    endif
+#        endif
         if (tstate->gilstate_counter == 0) {
-#    if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+#        if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
             if (!release) {
                 pybind11_fail("scoped_acquire::dec_ref(): internal error!");
             }
-#    endif
+#        endif
             PyThreadState_Clear(tstate);
             if (active) {
                 PyThreadState_DeleteCurrent();
@@ -178,12 +183,14 @@ private:
     bool disassoc;
     bool active = true;
 };
-#elif defined(PYPY_VERSION)
+
+#    else // PYBIND11_SIMPLE_GIL_MANAGEMENT
+
 class gil_scoped_acquire {
     PyGILState_STATE state;
 
 public:
-    gil_scoped_acquire() { state = PyGILState_Ensure(); }
+    gil_scoped_acquire() : state{PyGILState_Ensure()} {}
     gil_scoped_acquire(const gil_scoped_acquire &) = delete;
     gil_scoped_acquire &operator=(const gil_scoped_acquire &) = delete;
     ~gil_scoped_acquire() { PyGILState_Release(state); }
@@ -194,19 +201,39 @@ class gil_scoped_release {
     PyThreadState *state;
 
 public:
-    gil_scoped_release() { state = PyEval_SaveThread(); }
+    gil_scoped_release() : state{PyEval_SaveThread()} {}
     gil_scoped_release(const gil_scoped_release &) = delete;
     gil_scoped_release &operator=(const gil_scoped_acquire &) = delete;
     ~gil_scoped_release() { PyEval_RestoreThread(state); }
     void disarm() {}
 };
-#else
+
+#    endif // PYBIND11_SIMPLE_GIL_MANAGEMENT
+
+#else // WITH_THREAD
+
 class gil_scoped_acquire {
+public:
+    gil_scoped_acquire() {
+        // Trick to suppress `unused variable` error messages (at call sites).
+        (void) (this != (this + 1));
+    }
+    gil_scoped_acquire(const gil_scoped_acquire &) = delete;
+    gil_scoped_acquire &operator=(const gil_scoped_acquire &) = delete;
     void disarm() {}
 };
+
 class gil_scoped_release {
+public:
+    gil_scoped_release() {
+        // Trick to suppress `unused variable` error messages (at call sites).
+        (void) (this != (this + 1));
+    }
+    gil_scoped_release(const gil_scoped_release &) = delete;
+    gil_scoped_release &operator=(const gil_scoped_acquire &) = delete;
     void disarm() {}
 };
-#endif
+
+#endif // WITH_THREAD
 
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

tests/conftest.py

@@ -210,4 +210,5 @@ def pytest_report_header(config):
         f" {pybind11_tests.compiler_info}"
         f" {pybind11_tests.cpp_std}"
         f" {pybind11_tests.PYBIND11_INTERNALS_ID}"
+        f" PYBIND11_SIMPLE_GIL_MANAGEMENT={pybind11_tests.PYBIND11_SIMPLE_GIL_MANAGEMENT}"
     )

tests/cross_module_gil_utils.cpp

@@ -6,9 +6,15 @@
     All rights reserved. Use of this source code is governed by a
     BSD-style license that can be found in the LICENSE file.
 */
+#if defined(PYBIND11_INTERNALS_VERSION)
+#    undef PYBIND11_INTERNALS_VERSION
+#endif
+#define PYBIND11_INTERNALS_VERSION 21814642 // Ensure this module has its own `internals` instance.
 #include <pybind11/pybind11.h>
 
 #include <cstdint>
+#include <string>
+#include <thread>
 
 // This file mimics a DSO that makes pybind11 calls but does not define a
 // PYBIND11_MODULE. The purpose is to test that such a DSO can create a
@@ -21,8 +27,54 @@
 namespace {
 
 namespace py = pybind11;
+
 void gil_acquire() { py::gil_scoped_acquire gil; }
 
+std::string gil_multi_acquire_release(unsigned bits) {
+    if ((bits & 0x1u) != 0u) {
+        py::gil_scoped_acquire gil;
+    }
+    if ((bits & 0x2u) != 0u) {
+        py::gil_scoped_release gil;
+    }
+    if ((bits & 0x4u) != 0u) {
+        py::gil_scoped_acquire gil;
+    }
+    if ((bits & 0x8u) != 0u) {
+        py::gil_scoped_release gil;
+    }
+    return PYBIND11_INTERNALS_ID;
+}
+
+struct CustomAutoGIL {
+    CustomAutoGIL() : gstate(PyGILState_Ensure()) {}
+    ~CustomAutoGIL() { PyGILState_Release(gstate); }
+
+    PyGILState_STATE gstate;
+};
+struct CustomAutoNoGIL {
+    CustomAutoNoGIL() : save(PyEval_SaveThread()) {}
+    ~CustomAutoNoGIL() { PyEval_RestoreThread(save); }
+
+    PyThreadState *save;
+};
+
+template <typename Acquire, typename Release>
+void gil_acquire_inner() {
+    Acquire acquire_outer;
+    Acquire acquire_inner;
+    Release release;
+}
+
+template <typename Acquire, typename Release>
+void gil_acquire_nested() {
+    Acquire acquire_outer;
+    Acquire acquire_inner;
+    Release release;
+    auto thread = std::thread(&gil_acquire_inner<Acquire, Release>);
+    thread.join();
+}
+
 constexpr char kModuleName[] = "cross_module_gil_utils";
 
 struct PyModuleDef moduledef = {
@@ -30,6 +82,9 @@ struct PyModuleDef moduledef = {
 
 } // namespace
 
+#define ADD_FUNCTION(Name, ...)                                                                   \
+    PyModule_AddObject(m, Name, PyLong_FromVoidPtr(reinterpret_cast<void *>(&__VA_ARGS__)));
+
 extern "C" PYBIND11_EXPORT PyObject *PyInit_cross_module_gil_utils() {
 
     PyObject *m = PyModule_Create(&moduledef);
@@ -37,8 +92,16 @@ extern "C" PYBIND11_EXPORT PyObject *PyInit_cross_module_gil_utils() {
     if (m != nullptr) {
         static_assert(sizeof(&gil_acquire) == sizeof(void *),
                       "Function pointer must have the same size as void*");
-        PyModule_AddObject(
+        ADD_FUNCTION("gil_acquire_funcaddr", gil_acquire)
-            m, "gil_acquire_funcaddr", PyLong_FromVoidPtr(reinterpret_cast<void *>(&gil_acquire)));
+        ADD_FUNCTION("gil_multi_acquire_release_funcaddr", gil_multi_acquire_release)
+        ADD_FUNCTION("gil_acquire_inner_custom_funcaddr",
+                     gil_acquire_inner<CustomAutoGIL, CustomAutoNoGIL>)
+        ADD_FUNCTION("gil_acquire_nested_custom_funcaddr",
+                     gil_acquire_nested<CustomAutoGIL, CustomAutoNoGIL>)
+        ADD_FUNCTION("gil_acquire_inner_pybind11_funcaddr",
+                     gil_acquire_inner<py::gil_scoped_acquire, py::gil_scoped_release>)
+        ADD_FUNCTION("gil_acquire_nested_pybind11_funcaddr",
+                     gil_acquire_nested<py::gil_scoped_acquire, py::gil_scoped_release>)
     }
 
     return m;

tests/pybind11_tests.cpp

@@ -89,6 +89,12 @@ PYBIND11_MODULE(pybind11_tests, m) {
 #endif
     m.attr("cpp_std") = cpp_std();
     m.attr("PYBIND11_INTERNALS_ID") = PYBIND11_INTERNALS_ID;
+    m.attr("PYBIND11_SIMPLE_GIL_MANAGEMENT") =
+#if defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
+        true;
+#else
+        false;
+#endif
 
     bind_ConstructorStats(m);
 

tests/test_embed/test_interpreter.cpp

@@ -293,7 +293,6 @@ TEST_CASE("Threads") {
 
     {
         py::gil_scoped_release gil_release{};
-        REQUIRE(has_pybind11_internals_static());
 
         auto threads = std::vector<std::thread>();
         for (auto i = 0; i < num_threads; ++i) {

tests/test_gil_scoped.cpp

@@ -11,6 +11,13 @@
 
 #include "pybind11_tests.h"
 
+#include <string>
+#include <thread>
+
+#define CROSS_MODULE(Function)                                                                    \
+    auto cm = py::module_::import("cross_module_gil_utils");                                      \
+    auto target = reinterpret_cast<void (*)()>(PyLong_AsVoidPtr(cm.attr(Function).ptr()));
+
 class VirtClass {
 public:
     virtual ~VirtClass() = default;
@@ -28,6 +35,16 @@ class PyVirtClass : public VirtClass {
 };
 
 TEST_SUBMODULE(gil_scoped, m) {
+    m.attr("defined_THREAD_SANITIZER") =
+#if defined(THREAD_SANITIZER)
+        true;
+#else
+        false;
+#endif
+
+    m.def("intentional_deadlock",
+          []() { std::thread([]() { py::gil_scoped_acquire gil_acquired; }).join(); });
+
     py::class_<VirtClass, PyVirtClass>(m, "VirtClass")
         .def(py::init<>())
         .def("virtual_func", &VirtClass::virtual_func)
@@ -37,11 +54,91 @@ TEST_SUBMODULE(gil_scoped, m) {
     m.def("test_callback_std_func", [](const std::function<void()> &func) { func(); });
     m.def("test_callback_virtual_func", [](VirtClass &virt) { virt.virtual_func(); });
     m.def("test_callback_pure_virtual_func", [](VirtClass &virt) { virt.pure_virtual_func(); });
-    m.def("test_cross_module_gil", []() {
+    m.def("test_cross_module_gil_released", []() {
-        auto cm = py::module_::import("cross_module_gil_utils");
+        CROSS_MODULE("gil_acquire_funcaddr")
-        auto gil_acquire = reinterpret_cast<void (*)()>(
-            PyLong_AsVoidPtr(cm.attr("gil_acquire_funcaddr").ptr()));
         py::gil_scoped_release gil_release;
-        gil_acquire();
+        target();
+    });
+    m.def("test_cross_module_gil_acquired", []() {
+        CROSS_MODULE("gil_acquire_funcaddr")
+        py::gil_scoped_acquire gil_acquire;
+        target();
+    });
+    m.def("test_cross_module_gil_inner_custom_released", []() {
+        CROSS_MODULE("gil_acquire_inner_custom_funcaddr")
+        py::gil_scoped_release gil_release;
+        target();
+    });
+    m.def("test_cross_module_gil_inner_custom_acquired", []() {
+        CROSS_MODULE("gil_acquire_inner_custom_funcaddr")
+        py::gil_scoped_acquire gil_acquire;
+        target();
+    });
+    m.def("test_cross_module_gil_inner_pybind11_released", []() {
+        CROSS_MODULE("gil_acquire_inner_pybind11_funcaddr")
+        py::gil_scoped_release gil_release;
+        target();
+    });
+    m.def("test_cross_module_gil_inner_pybind11_acquired", []() {
+        CROSS_MODULE("gil_acquire_inner_pybind11_funcaddr")
+        py::gil_scoped_acquire gil_acquire;
+        target();
+    });
+    m.def("test_cross_module_gil_nested_custom_released", []() {
+        CROSS_MODULE("gil_acquire_nested_custom_funcaddr")
+        py::gil_scoped_release gil_release;
+        target();
+    });
+    m.def("test_cross_module_gil_nested_custom_acquired", []() {
+        CROSS_MODULE("gil_acquire_nested_custom_funcaddr")
+        py::gil_scoped_acquire gil_acquire;
+        target();
+    });
+    m.def("test_cross_module_gil_nested_pybind11_released", []() {
+        CROSS_MODULE("gil_acquire_nested_pybind11_funcaddr")
+        py::gil_scoped_release gil_release;
+        target();
+    });
+    m.def("test_cross_module_gil_nested_pybind11_acquired", []() {
+        CROSS_MODULE("gil_acquire_nested_pybind11_funcaddr")
+        py::gil_scoped_acquire gil_acquire;
+        target();
+    });
+    m.def("test_release_acquire", [](const py::object &obj) {
+        py::gil_scoped_release gil_released;
+        py::gil_scoped_acquire gil_acquired;
+        return py::str(obj);
+    });
+    m.def("test_nested_acquire", [](const py::object &obj) {
+        py::gil_scoped_release gil_released;
+        py::gil_scoped_acquire gil_acquired_outer;
+        py::gil_scoped_acquire gil_acquired_inner;
+        return py::str(obj);
+    });
+    m.def("test_multi_acquire_release_cross_module", [](unsigned bits) {
+        py::set internals_ids;
+        internals_ids.add(PYBIND11_INTERNALS_ID);
+        {
+            py::gil_scoped_release gil_released;
+            auto thread_f = [bits, &internals_ids]() {
+                py::gil_scoped_acquire gil_acquired;
+                auto cm = py::module_::import("cross_module_gil_utils");
+                auto target = reinterpret_cast<std::string (*)(unsigned)>(
+                    PyLong_AsVoidPtr(cm.attr("gil_multi_acquire_release_funcaddr").ptr()));
+                std::string cm_internals_id = target(bits >> 3);
+                internals_ids.add(cm_internals_id);
+            };
+            if ((bits & 0x1u) != 0u) {
+                thread_f();
+            }
+            if ((bits & 0x2u) != 0u) {
+                std::thread non_python_thread(thread_f);
+                non_python_thread.join();
+            }
+            if ((bits & 0x4u) != 0u) {
+                thread_f();
+            }
+        }
+        return internals_ids;
     });
 }

tests/test_gil_scoped.py

@@ -1,45 +1,199 @@
 import multiprocessing
+import sys
 import threading
+import time
+
+import pytest
 
 from pybind11_tests import gil_scoped as m
 
 
+class ExtendedVirtClass(m.VirtClass):
+    def virtual_func(self):
+        pass
+
+    def pure_virtual_func(self):
+        pass
+
+
+def test_callback_py_obj():
+    m.test_callback_py_obj(lambda: None)
+
+
+def test_callback_std_func():
+    m.test_callback_std_func(lambda: None)
+
+
+def test_callback_virtual_func():
+    extended = ExtendedVirtClass()
+    m.test_callback_virtual_func(extended)
+
+
+def test_callback_pure_virtual_func():
+    extended = ExtendedVirtClass()
+    m.test_callback_pure_virtual_func(extended)
+
+
+def test_cross_module_gil_released():
+    """Makes sure that the GIL can be acquired by another module from a GIL-released state."""
+    m.test_cross_module_gil_released()  # Should not raise a SIGSEGV
+
+
+def test_cross_module_gil_acquired():
+    """Makes sure that the GIL can be acquired by another module from a GIL-acquired state."""
+    m.test_cross_module_gil_acquired()  # Should not raise a SIGSEGV
+
+
+def test_cross_module_gil_inner_custom_released():
+    """Makes sure that the GIL can be acquired/released by another module
+    from a GIL-released state using custom locking logic."""
+    m.test_cross_module_gil_inner_custom_released()
+
+
+def test_cross_module_gil_inner_custom_acquired():
+    """Makes sure that the GIL can be acquired/acquired by another module
+    from a GIL-acquired state using custom locking logic."""
+    m.test_cross_module_gil_inner_custom_acquired()
+
+
+def test_cross_module_gil_inner_pybind11_released():
+    """Makes sure that the GIL can be acquired/released by another module
+    from a GIL-released state using pybind11 locking logic."""
+    m.test_cross_module_gil_inner_pybind11_released()
+
+
+def test_cross_module_gil_inner_pybind11_acquired():
+    """Makes sure that the GIL can be acquired/acquired by another module
+    from a GIL-acquired state using pybind11 locking logic."""
+    m.test_cross_module_gil_inner_pybind11_acquired()
+
+
+def test_cross_module_gil_nested_custom_released():
+    """Makes sure that the GIL can be nested acquired/released by another module
+    from a GIL-released state using custom locking logic."""
+    m.test_cross_module_gil_nested_custom_released()
+
+
+def test_cross_module_gil_nested_custom_acquired():
+    """Makes sure that the GIL can be nested acquired/acquired by another module
+    from a GIL-acquired state using custom locking logic."""
+    m.test_cross_module_gil_nested_custom_acquired()
+
+
+def test_cross_module_gil_nested_pybind11_released():
+    """Makes sure that the GIL can be nested acquired/released by another module
+    from a GIL-released state using pybind11 locking logic."""
+    m.test_cross_module_gil_nested_pybind11_released()
+
+
+def test_cross_module_gil_nested_pybind11_acquired():
+    """Makes sure that the GIL can be nested acquired/acquired by another module
+    from a GIL-acquired state using pybind11 locking logic."""
+    m.test_cross_module_gil_nested_pybind11_acquired()
+
+
+def test_release_acquire():
+    assert m.test_release_acquire(0xAB) == "171"
+
+
+def test_nested_acquire():
+    assert m.test_nested_acquire(0xAB) == "171"
+
+
+def test_multi_acquire_release_cross_module():
+    for bits in range(16 * 8):
+        internals_ids = m.test_multi_acquire_release_cross_module(bits)
+        assert len(internals_ids) == 2 if bits % 8 else 1
+
+
+# Intentionally putting human review in the loop here, to guard against accidents.
+VARS_BEFORE_ALL_BASIC_TESTS = dict(vars())  # Make a copy of the dict (critical).
+ALL_BASIC_TESTS = (
+    test_callback_py_obj,
+    test_callback_std_func,
+    test_callback_virtual_func,
+    test_callback_pure_virtual_func,
+    test_cross_module_gil_released,
+    test_cross_module_gil_acquired,
+    test_cross_module_gil_inner_custom_released,
+    test_cross_module_gil_inner_custom_acquired,
+    test_cross_module_gil_inner_pybind11_released,
+    test_cross_module_gil_inner_pybind11_acquired,
+    test_cross_module_gil_nested_custom_released,
+    test_cross_module_gil_nested_custom_acquired,
+    test_cross_module_gil_nested_pybind11_released,
+    test_cross_module_gil_nested_pybind11_acquired,
+    test_release_acquire,
+    test_nested_acquire,
+    test_multi_acquire_release_cross_module,
+)
+
+
+def test_all_basic_tests_completeness():
+    num_found = 0
+    for key, value in VARS_BEFORE_ALL_BASIC_TESTS.items():
+        if not key.startswith("test_"):
+            continue
+        assert value in ALL_BASIC_TESTS
+        num_found += 1
+    assert len(ALL_BASIC_TESTS) == num_found
+
+
+def _intentional_deadlock():
+    m.intentional_deadlock()
+
+
+ALL_BASIC_TESTS_PLUS_INTENTIONAL_DEADLOCK = ALL_BASIC_TESTS + (_intentional_deadlock,)
+SKIP_IF_DEADLOCK = True  # See PR #4216
+
+
 def _run_in_process(target, *args, **kwargs):
-    """Runs target in process and returns its exitcode after 10s (None if still alive)."""
+    if len(args) == 0:
+        test_fn = target
+    else:
+        test_fn = args[0]
+    # Do not need to wait much, 10s should be more than enough.
+    timeout = 0.1 if test_fn is _intentional_deadlock else 10
     process = multiprocessing.Process(target=target, args=args, kwargs=kwargs)
     process.daemon = True
     try:
+        t_start = time.time()
         process.start()
-        # Do not need to wait much, 10s should be more than enough.
+        if timeout >= 100:  # For debugging.
-        process.join(timeout=10)
+            print(
+                "\nprocess.pid STARTED", process.pid, (sys.argv, target, args, kwargs)
+            )
+            print(f"COPY-PASTE-THIS: gdb {sys.argv[0]} -p {process.pid}", flush=True)
+        process.join(timeout=timeout)
+        if timeout >= 100:
+            print("\nprocess.pid JOINED", process.pid, flush=True)
+        t_delta = time.time() - t_start
+        if process.exitcode == 66 and m.defined_THREAD_SANITIZER:  # Issue #2754
+            # WOULD-BE-NICE-TO-HAVE: Check that the message below is actually in the output.
+            # Maybe this could work:
+            # https://gist.github.com/alexeygrigorev/01ce847f2e721b513b42ea4a6c96905e
+            pytest.skip(
+                "ThreadSanitizer: starting new threads after multi-threaded fork is not supported."
+            )
+        elif test_fn is _intentional_deadlock:
+            assert process.exitcode is None
+            return 0
+        elif process.exitcode is None:
+            assert t_delta > 0.9 * timeout
+            msg = "DEADLOCK, most likely, exactly what this test is meant to detect."
+            if SKIP_IF_DEADLOCK:
+                pytest.skip(msg)
+            raise RuntimeError(msg)
         return process.exitcode
     finally:
         if process.is_alive():
             process.terminate()
 
 
-def _python_to_cpp_to_python():
+def _run_in_threads(test_fn, num_threads, parallel):
-    """Calls different C++ functions that come back to Python."""
-
-    class ExtendedVirtClass(m.VirtClass):
-        def virtual_func(self):
-            pass
-
-        def pure_virtual_func(self):
-            pass
-
-    extended = ExtendedVirtClass()
-    m.test_callback_py_obj(lambda: None)
-    m.test_callback_std_func(lambda: None)
-    m.test_callback_virtual_func(extended)
-    m.test_callback_pure_virtual_func(extended)
-
-
-def _python_to_cpp_to_python_from_threads(num_threads, parallel=False):
-    """Calls different C++ functions that come back to Python, from Python threads."""
     threads = []
     for _ in range(num_threads):
-        thread = threading.Thread(target=_python_to_cpp_to_python)
+        thread = threading.Thread(target=test_fn)
         thread.daemon = True
         thread.start()
         if parallel:
@@ -51,43 +205,40 @@ def _python_to_cpp_to_python_from_threads(num_threads, parallel=False):
 
 
 # TODO: FIXME, sometimes returns -11 (segfault) instead of 0 on macOS Python 3.9
-def test_python_to_cpp_to_python_from_thread():
+@pytest.mark.parametrize("test_fn", ALL_BASIC_TESTS_PLUS_INTENTIONAL_DEADLOCK)
+def test_run_in_process_one_thread(test_fn):
     """Makes sure there is no GIL deadlock when running in a thread.
 
     It runs in a separate process to be able to stop and assert if it deadlocks.
     """
-    assert _run_in_process(_python_to_cpp_to_python_from_threads, 1) == 0
+    assert _run_in_process(_run_in_threads, test_fn, num_threads=1, parallel=False) == 0
 
 
 # TODO: FIXME on macOS Python 3.9
-def test_python_to_cpp_to_python_from_thread_multiple_parallel():
+@pytest.mark.parametrize("test_fn", ALL_BASIC_TESTS_PLUS_INTENTIONAL_DEADLOCK)
+def test_run_in_process_multiple_threads_parallel(test_fn):
     """Makes sure there is no GIL deadlock when running in a thread multiple times in parallel.
 
     It runs in a separate process to be able to stop and assert if it deadlocks.
     """
-    assert _run_in_process(_python_to_cpp_to_python_from_threads, 8, parallel=True) == 0
+    assert _run_in_process(_run_in_threads, test_fn, num_threads=8, parallel=True) == 0
 
 
 # TODO: FIXME on macOS Python 3.9
-def test_python_to_cpp_to_python_from_thread_multiple_sequential():
+@pytest.mark.parametrize("test_fn", ALL_BASIC_TESTS_PLUS_INTENTIONAL_DEADLOCK)
+def test_run_in_process_multiple_threads_sequential(test_fn):
     """Makes sure there is no GIL deadlock when running in a thread multiple times sequentially.
 
     It runs in a separate process to be able to stop and assert if it deadlocks.
     """
-    assert (
+    assert _run_in_process(_run_in_threads, test_fn, num_threads=8, parallel=False) == 0
-        _run_in_process(_python_to_cpp_to_python_from_threads, 8, parallel=False) == 0
-    )
 
 
 # TODO: FIXME on macOS Python 3.9
-def test_python_to_cpp_to_python_from_process():
+@pytest.mark.parametrize("test_fn", ALL_BASIC_TESTS_PLUS_INTENTIONAL_DEADLOCK)
+def test_run_in_process_direct(test_fn):
     """Makes sure there is no GIL deadlock when using processes.
 
     This test is for completion, but it was never an issue.
     """
-    assert _run_in_process(_python_to_cpp_to_python) == 0
+    assert _run_in_process(test_fn) == 0
-
-
-def test_cross_module_gil():
-    """Makes sure that the GIL can be acquired by another module from a GIL-released state."""
-    m.test_cross_module_gil()  # Should not raise a SIGSEGV