pybind11/include/pybind11/gil.h

/*
    pybind11/gil.h: RAII helpers for managing the GIL

    Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>

    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 "detail/common.h"
#include "detail/internals.h"

PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)


PYBIND11_NAMESPACE_BEGIN(detail)

// forward declarations
PyThreadState *get_thread_state_unchecked();

PYBIND11_NAMESPACE_END(detail)


#if defined(WITH_THREAD) && !defined(PYPY_VERSION)

/* The functions below essentially reproduce the PyGILState_* API using a RAII
 * pattern, but there are a few important differences:
 *
 * 1. When acquiring the GIL from an non-main thread during the finalization
 *    phase, the GILState API blindly terminates the calling thread, which
 *    is often not what is wanted. This API does not do this.
 *
 * 2. The gil_scoped_release function can optionally cut the relationship
 *    of a PyThreadState and its associated thread, which allows moving it to
 *    another thread (this is a fairly rare/advanced use case).
 *
 * 3. The reference count of an acquired thread state can be controlled. This
 *    can be handy to prevent cases where callbacks issued from an external
 *    thread would otherwise constantly construct and destroy thread state data
 *    structures.
 *
 * See the Python bindings of NanoGUI (http://github.com/wjakob/nanogui) for an
 * example which uses features 2 and 3 to migrate the Python thread of
 * execution to another thread (to run the event loop on the original thread,
 * in this case).
 */

class gil_scoped_acquire {
public:
    PYBIND11_NOINLINE gil_scoped_acquire() {
        auto const &internals = detail::get_internals();
        tstate = (PyThreadState *) PYBIND11_TLS_GET_VALUE(internals.tstate);

        if (!tstate) {
            /* Check if the GIL was acquired using the PyGILState_* API instead (e.g. if
               calling from a Python thread). Since we use a different key, this ensures
               we don't create a new thread state and deadlock in PyEval_AcquireThread
               below. Note we don't save this state with internals.tstate, since we don't
               create it we would fail to clear it (its reference count should be > 0). */
            tstate = PyGILState_GetThisThreadState();
        }

        if (!tstate) {
            tstate = PyThreadState_New(internals.istate);
            #if !defined(NDEBUG)
                if (!tstate)
                    pybind11_fail("scoped_acquire: could not create thread state!");
            #endif
            tstate->gilstate_counter = 0;
            PYBIND11_TLS_REPLACE_VALUE(internals.tstate, tstate);
        } else {
            release = detail::get_thread_state_unchecked() != tstate;
        }

        if (release) {
            PyEval_AcquireThread(tstate);
        }

        inc_ref();
    }

    void inc_ref() {
        ++tstate->gilstate_counter;
    }

    PYBIND11_NOINLINE void dec_ref() {
        --tstate->gilstate_counter;
        #if !defined(NDEBUG)
            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
        if (tstate->gilstate_counter == 0) {
            #if !defined(NDEBUG)
                if (!release)
                    pybind11_fail("scoped_acquire::dec_ref(): internal error!");
            #endif
            PyThreadState_Clear(tstate);
            if (active)
                PyThreadState_DeleteCurrent();
            PYBIND11_TLS_DELETE_VALUE(detail::get_internals().tstate);
            release = false;
        }
    }

    /// This method will disable the PyThreadState_DeleteCurrent call and the
    /// GIL won't be acquired. This method should be used if the interpreter
    /// could be shutting down when this is called, as thread deletion is not
    /// allowed during shutdown. Check _Py_IsFinalizing() on Python 3.7+, and
    /// protect subsequent code.
    PYBIND11_NOINLINE void disarm() {
        active = false;
    }

    PYBIND11_NOINLINE ~gil_scoped_acquire() {
        dec_ref();
        if (release)
           PyEval_SaveThread();
    }
private:
    PyThreadState *tstate = nullptr;
    bool release = true;
    bool active = true;
};

class gil_scoped_release {
public:
    explicit gil_scoped_release(bool disassoc = false) : disassoc(disassoc) {
        // `get_internals()` must be called here unconditionally in order to initialize
        // `internals.tstate` for subsequent `gil_scoped_acquire` calls. Otherwise, an
        // initialization race could occur as multiple threads try `gil_scoped_acquire`.
        const auto &internals = detail::get_internals();
        tstate = PyEval_SaveThread();
        if (disassoc) {
            auto key = internals.tstate;
            PYBIND11_TLS_DELETE_VALUE(key);
        }
    }

    /// This method will disable the PyThreadState_DeleteCurrent call and the
    /// GIL won't be acquired. This method should be used if the interpreter
    /// could be shutting down when this is called, as thread deletion is not
    /// allowed during shutdown. Check _Py_IsFinalizing() on Python 3.7+, and
    /// protect subsequent code.
    PYBIND11_NOINLINE void disarm() {
        active = false;
    }

    ~gil_scoped_release() {
        if (!tstate)
            return;
        // `PyEval_RestoreThread()` should not be called if runtime is finalizing
        if (active)
            PyEval_RestoreThread(tstate);
        if (disassoc) {
            auto key = detail::get_internals().tstate;
            PYBIND11_TLS_REPLACE_VALUE(key, tstate);
        }
    }
private:
    PyThreadState *tstate;
    bool disassoc;
    bool active = true;
};
#elif defined(PYPY_VERSION)
class gil_scoped_acquire {
    PyGILState_STATE state;
public:
    gil_scoped_acquire() { state = PyGILState_Ensure(); }
    ~gil_scoped_acquire() { PyGILState_Release(state); }
    void disarm() {}
};

class gil_scoped_release {
    PyThreadState *state;
public:
    gil_scoped_release() { state = PyEval_SaveThread(); }
    ~gil_scoped_release() { PyEval_RestoreThread(state); }
    void disarm() {}
};
#else
class gil_scoped_acquire {
    void disarm() {}
};
class gil_scoped_release {
    void disarm() {}
};
#endif

PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)