PyGILState_Ensure() deadlocks, why?

[Tomas Ukkonen]

Hello
Is this python c api bug? The following C++ code (g++) deadlocks on Ubuntu Linux.

/* * This code deadlocks on python3-dev 3.12.3 (ubuntu 24.04 lts)
 *
 * g++ python_thread_test.cpp `python3-config --cflags --libs --embed`
 * ./a.out
 *
 * uname:
 * Linux softice 6.8.0-36-generic SMP PREEMPT_DYNAMIC x86_64 GNU/Linux
 */

#include <Python.h>
#include <thread>
#include <vector>
#include <iostream>

void perform_long_operation() {
    // Simulate a long-running task
    std::this_thread::sleep_for(std::chrono::seconds(5));
}

void thread_function() {
    // Ensure this thread has the GIL
    PyGILState_STATE gstate = PyGILState_Ensure();

    // Execute some Python code
    PyRun_SimpleString("print('Hello from std::thread!')");

    // Release the GIL for long operation
    Py_BEGIN_ALLOW_THREADS
    perform_long_operation();
    Py_END_ALLOW_THREADS

    // Re-acquire the GIL and execute more Python code
    gstate = PyGILState_Ensure();
    PyRun_SimpleString("print('Thread operation completed!')");

    // Release the GIL
    PyGILState_Release(gstate);
}

int main() {
    // Initialize the Python Interpreter
    Py_Initialize();

    // Create a vector of threads
    std::vector<std::thread> threads;

    // Launch threads
    for (int i = 0; i < 5; ++i) {
        threads.push_back(std::thread(thread_function));
    }

    // Join threads
    for (auto& t : threads) {
        t.join();
    }

    // Finalize the Python Interpreter
    Py_Finalize();

    return 0;
}



Tomas Ukkonen
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