feat: to get the running event-loop in Python 3.13 is as simple as thread_state->asyncio_running_loop

Author: Xmader

src/PyEventLoop.cc

@@ -147,17 +147,26 @@ PyEventLoop PyEventLoop::_getLoopOnThread(PyThreadState *tstate) {
   // However, simply replacing it with `PyThreadState_GetDict()` does not work,
   //   since the public `PyThreadState_GetDict()` API can only get from the current thread.
   // We need to somehow get the thread dictionary on the main thread instead of the current thread.
-  if (!tstate->dict) {
-    // Modified from https://github.com/python/cpython/blob/v3.13.0rc1/Python/pystate.c#L1934-L1951
-    tstate->dict = PyDict_New();
-    if (tstate->dict == NULL) { // when it's still null, no per-thread state is available, and an exception should not be raised
-      PyObject *old_exc = tstate->current_exception;
-      tstate->current_exception = NULL; // _PyErr_Clear(tstate)
-      Py_XDECREF(old_exc);
+  //
+  // UPDATE: We don't need the thread dictionary anymore.
+  // To get the thread's running event-loop in Python 3.13 is as simple as `thread_state->asyncio_running_loop`
+  {
+    // Every `PyThreadState` is actually allocated with extra fields as a `_PyThreadStateImpl` struct
+    // See https://github.com/python/cpython/blob/v3.13.0rc1/Include/internal/pycore_tstate.h#L17-L24
+    using PyThreadStateHolder = struct { // _PyThreadStateImpl
+      PyThreadState base;
+      PyObject *asyncio_running_loop; // we only need the first field of `_PyThreadStateImpl`
+    };
+
+    // Modified from https://github.com/python/cpython/blob/v3.13.0rc1/Modules/_asynciomodule.c#L3205-L3210
+    PyObject *loop = ((PyThreadStateHolder *)tstate)->asyncio_running_loop;
+    if (loop == NULL) {
       return _loopNotFound();
     }
+
+    Py_INCREF(loop);
+    return PyEventLoop(loop);
   }
-  ts_dict = tstate->dict;
   #elif PY_VERSION_HEX >= 0x03090000 // Python version is greater than 3.9
   ts_dict = _PyThreadState_GetDict(tstate);  // borrowed reference
   #else // Python 3.8