gh-100344: Provide C implementation for asyncio.current_task

Implementing it in C makes it about 4x-6x faster

Microbenchmark:

```
 # bench.py

import time
import timeit
import asyncio

ITERS: int = 10**6
NANO: int = 10**9
NANO_PER_ITER: float = NANO / ITERS

async def main():
   # avoid load attr noise
   py_current_task = asyncio.tasks._py_current_task
   c_current_task = asyncio.tasks._c_current_task

   asyncio.current_task() # warmup
   py_current_task() # warmup
   c_current_task() # warmup

   print(
      "current_task: {}ns".format(timeit.timeit(py_current_task, number=ITERS, timer=time.process_time) * NANO_PER_ITER)
   )
   print(
      "current_task: {}ns".format(timeit.timeit(c_current_task, number=ITERS, timer=time.process_time) * NANO_PER_ITER)
   )

asyncio.run(main())
```

a few runs on MacBook Pro
2.4 GHz 8-Core Intel Core i9
64 GB 2667 MHz DDR4:

debug build:

```
~/work/pyexe/main-dbg ⌚ 9:57:34
$ ./python.exe bench.py
[py] current_task: 606.234ns
[c] current_task: 104.47699999999993ns

~/work/pyexe/main-dbg ⌚ 9:57:59
$ ./python.exe bench.py
[py] current_task: 631.856ns
[c] current_task: 110.22500000000002ns

~/work/pyexe/main-dbg ⌚ 9:58:08
$ ./python.exe bench.py
[py] current_task: 637.746ns
[c] current_task: 105.03899999999999ns

~/work/pyexe/main-dbg ⌚ 9:58:16
$ ./python.exe bench.py
[py] current_task: 621.3169999999999ns
[c] current_task: 103.01300000000002ns
```

opt build:

```
~/work/pyexe/main-opt ⌚ 10:33:17
$ ./python.exe bench.py
[py] current_task: 128.743ns
[c] current_task: 31.997999999999998ns

~/work/pyexe/main-opt ⌚ 10:33:24
$ ./python.exe bench.py
[py] current_task: 126.388ns
[c] current_task: 32.64599999999998ns

~/work/pyexe/main-opt ⌚ 10:33:26
$ ./python.exe bench.py
[py] current_task: 137.053ns
[c] current_task: 32.066999999999986ns

~/work/pyexe/main-opt ⌚ 10:33:28
$ ./python.exe bench.py
[py] current_task: 131.17800000000003ns
[c] current_task: 32.06600000000001ns
```

Author: itamaro

Doc/whatsnew/3.12.rst

@@ -225,6 +225,9 @@ asyncio
   a custom event loop factory.
   (Contributed by Kumar Aditya in :gh:`99388`.)
 
+* Add C implementation of :func:`asyncio.current_task` for 4x-6x speedup.
+  (Contributed by Itamar Ostricher and Pranav Thulasiram Bhat in :gh:`100344`.)
+
 inspect
 -------
 

Lib/asyncio/tasks.py

@@ -964,6 +964,7 @@ def _unregister_task(task):
     _all_tasks.discard(task)
 
 
+_py_current_task = current_task
 _py_register_task = _register_task
 _py_unregister_task = _unregister_task
 _py_enter_task = _enter_task
@@ -973,10 +974,12 @@ def _unregister_task(task):
 try:
     from _asyncio import (_register_task, _unregister_task,
                           _enter_task, _leave_task,
-                          _all_tasks, _current_tasks)
+                          _all_tasks, _current_tasks,
+                          current_task)
 except ImportError:
     pass
 else:
+    _c_current_task = current_task
     _c_register_task = _register_task
     _c_unregister_task = _unregister_task
     _c_enter_task = _enter_task

Misc/NEWS.d/next/Library/2022-12-19-12-18-28.gh-issue-100344.lfCqpE.rst

@@ -0,0 +1,2 @@
+Provide C implementation for :func:`asyncio.current_task` for a 4x-6x
+speedup.

Modules/_asynciomodule.c

@@ -3344,6 +3344,44 @@ _asyncio__leave_task_impl(PyObject *module, PyObject *loop, PyObject *task)
 }
 
 
+/*[clinic input]
+_asyncio.current_task
+
+    loop: object = None
+
+Return a currently executed task.
+
+[clinic start generated code]*/
+
+static PyObject *
+_asyncio_current_task_impl(PyObject *module, PyObject *loop)
+/*[clinic end generated code: output=fe15ac331a7f981a input=58910f61a5627112]*/
+{
+    PyObject *ret;
+    asyncio_state *state = get_asyncio_state(module);
+
+    if (loop == Py_None) {
+        loop = _asyncio_get_running_loop_impl(module);
+    }
+
+    if (loop == NULL) {
+        return NULL;
+    }
+
+    ret = PyDict_GetItemWithError(state->current_tasks, loop);
+    if (ret == NULL && PyErr_Occurred()) {
+        return NULL;
+    }
+    else if (ret == NULL) {
+        Py_RETURN_NONE;
+    }
+    else {
+        Py_INCREF(ret);
+        return ret;
+    }
+}
+
+
 /*********************** PyRunningLoopHolder ********************/
 
 
@@ -3599,6 +3637,7 @@ module_init(asyncio_state *state)
 PyDoc_STRVAR(module_doc, "Accelerator module for asyncio");
 
 static PyMethodDef asyncio_methods[] = {
+    _ASYNCIO_CURRENT_TASK_METHODDEF
     _ASYNCIO_GET_EVENT_LOOP_METHODDEF
     _ASYNCIO_GET_RUNNING_LOOP_METHODDEF
     _ASYNCIO__GET_RUNNING_LOOP_METHODDEF