asyncio: fix race conditions in enter_task and leave_task

The fix relies on the internal locking of PyDictObject. It
would be nice, in the future, to use an external lock or
move the store task to the loop object.

Modules/_asynciomodule.c

@@ -1969,15 +1969,13 @@ unregister_task(asyncio_state *state, PyObject *task)
 static int
 enter_task(asyncio_state *state, PyObject *loop, PyObject *task)
 {
-    PyObject *item;
-    Py_hash_t hash;
-    hash = PyObject_Hash(loop);
-    if (hash == -1) {
+    int is_insert;
+    PyObject *item = _PyDict_SetDefault(state->current_tasks, loop, task, 1,
+                                        &is_insert);
+    if (item == NULL) {
         return -1;
     }
-    item = _PyDict_GetItem_KnownHash(state->current_tasks, loop, hash);
-    if (item != NULL) {
-        Py_INCREF(item);
+    if (!is_insert) {
         PyErr_Format(
             PyExc_RuntimeError,
             "Cannot enter into task %R while another " \
@@ -1986,36 +1984,51 @@ enter_task(asyncio_state *state, PyObject *loop, PyObject *task)
         Py_DECREF(item);
         return -1;
     }
-    if (PyErr_Occurred()) {
-        return -1;
-    }
-    return _PyDict_SetItem_KnownHash(state->current_tasks, loop, task, hash);
+    assert(item == task);
+    Py_DECREF(item);
+    return 0;
 }
 
+struct task_matches_arg {
+    PyObject *task;
+    PyObject *item;
+};
+
+static int
+task_matches(PyObject *item, void *data)
+{
+    struct task_matches_arg *arg = (struct task_matches_arg *)data;
+    arg->item = Py_NewRef(item);
+    return arg->task == item;
+}
 
 static int
 leave_task(asyncio_state *state, PyObject *loop, PyObject *task)
 /*[clinic end generated code: output=0ebf6db4b858fb41 input=51296a46313d1ad8]*/
 {
-    PyObject *item;
-    Py_hash_t hash;
-    hash = PyObject_Hash(loop);
-    if (hash == -1) {
-        return -1;
-    }
-    item = _PyDict_GetItem_KnownHash(state->current_tasks, loop, hash);
-    if (item != task) {
-        if (item == NULL) {
-            /* Not entered, replace with None */
-            item = Py_None;
+    struct task_matches_arg arg;
+    arg.task = task;
+    arg.item = Py_None;
+    int res = _PyDict_DelItemIf(state->current_tasks, loop, &task_matches, &arg);
+    if (res != 0) {
+        if (PyErr_ExceptionMatches(PyExc_KeyError)) {
+            // use custom error message if loop is not present
+            goto fail;
         }
-        PyErr_Format(
-            PyExc_RuntimeError,
-            "Leaving task %R does not match the current task %R.",
-            task, item, NULL);
         return -1;
     }
-    return _PyDict_DelItem_KnownHash(state->current_tasks, loop, hash);
+    if (arg.item == task) {
+        Py_XDECREF(arg.item);
+        return 0;
+    }
+
+fail:
+    PyErr_Format(
+        PyExc_RuntimeError,
+        "Leaving task %R does not match the current task %R.",
+        task, arg.item, NULL);
+    Py_XDECREF(arg.item);
+    return -1;
 }
 
 /* ----- Task */