pythongh-108240: Fix a reference cycle in _socket.CAPI capsule

_socket.CAPI capsule contains a strong reference to _socket.socket
type. The garbage collector cannot visit this reference and so cannot
create the reference cycle involving _socket.socket (a heap type
creates a reference cycle to inside, via MRO and methods). At Python,
_PyImport_ClearModules() sets _socket attributes to None which works
around the issue.

If the module is cleared from sys.modules manually,
_PyImport_ClearModules() cannot set _socket.CAPI to None and so the
issue cannot be worked around.

Change _socket.CAPI to use a borrowed reference instead of a strong
reference to allow clearing _socket.socket in this case.

Co-authored-by: Kirill Podoprigora <[email protected]>

Author: vstinner

Modules/socketmodule.c

@@ -7317,7 +7317,6 @@ os_init(void)
 static void
 sock_free_api(PySocketModule_APIObject *capi)
 {
-    Py_DECREF(capi->Sock_Type);
     Py_DECREF(capi->error);
     Py_DECREF(capi->timeout_error);
     PyMem_Free(capi);
@@ -7339,7 +7338,7 @@ sock_get_api(socket_state *state)
         return NULL;
     }
 
-    capi->Sock_Type = (PyTypeObject *)Py_NewRef(state->sock_type);
+    capi->Sock_Type = state->sock_type;
     capi->error = Py_NewRef(PyExc_OSError);
     capi->timeout_error = Py_NewRef(PyExc_TimeoutError);
     return capi;

Modules/socketmodule.h

@@ -382,7 +382,7 @@ typedef struct {
 /* C API for usage by other Python modules.
  * Always add new things to the end for binary compatibility. */
 typedef struct {
-    PyTypeObject *Sock_Type;
+    PyTypeObject *Sock_Type;  // borrowed reference (gh-108240)
     PyObject *error;
     PyObject *timeout_error;
 } PySocketModule_APIObject;