gh-119333: Change PyContext_WatchCallback to take PyObject

The PyContext struct is not intended to be public, and users of the
API don't need anything more specific than PyObject.  Also see
gh-78943.

Author: rhansen

Doc/c-api/contextvars.rst

@@ -127,7 +127,7 @@ Context object management functions:
 
    .. versionadded:: 3.14
 
-.. c:type:: int (*PyContext_WatchCallback)(PyContextEvent event, PyContext* ctx)
+.. c:type:: int (*PyContext_WatchCallback)(PyContextEvent event, PyObject* ctx)
 
    Type of a context object watcher callback function.
    If *event* is ``Py_CONTEXT_EVENT_ENTER``, then the callback is invoked

Include/cpython/context.h

@@ -41,7 +41,7 @@ typedef enum {
  * if the callback returns with an exception set, it must return -1. Otherwise
  * it should return 0
  */
-typedef int (*PyContext_WatchCallback)(PyContextEvent, PyContext *);
+typedef int (*PyContext_WatchCallback)(PyContextEvent, PyObject *);
 
 /*
  * Register a per-interpreter callback that will be invoked for context object

Python/context.c

@@ -124,7 +124,7 @@ static void notify_context_watchers(PyContextEvent event, PyContext *ctx, PyThre
         if (bits & 1) {
             PyContext_WatchCallback cb = interp->context_watchers[i];
             assert(cb != NULL);
-            if (cb(event, ctx) < 0) {
+            if (cb(event, (PyObject *)ctx) < 0) {
                 PyErr_FormatUnraisable(
                     "Exception ignored in %s watcher callback for %R",
                     context_event_name(event), ctx);