pythongh-124872: Change PyContext_WatchCallback return type to void

The callback cannot prevent the event from occurring by raising an
exception.  Nor should it be able to; any failure to switch contexts
would be difficult if not impossible to handle correctly.  The API
should reflect that the callback is purely informational, and that the
context switch will happen even if the callback doesn't want it to.

Doc/c-api/contextvars.rst

@@ -136,18 +136,16 @@ Context object management functions:
 
    .. versionadded:: 3.14
 
-.. c:type:: int (*PyContext_WatchCallback)(PyContextEvent event, PyObject *obj)
+.. c:type:: void (*PyContext_WatchCallback)(PyContextEvent event, PyObject *obj)
 
    Context object watcher callback function.  The object passed to the callback
    is event-specific; see :c:type:`PyContextEvent` for details.
 
    Any pending exception is cleared before the callback is called and restored
    after the callback returns.
 
-   If the callback raises an exception it must return ``-1``; the exception
-   will be printed as an unraisable exception using
-   :c:func:`PyErr_FormatUnraisable` and discarded.  Otherwise it must return
-   ``0``.
+   If the callback raises an exception it will be printed as an unraisable
+   exception using :c:func`PyErr_FormatUnraisable` and discarded.
 
    .. versionadded:: 3.14
 

Include/cpython/context.h

@@ -52,11 +52,10 @@ typedef enum {
  * Any pending exception is cleared before the callback is called and restored
  * after the callback returns.
  *
- * If the callback raises an exception it must return -1; the exception will be
- * printed as an unraisable exception using PyErr_FormatUnraisable and
- * discarded.  Otherwise it must return 0.
+ * If the callback raises an exception it will be printed as an unraisable
+ * exception using PyErr_FormatUnraisable and discarded.
  */
-typedef int (*PyContext_WatchCallback)(PyContextEvent, PyObject *);
+typedef void (*PyContext_WatchCallback)(PyContextEvent, PyObject *);
 
 /*
  * Register a per-interpreter callback that will be invoked for context object

Modules/_testcapi/watchers.c

@@ -629,7 +629,7 @@ static int context_watcher_ids[NUM_CONTEXT_WATCHERS] = {-1, -1};
 static int num_context_object_enter_events[NUM_CONTEXT_WATCHERS] = {0, 0};
 static int num_context_object_exit_events[NUM_CONTEXT_WATCHERS] = {0, 0};
 
-static int
+static void
 handle_context_watcher_event(int which_watcher, PyContextEvent event, PyObject *ctx) {
     if (event == Py_CONTEXT_EVENT_ENTER) {
         num_context_object_enter_events[which_watcher]++;
@@ -638,30 +638,27 @@ handle_context_watcher_event(int which_watcher, PyContextEvent event, PyObject *
         num_context_object_exit_events[which_watcher]++;
     }
     else {
-        return -1;
+        Py_UNREACHABLE();
     }
-    return 0;
 }
 
-static int
+static void
 first_context_watcher_callback(PyContextEvent event, PyObject *ctx) {
-    return handle_context_watcher_event(0, event, ctx);
+    handle_context_watcher_event(0, event, ctx);
 }
 
-static int
+static void
 second_context_watcher_callback(PyContextEvent event, PyObject *ctx) {
-    return handle_context_watcher_event(1, event, ctx);
+    handle_context_watcher_event(1, event, ctx);
 }
 
-static int
+static void
 noop_context_event_handler(PyContextEvent event, PyObject *ctx) {
-    return 0;
 }
 
-static int
+static void
 error_context_event_handler(PyContextEvent event, PyObject *ctx) {
     PyErr_SetString(PyExc_RuntimeError, "boom!");
-    return -1;
 }
 
 static PyObject *

Python/context.c

@@ -126,7 +126,8 @@ notify_context_watchers(PyThreadState *ts, PyContextEvent event, PyObject *ctx)
             PyContext_WatchCallback cb = interp->context_watchers[i];
             assert(cb != NULL);
             PyObject *exc = _PyErr_GetRaisedException(ts);
-            if (cb(event, ctx) < 0) {
+            cb(event, ctx);
+            if (_PyErr_Occurred(ts) != NULL) {
                 PyErr_FormatUnraisable(
                     "Exception ignored in %s watcher callback for %R",
                     context_event_name(event), ctx);