gh-124872: Change PyContext_WatchCallback return type to void

The exception is ignored so change the return type from `int` to
`void` to discourage callbacks from raising an exception in the first
place.

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:: void (*PyContext_WatchCallback)(PyContextEvent event, PyContext* ctx)
 
    Type of a context object watcher callback function.
    If *event* is ``Py_CONTEXT_EVENT_ENTER``, then the callback is invoked
@@ -138,9 +138,7 @@ Context object management functions:
    Any pending exception is cleared before the callback is called and restored
    after the callback returns.
 
-   If the callback returns with an exception set, it must return ``-1``; this
-   exception will be printed as an unraisable exception using
-   :c:func:`PyErr_FormatUnraisable`. Otherwise it should return ``0``.
+   If the callback raises an exception it will be ignored.
 
    .. versionadded:: 3.14
 

Include/cpython/context.h

@@ -41,10 +41,9 @@ typedef enum {
  * Any pending exception is cleared before the callback is called and restored
  * after the callback returns.
  *
- * if the callback returns with an exception set, it must return -1. Otherwise
- * it should return 0
+ * If the callback raises an exception it will be ignored.
  */
-typedef int (*PyContext_WatchCallback)(PyContextEvent, PyContext *);
+typedef void (*PyContext_WatchCallback)(PyContextEvent, PyContext *);
 
 /*
  * 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, PyContext *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, PyContext
         num_context_object_exit_events[which_watcher]++;
     }
     else {
-        return -1;
+        Py_UNREACHABLE();
     }
-    return 0;
 }
 
-static int
+static void
 first_context_watcher_callback(PyContextEvent event, PyContext *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, PyContext *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, PyContext *ctx) {
-    return 0;
 }
 
-static int
+static void
 error_context_event_handler(PyContextEvent event, PyContext *ctx) {
     PyErr_SetString(PyExc_RuntimeError, "boom!");
-    return -1;
 }
 
 static PyObject *

Python/context.c

@@ -125,7 +125,8 @@ static void notify_context_watchers(PyContextEvent event, PyContext *ctx, PyThre
             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);