gh-124872: Back up exception before calling PyContext_WatchCallback

Doc/c-api/contextvars.rst

@@ -136,20 +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.
 
-   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``.
+   Any pending exception is cleared before the callback is called and restored
+   after the callback returns.
 
-   There may already be a pending exception set on entry to the callback. In
-   this case, the callback should return ``0`` with the same exception still
-   set. This means the callback may not call any other API that can set an
-   exception unless it saves and clears the exception state first, and restores
-   it before returning.
+   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

@@ -49,10 +49,13 @@ typedef enum {
  * Context object watcher callback function.  The object passed to the callback
  * is event-specific; see PyContextEvent for details.
  *
- * if the callback returns with an exception set, it must return -1. Otherwise
- * it should return 0
+ * Any pending exception is cleared before the callback is called and restored
+ * after the callback returns.
+ *
+ * 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

Lib/test/test_capi/test_watchers.py

@@ -640,6 +640,16 @@ def _in_context(stack):
                 ctx.run(_in_context, stack)
             self.assertEqual(str(cm.unraisable.exc_value), "boom!")
 
+    def test_exception_save(self):
+        with self.context_watcher(2):
+            with catch_unraisable_exception() as cm:
+                def _in_context():
+                    raise RuntimeError("test")
+
+                with self.assertRaisesRegex(RuntimeError, "test"):
+                    contextvars.copy_context().run(_in_context)
+                self.assertEqual(str(cm.unraisable.exc_value), "boom!")
+
     def test_clear_out_of_range_watcher_id(self):
         with self.assertRaisesRegex(ValueError, r"Invalid context watcher ID -1"):
             _testcapi.clear_context_watcher(-1)

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

@@ -125,11 +125,14 @@ notify_context_watchers(PyThreadState *ts, PyContextEvent event, PyObject *ctx)
         if (bits & 1) {
             PyContext_WatchCallback cb = interp->context_watchers[i];
             assert(cb != NULL);
-            if (cb(event, ctx) < 0) {
+            PyObject *exc = _PyErr_GetRaisedException(ts);
+            cb(event, ctx);
+            if (_PyErr_Occurred(ts) != NULL) {
                 PyErr_FormatUnraisable(
                     "Exception ignored in %s watcher callback for %R",
                     context_event_name(event), ctx);
             }
+            _PyErr_SetRaisedException(ts, exc);
         }
         i++;
         bits >>= 1;