python · 1st1 · Oct 14, 2024 · Sep 28, 2024 · 1st1 · Oct 14, 2024
@@ -123,16 +123,10 @@ Context object management functions:
 
    Enumeration of possible context object watcher events:
 
-   - ``Py_CONTEXT_EVENT_ENTER``: A context has been entered, causing the
-     :term:`current context` to switch to it.  The object passed to the watch
-     callback is the now-current :class:`contextvars.Context` object.  Each
-     enter event will eventually have a corresponding exit event for the same
-     context object after any subsequently entered contexts have themselves been
-     exited.
-   - ``Py_CONTEXT_EVENT_EXIT``: A context is about to be exited, which will
-     cause the :term:`current context` to switch back to what it was before the
-     context was entered.  The object passed to the watch callback is the
-     still-current :class:`contextvars.Context` object.
+   - ``Py_CONTEXT_SWITCHED``: The :term:`current context` has switched to a
+     different context.  The object passed to the watch callback is the
+     now-current :class:`contextvars.Context` object, or None if no context is
+     current.
 
    .. versionadded:: 3.14
 

@@ -29,20 +29,11 @@ PyAPI_FUNC(int) PyContext_Exit(PyObject *);
 
 typedef enum {
     /*
-     * A context has been entered, causing the "current context" to switch to
-     * it.  The object passed to the watch callback is the now-current
-     * contextvars.Context object.  Each enter event will eventually have a
-     * corresponding exit event for the same context object after any
-     * subsequently entered contexts have themselves been exited.
+     * The current context has switched to a different context.  The object
+     * passed to the watch callback is the now-current contextvars.Context
+     * object, or None if no context is current.
      */
-    Py_CONTEXT_EVENT_ENTER,
-    /*
-     * A context is about to be exited, which will cause the "current context"
-     * to switch back to what it was before the context was entered.  The
-     * object passed to the watch callback is the still-current
-     * contextvars.Context object.
-     */
-    Py_CONTEXT_EVENT_EXIT,
+    Py_CONTEXT_SWITCHED = 1,
 } PyContextEvent;
 
 /*

diff --git a/Lib/test/test_capi/test_watchers.py b/Lib/test/test_capi/test_watchers.py
@@ -577,68 +577,62 @@ class TestContextObjectWatchers(unittest.TestCase):
     def context_watcher(self, which_watcher):
         wid = _testcapi.add_context_watcher(which_watcher)
         try:
-            yield wid
+            switches = _testcapi.get_context_switches(which_watcher)
+        except ValueError:
+            switches = None
+        try:
+            yield switches
         finally:
             _testcapi.clear_context_watcher(wid)
 
-    def assert_event_counts(self, exp_enter_0, exp_exit_0,
-                            exp_enter_1, exp_exit_1):
-        self.assertEqual(
-            exp_enter_0, _testcapi.get_context_watcher_num_enter_events(0))
-        self.assertEqual(
-            exp_exit_0, _testcapi.get_context_watcher_num_exit_events(0))
-        self.assertEqual(
-            exp_enter_1, _testcapi.get_context_watcher_num_enter_events(1))
-        self.assertEqual(
-            exp_exit_1, _testcapi.get_context_watcher_num_exit_events(1))
+    def assert_event_counts(self, want_0, want_1):
+        self.assertEqual(len(_testcapi.get_context_switches(0)), want_0)
+        self.assertEqual(len(_testcapi.get_context_switches(1)), want_1)
 
     def test_context_object_events_dispatched(self):
         # verify that all counts are zero before any watchers are registered
-        self.assert_event_counts(0, 0, 0, 0)
+        self.assert_event_counts(0, 0)
 
         # verify that all counts remain zero when a context object is
         # entered and exited with no watchers registered
         ctx = contextvars.copy_context()
-        ctx.run(self.assert_event_counts, 0, 0, 0, 0)
-        self.assert_event_counts(0, 0, 0, 0)
+        ctx.run(self.assert_event_counts, 0, 0)
+        self.assert_event_counts(0, 0)
 
         # verify counts are as expected when first watcher is registered
         with self.context_watcher(0):
-            self.assert_event_counts(0, 0, 0, 0)
-            ctx.run(self.assert_event_counts, 1, 0, 0, 0)
-            self.assert_event_counts(1, 1, 0, 0)
+            self.assert_event_counts(0, 0)
+            ctx.run(self.assert_event_counts, 1, 0)
+            self.assert_event_counts(2, 0)
 
             # again with second watcher registered
             with self.context_watcher(1):
-                self.assert_event_counts(1, 1, 0, 0)
-                ctx.run(self.assert_event_counts, 2, 1, 1, 0)
-                self.assert_event_counts(2, 2, 1, 1)
+                self.assert_event_counts(2, 0)
+                ctx.run(self.assert_event_counts, 3, 1)
+                self.assert_event_counts(4, 2)
 
         # verify counts are reset and don't change after both watchers are cleared
-        ctx.run(self.assert_event_counts, 0, 0, 0, 0)
-        self.assert_event_counts(0, 0, 0, 0)
-
-    def test_enter_error(self):
-        with self.context_watcher(2):
-            with catch_unraisable_exception() as cm:
-                ctx = contextvars.copy_context()
-                ctx.run(int, 0)
-                self.assertEqual(
-                    cm.unraisable.err_msg,
-                    "Exception ignored in "
-                    f"Py_CONTEXT_EVENT_EXIT watcher callback for {ctx!r}"
-                )
-                self.assertEqual(str(cm.unraisable.exc_value), "boom!")
-
-    def test_exit_error(self):
-        ctx = contextvars.copy_context()
-        def _in_context(stack):
-            stack.enter_context(self.context_watcher(2))
-
-        with catch_unraisable_exception() as cm:
-            with ExitStack() as stack:
-                ctx.run(_in_context, stack)
-            self.assertEqual(str(cm.unraisable.exc_value), "boom!")
+        ctx.run(self.assert_event_counts, 0, 0)
+        self.assert_event_counts(0, 0)
+
+    def test_callback_error(self):
+        ctx_outer = contextvars.copy_context()
+        ctx_inner = contextvars.copy_context()
+        unraisables = []
+
+        def _in_outer():
+            with self.context_watcher(2):
+                with catch_unraisable_exception() as cm:
+                    ctx_inner.run(lambda: unraisables.append(cm.unraisable))
+                    unraisables.append(cm.unraisable)
+
+        ctx_outer.run(_in_outer)
+        self.assertEqual([x.err_msg for x in unraisables],
+                         ["Exception ignored in Py_CONTEXT_SWITCHED "
+                          f"watcher callback for {ctx!r}"
+                          for ctx in [ctx_inner, ctx_outer]])
+        self.assertEqual([str(x.exc_value) for x in unraisables],
+                         ["boom!", "boom!"])
 
     def test_clear_out_of_range_watcher_id(self):
         with self.assertRaisesRegex(ValueError, r"Invalid context watcher ID -1"):
@@ -654,5 +648,12 @@ def test_allocate_too_many_watchers(self):
         with self.assertRaisesRegex(RuntimeError, r"no more context watcher IDs available"):
             _testcapi.allocate_too_many_context_watchers()
 
+    def test_exit_base_context(self):
+        ctx = contextvars.Context()
+        _testcapi.clear_context_stack()
+        with self.context_watcher(0) as switches:
+            ctx.run(lambda: None)
+        self.assertEqual(switches, [ctx, None])
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Modules/_testcapi/watchers.c b/Modules/_testcapi/watchers.c
@@ -626,16 +626,12 @@ allocate_too_many_func_watchers(PyObject *self, PyObject *args)
 // Test contexct object watchers
 #define NUM_CONTEXT_WATCHERS 2
 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 PyObject *context_switches[NUM_CONTEXT_WATCHERS];
 
 static int
 handle_context_watcher_event(int which_watcher, PyContextEvent event, PyObject *ctx) {
-    if (event == Py_CONTEXT_EVENT_ENTER) {
-        num_context_object_enter_events[which_watcher]++;
-    }
-    else if (event == Py_CONTEXT_EVENT_EXIT)  {
-        num_context_object_exit_events[which_watcher]++;
+    if (event == Py_CONTEXT_SWITCHED) {
+        PyList_Append(context_switches[which_watcher], ctx);
     }
     else {
         return -1;
@@ -667,31 +663,28 @@ error_context_event_handler(PyContextEvent event, PyObject *ctx) {
 static PyObject *
 add_context_watcher(PyObject *self, PyObject *which_watcher)
 {
-    int watcher_id;
+    static const PyContext_WatchCallback callbacks[] = {
+        &first_context_watcher_callback,
+        &second_context_watcher_callback,
+        &error_context_event_handler,
+    };
     assert(PyLong_Check(which_watcher));
     long which_l = PyLong_AsLong(which_watcher);
-    if (which_l == 0) {
-        watcher_id = PyContext_AddWatcher(first_context_watcher_callback);
-        context_watcher_ids[0] = watcher_id;
-        num_context_object_enter_events[0] = 0;
-        num_context_object_exit_events[0] = 0;
-    }
-    else if (which_l == 1) {
-        watcher_id = PyContext_AddWatcher(second_context_watcher_callback);
-        context_watcher_ids[1] = watcher_id;
-        num_context_object_enter_events[1] = 0;
-        num_context_object_exit_events[1] = 0;
-    }
-    else if (which_l == 2) {
-        watcher_id = PyContext_AddWatcher(error_context_event_handler);
-    }
-    else {
+    if (which_l < 0 || which_l >= (long)Py_ARRAY_LENGTH(callbacks)) {
         PyErr_Format(PyExc_ValueError, "invalid watcher %d", which_l);
         return NULL;
     }
+    int watcher_id = PyContext_AddWatcher(callbacks[which_l]);
     if (watcher_id < 0) {
         return NULL;
     }
+    if (which_l >= 0 && which_l < NUM_CONTEXT_WATCHERS) {
+        context_watcher_ids[which_l] = watcher_id;
+        Py_XSETREF(context_switches[which_l], PyList_New(0));
+        if (context_switches[which_l] == NULL) {
+            return NULL;
+        }
+    }
     return PyLong_FromLong(watcher_id);
 }
 
@@ -708,30 +701,42 @@ clear_context_watcher(PyObject *self, PyObject *watcher_id)
         for (int i = 0; i < NUM_CONTEXT_WATCHERS; i++) {
             if (watcher_id_l == context_watcher_ids[i]) {
                 context_watcher_ids[i] = -1;
-                num_context_object_enter_events[i] = 0;
-                num_context_object_exit_events[i] = 0;
+                Py_CLEAR(context_switches[i]);
             }
         }
     }
     Py_RETURN_NONE;
 }
 
 static PyObject *
-get_context_watcher_num_enter_events(PyObject *self, PyObject *watcher_id)
+clear_context_stack(PyObject *Py_UNUSED(self), PyObject *Py_UNUSED(args))
 {
-    assert(PyLong_Check(watcher_id));
-    long watcher_id_l = PyLong_AsLong(watcher_id);
-    assert(watcher_id_l >= 0 && watcher_id_l < NUM_CONTEXT_WATCHERS);
-    return PyLong_FromLong(num_context_object_enter_events[watcher_id_l]);
+    PyThreadState *tstate = PyThreadState_Get();
+    if (tstate->context == NULL) {
+        Py_RETURN_NONE;
+    }
+    if (((PyContext *)tstate->context)->ctx_prev != NULL) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "must first exit all non-base contexts");
+        return NULL;
+    }
+    Py_CLEAR(tstate->context);
+    Py_RETURN_NONE;
 }
 
 static PyObject *
-get_context_watcher_num_exit_events(PyObject *self, PyObject *watcher_id)
+get_context_switches(PyObject *Py_UNUSED(self), PyObject *watcher_id)
 {
     assert(PyLong_Check(watcher_id));
     long watcher_id_l = PyLong_AsLong(watcher_id);
-    assert(watcher_id_l >= 0 && watcher_id_l < NUM_CONTEXT_WATCHERS);
-    return PyLong_FromLong(num_context_object_exit_events[watcher_id_l]);
+    if (watcher_id_l < 0 || watcher_id_l >= NUM_CONTEXT_WATCHERS) {
+        PyErr_Format(PyExc_ValueError, "invalid watcher %ld", watcher_id_l);
+        return NULL;
+    }
+    if (context_switches[watcher_id_l] == NULL) {
+        return PyList_New(0);
+    }
+    return Py_NewRef(context_switches[watcher_id_l]);
 }
 
 static PyObject *
@@ -835,10 +840,8 @@ static PyMethodDef test_methods[] = {
     // Code object watchers.
     {"add_context_watcher",         add_context_watcher,        METH_O,       NULL},
     {"clear_context_watcher",       clear_context_watcher,      METH_O,       NULL},
-    {"get_context_watcher_num_enter_events",
-     get_context_watcher_num_enter_events,                 METH_O,       NULL},
-    {"get_context_watcher_num_exit_events",
-     get_context_watcher_num_exit_events,               METH_O,       NULL},
+    {"clear_context_stack",      clear_context_stack,     METH_NOARGS,  NULL},
+    {"get_context_switches",     get_context_switches,    METH_O,       NULL},
     {"allocate_too_many_context_watchers",
      (PyCFunction) allocate_too_many_context_watchers,       METH_NOARGS,  NULL},
     {NULL},

@@ -102,10 +102,8 @@ PyContext_CopyCurrent(void)
 static const char *
 context_event_name(PyContextEvent event) {
     switch (event) {
-        case Py_CONTEXT_EVENT_ENTER:
-            return "Py_CONTEXT_EVENT_ENTER";
-        case Py_CONTEXT_EVENT_EXIT:
-            return "Py_CONTEXT_EVENT_EXIT";
+        case Py_CONTEXT_SWITCHED:
+            return "Py_CONTEXT_SWITCHED";
         default:
             return "?";
     }
@@ -115,6 +113,13 @@ context_event_name(PyContextEvent event) {
 static void
 notify_context_watchers(PyThreadState *ts, PyContextEvent event, PyObject *ctx)
 {
+    if (ctx == NULL) {
+        // This will happen after exiting the last context in the stack, which
+        // can occur if context_get was never called before entering a context
+        // (e.g., called `contextvars.Context().run()` on a fresh thread, as
+        // PyContext_Enter doesn't call context_get).
+        ctx = Py_None;
+    }
     assert(Py_REFCNT(ctx) > 0);
     PyInterpreterState *interp = ts->interp;
     assert(interp->_initialized);
@@ -175,6 +180,16 @@ PyContext_ClearWatcher(int watcher_id)
 }
 
 
+static inline void
+context_switched(PyThreadState *ts)
+{
+    ts->context_ver++;
+    // ts->context is used instead of context_get() because context_get() might
+    // throw if ts->context is NULL.
+    notify_context_watchers(ts, Py_CONTEXT_SWITCHED, ts->context);
+}
+
+
 static int
 _PyContext_Enter(PyThreadState *ts, PyObject *octx)
 {
@@ -191,9 +206,7 @@ _PyContext_Enter(PyThreadState *ts, PyObject *octx)
     ctx->ctx_entered = 1;
 
     ts->context = Py_NewRef(ctx);
-    ts->context_ver++;
-
-    notify_context_watchers(ts, Py_CONTEXT_EVENT_ENTER, octx);
+    context_switched(ts);
     return 0;
 }
 
@@ -227,13 +240,11 @@ _PyContext_Exit(PyThreadState *ts, PyObject *octx)
         return -1;
     }
 
-    notify_context_watchers(ts, Py_CONTEXT_EVENT_EXIT, octx);
     Py_SETREF(ts->context, (PyObject *)ctx->ctx_prev);
-    ts->context_ver++;
 
     ctx->ctx_prev = NULL;
     ctx->ctx_entered = 0;
-
+    context_switched(ts);
     return 0;
 }
 

@@ -455,8 +455,8 @@ Modules/_testcapi/watchers.c	-	pyfunc_watchers	-
 Modules/_testcapi/watchers.c	-	func_watcher_ids	-
 Modules/_testcapi/watchers.c	-	func_watcher_callbacks	-
 Modules/_testcapi/watchers.c	-	context_watcher_ids	-
-Modules/_testcapi/watchers.c	-	num_context_object_enter_events	-
-Modules/_testcapi/watchers.c	-	num_context_object_exit_events	-
+Modules/_testcapi/watchers.c	-	context_switches	-
+Modules/_testcapi/watchers.c	add_context_watcher	callbacks	-
 Modules/_testcapimodule.c	-	BasicStaticTypes	-
 Modules/_testcapimodule.c	-	num_basic_static_types_used	-
 Modules/_testcapimodule.c	-	ContainerNoGC_members	-