[3.12] gh-91051: fix segfault when using all 8 type watchers (GH-107853) (#107876)

* gh-91051: fix segfault when using all 8 type watchers (GH-107853)
(cherry picked from commit 66e4edd734)

Co-authored-by: Carl Meyer <carl@oddbird.net>

Doc/c-api/typeobj.rst

@@ -147,7 +147,7 @@ Quick Reference
    +------------------------------------------------+-----------------------------------+-------------------+---+---+---+---+
    | :c:member:`~PyTypeObject.tp_vectorcall`        | :c:type:`vectorcallfunc`          |                   |   |   |   |   |
    +------------------------------------------------+-----------------------------------+-------------------+---+---+---+---+
-   | [:c:member:`~PyTypeObject.tp_watched`]         | char                              |                   |   |   |   |   |
+   | [:c:member:`~PyTypeObject.tp_watched`]         | unsigned char                     |                   |   |   |   |   |
    +------------------------------------------------+-----------------------------------+-------------------+---+---+---+---+
 
 .. [#slots]
@@ -2141,7 +2141,7 @@ and :c:data:`PyType_Type` effectively act as defaults.)
    .. versionadded:: 3.9 (the field exists since 3.8 but it's only used since 3.9)
 
 
-.. c:member:: char PyTypeObject.tp_watched
+.. c:member:: unsigned char PyTypeObject.tp_watched
 
    Internal. Do not use.
 

Doc/includes/typestruct.h

@@ -82,5 +82,5 @@ typedef struct _typeobject {
     vectorcallfunc tp_vectorcall;
 
     /* bitset of which type-watchers care about this type */
-    char tp_watched;
+    unsigned char tp_watched;
 } PyTypeObject;

Include/cpython/object.h

@@ -227,7 +227,7 @@ struct _typeobject {
     vectorcallfunc tp_vectorcall;
 
     /* bitset of which type-watchers care about this type */
-    char tp_watched;
+    unsigned char tp_watched;
 };
 
 /* This struct is used by the specializer

Lib/test/test_capi/test_watchers.py

@@ -294,6 +294,18 @@ class TestTypeWatchers(unittest.TestCase):
                 C2.hmm = "baz"
                 self.assert_events([C1, [C2]])
 
+    def test_all_watchers(self):
+        class C: pass
+        with ExitStack() as stack:
+            last_wid = -1
+            # don't make assumptions about how many watchers are already
+            # registered, just go until we reach the max ID
+            while last_wid < self.TYPE_MAX_WATCHERS - 1:
+                last_wid = stack.enter_context(self.watcher())
+            self.watch(last_wid, C)
+            C.foo = "bar"
+            self.assert_events([C])
+
     def test_watch_non_type(self):
         with self.watcher() as wid:
             with self.assertRaisesRegex(ValueError, r"Cannot watch non-type"):

Misc/NEWS.d/next/Core and Builtins/2023-08-10-17-36-27.gh-issue-91051.LfaeNW.rst

@@ -0,0 +1,2 @@
+Fix abort / segfault when using all eight type watcher slots, on platforms
+where ``char`` is signed by default.