Backport to 2.2.1.

Fix an issue that was reported in but unrelated to the main problem of
SF bug 535905 (Evil Trashcan and GC interaction).

The SETLOCAL() macro should not DECREF the local variable in-place and
then store the new value; it should copy the old value to a temporary
value, then store the new value, and then DECREF the temporary value.
This is because it is possible that during the DECREF the frame is
accessed by other code (e.g. a __del__ method or gc.collect()) and the
variable would be pointing to already-freed memory.

BUGFIX CANDIDATE!

diff --git a/Python/ceval.c b/Python/ceval.c
index e7ca82bc1a422d2648fe04709fc79c17163822d3..0fa58877819122a5a77399a9b797f18186e1ce20 100644 (file)
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -554,8 +554,16 @@ eval_frame(PyFrameObject *f)
 /* Local variable macros */
 
 #define GETLOCAL(i)    (fastlocals[i])
-#define SETLOCAL(i, value)     do { Py_XDECREF(GETLOCAL(i)); \
-                                    GETLOCAL(i) = value; } while (0)
+
+/* The SETLOCAL() macro must not DECREF the local variable in-place and
+   then store the new value; it must copy the old value to a temporary
+   value, then store the new value, and then DECREF the temporary value.
+   This is because it is possible that during the DECREF the frame is
+   accessed by other code (e.g. a __del__ method or gc.collect()) and the
+   variable would be pointing to already-freed memory. */
+#define SETLOCAL(i, value)     do { PyObject *tmp = GETLOCAL(i); \
+                                    GETLOCAL(i) = value; \
+                                     Py_XDECREF(tmp); } while (0)
 
 /* Start of code */