Checkin Tim's fix to an error discussed on python-dev.

Also, add a testcase.

Formerly, the list_extend() code used several local variables to remember
its state across iterations.  Since an iteration could call arbitrary
Python code, it was possible for the list state to be changed.  The new
code uses dynamic structure references instead of C locals.  So, they
are always up-to-date.

After list_resize() is called, its size has been updated but the new
cells are filled with NULLs.  These needed to be filled before arbitrary
iteration code was called; otherwise, that code could attempt to modify
a list that was in a semi-invalid state.  The solution was to change
the ob->size field back to a value reflecting the actual number of valid
cells.

1 files changed, 20 insertions, 10 deletions

diff --git a/Objects/listobject.c b/Objects/listobject.c
index 1fb77b9..44616e5 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -769,12 +769,20 @@ listextend(PyListObject *self, PyObject *b)
 	}
 	m = self->ob_size;
 	mn = m + n;
-	if (list_resize(self, mn) == -1)
-		goto error;
-	memset(&(self->ob_item[m]), 0, sizeof(*self->ob_item) * n);
+	if (mn >= m) {
+		/* Make room. */
+		if (list_resize(self, mn) == -1)
+			goto error;
+		/* Make the list sane again. */
+		self->ob_size = m;
+	}
+	/* Else m + n overflowed; on the chance that n lied, and there really
+	 * is enough room, ignore it.  If n was telling the truth, we'll
+	 * eventually run out of memory during the loop.
+	 */
 
 	/* Run iterator to exhaustion. */
-	for (i = m; ; i++) {
+	for (;;) {
 		PyObject *item = iternext(it);
 		if (item == NULL) {
 			if (PyErr_Occurred()) {
@@ -785,8 +793,11 @@ listextend(PyListObject *self, PyObject *b)
 			}
 			break;
 		}
-		if (i < mn)
-			PyList_SET_ITEM(self, i, item); /* steals ref */
+		if (self->ob_size < self->allocated) {
+			/* steals ref */
+			PyList_SET_ITEM(self, self->ob_size, item);
+			++self->ob_size;
+		}
 		else {
 			int status = app1(self, item);
 			Py_DECREF(item);  /* append creates a new ref */
@@ -796,10 +807,9 @@ listextend(PyListObject *self, PyObject *b)
 	}
 
 	/* Cut back result list if initial guess was too large. */
-	if (i < mn && self != NULL) {
-		if (list_ass_slice(self, i, mn, (PyObject *)NULL) != 0)
-			goto error;
-	}
+	if (self->ob_size < self->allocated)
+		list_resize(self, self->ob_size);  /* shrinking can't fail */
+
 	Py_DECREF(it);
 	Py_RETURN_NONE;