This was quite a dark bug in my recent in-place string concatenation

hack: it would resize *interned* strings in-place!  This occurred because
their reference counts do not have their expected value -- stringobject.c
hacks them.  Mea culpa.

Objects/stringobject.c

@@ -3513,7 +3513,8 @@ _PyString_Resize(PyObject **pv, int newsize)
 	register PyObject *v;
 	register PyStringObject *sv;
 	v = *pv;
-	if (!PyString_Check(v) || v->ob_refcnt != 1 || newsize < 0) {
+	if (!PyString_Check(v) || v->ob_refcnt != 1 || newsize < 0 ||
+	    PyString_CHECK_INTERNED(v)) {
 		*pv = 0;
 		Py_DECREF(v);
 		PyErr_BadInternalCall();

Python/ceval.c

@@ -4253,7 +4253,7 @@ string_concatenate(PyObject *v, PyObject *w,
 		}
 	}
 
-	if (v->ob_refcnt == 1) {
+	if (v->ob_refcnt == 1 && !PyString_CHECK_INTERNED(v)) {
 		/* Now we own the last reference to 'v', so we can resize it
 		 * in-place.
 		 */