bpo-33817: Fix _PyBytes_Resize() for empty bytes object. (GH-11516)

Add also tests for PyUnicode_FromFormat() and PyBytes_FromFormat()
with empty result.
(cherry picked from commit 44cc4822bb3799858201e61294c5863f93ec12e2)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index 711faba645..5f9e1eccf2 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -2990,9 +2990,22 @@ _PyBytes_Resize(PyObject **pv, Py_ssize_t newsize)
         /* return early if newsize equals to v->ob_size */
         return 0;
     }
+    if (Py_SIZE(v) == 0) {
+        if (newsize == 0) {
+            return 0;
+        }
+        *pv = _PyBytes_FromSize(newsize, 0);
+        Py_DECREF(v);
+        return (*pv == NULL) ? -1 : 0;
+    }
     if (Py_REFCNT(v) != 1) {
         goto error;
     }
+    if (newsize == 0) {
+        *pv = _PyBytes_FromSize(0, 0);
+        Py_DECREF(v);
+        return (*pv == NULL) ? -1 : 0;
+    }
     /* XXX UNREF/NEWREF interface should be more symmetrical */
     _Py_DEC_REFTOTAL;
     _Py_ForgetReference(v);