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>

4 files changed, 26 insertions, 0 deletions

diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
index 145411efbb..274616bf99 100644
--- a/Lib/test/test_bytes.py
+++ b/Lib/test/test_bytes.py
@@ -999,6 +999,12 @@ class BytesTest(BaseBytesTest, unittest.TestCase):
         self.assertRaises(OverflowError,
                           PyBytes_FromFormat, b'%c', c_int(256))
 
+        # Issue #33817: empty strings
+        self.assertEqual(PyBytes_FromFormat(b''),
+                         b'')
+        self.assertEqual(PyBytes_FromFormat(b'%s', b''),
+                         b'')
+
     def test_bytes_blocking(self):
         class IterationBlocked(list):
             __bytes__ = None
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 3cc018c0cc..1aad933407 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -2676,6 +2676,12 @@ class CAPITest(unittest.TestCase):
         check_format('%.%s',
                      b'%.%s', b'abc')
 
+        # Issue #33817: empty strings
+        check_format('',
+                     b'')
+        check_format('',
+                     b'%s', b'')
+
     # Test PyUnicode_AsWideChar()
     @support.cpython_only
     def test_aswidechar(self):
diff --git a/Misc/NEWS.d/next/C API/2019-01-11-11-16-16.bpo-33817.nJ4yIj.rst b/Misc/NEWS.d/next/C API/2019-01-11-11-16-16.bpo-33817.nJ4yIj.rst
new file mode 100644
index 0000000000..ca4ccb26d3
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2019-01-11-11-16-16.bpo-33817.nJ4yIj.rst
@@ -0,0 +1 @@
+Fixed :c:func:`_PyBytes_Resize` for empty bytes objects.
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);