address review: ensure buffer unmodified on overflow

Author: skirpichev

Lib/test/test_struct.py

@@ -426,13 +426,20 @@ def test_705836(self):
 
     def test_float_complex_overflow(self):
         for value in (
-            1e300,   # big real
-            1e300j,  # big imag
+            1e300 + 0.5j,  # big real
+            1.5 + 1e300j,  # big imag
         ):
             for format in (">Zf", "<Zf", "Zf"):
                 with self.subTest(value=value, format=format):
                     self.assertRaises(OverflowError, struct.pack, format, value)
 
+                ba = bytearray(8)
+                try:
+                    struct.Struct(format).pack_into(ba, 0, value)
+                    assert False
+                except OverflowError:
+                    self.assertEqual(ba, bytearray(8))
+
     def test_1530559(self):
         for code, byteorder in iter_integer_formats():
             format = byteorder + code

Modules/_struct.c

@@ -797,12 +797,18 @@ np_float_complex(_structmodulestate *state, char *p, PyObject *v,
         return -1;
     }
 
-    int ret = PyFloat_Pack4(c.real, p, PY_LITTLE_ENDIAN);
+    char tmp[8];
+    int ret = PyFloat_Pack4(c.real, tmp, PY_LITTLE_ENDIAN);
 
     if (ret) {
         return ret;
     }
-    return PyFloat_Pack4(c.imag, p + sizeof(float), PY_LITTLE_ENDIAN);
+    ret = PyFloat_Pack4(c.imag, tmp + 4, PY_LITTLE_ENDIAN);
+    if (ret) {
+        return ret;
+    }
+    memcpy(p, tmp, 8);
+    return ret;
 }
 
 static int