Issue #19219: Speed up marshal.loads(), and make pyc files slightly (5% to 10%) smaller.

Include/marshal.h

@@ -7,7 +7,7 @@
 extern "C" {
 #endif
 
-#define Py_MARSHAL_VERSION 3
+#define Py_MARSHAL_VERSION 4
 
 PyAPI_FUNC(void) PyMarshal_WriteLongToFile(long, FILE *, int);
 PyAPI_FUNC(void) PyMarshal_WriteObjectToFile(PyObject *, FILE *, int);

Lib/test/test_marshal.py

@@ -262,11 +262,11 @@ class BugsTestCase(unittest.TestCase):
 
     def test_bad_reader(self):
         class BadReader(io.BytesIO):
-            def read(self, n=-1):
-                b = super().read(n)
+            def readinto(self, buf):
+                n = super().readinto(buf)
                 if n is not None and n > 4:
-                    b += b' ' * 10**6
-                return b
+                    n += 10**6
+                return n
         for value in (1.0, 1j, b'0123456789', '0123456789'):
             self.assertRaises(ValueError, marshal.load,
                               BadReader(marshal.dumps(value)))