#3664: The pickle module could segfault if a Pickler instance is not correctly initialized:

when a subclass forgets to call the base __init__ method,
or when __init__ is called a second time with invalid parameters

Patch by Alexandre Vassalotti.

Lib/test/pickletester.py

@@ -996,6 +996,20 @@ class AbstractPickleModuleTests(unittest.TestCase):
         pickle.Pickler(f, -1)
         pickle.Pickler(f, protocol=-1)
 
+    def test_bad_init(self):
+        # Test issue3664 (pickle can segfault from a badly initialized Pickler).
+        from io import BytesIO
+        # Override initialization without calling __init__() of the superclass.
+        class BadPickler(pickle.Pickler):
+            def __init__(self): pass
+
+        class BadUnpickler(pickle.Unpickler):
+            def __init__(self): pass
+
+        self.assertRaises(pickle.PicklingError, BadPickler().dump, 0)
+        self.assertRaises(pickle.UnpicklingError, BadUnpickler().load)
+
+
 class AbstractPersistentPicklerTests(unittest.TestCase):
 
     # This class defines persistent_id() and persistent_load()

Misc/NEWS

@@ -33,7 +33,10 @@ Core and Builtins
 Library
 -------
 
-- telnetlib now works completely in bytes.
+- Issue #3664: The pickle module could segfault if a subclass of Pickler fails
+  to call the base __init__ method.
+
+- Issue #3725: telnetlib now works completely in bytes.
 
 - Issue #4072: Restore build_py_2to3.
 

Modules/_pickle.c

@@ -421,6 +421,11 @@ pickler_write(PicklerObject *self, const char *s, Py_ssize_t n)
 {
     PyObject *data, *result;
 
+    if (self->write_buf == NULL) {
+        PyErr_SetString(PyExc_SystemError, "invalid write buffer");
+        return -1;
+    }
+
     if (s == NULL) {
         if (!(self->buf_size))
             return 0;
@@ -2378,6 +2383,16 @@ Pickler_dump(PicklerObject *self, PyObject *args)
 {
     PyObject *obj;
 
+    /* Check whether the Pickler was initialized correctly (issue3664).
+       Developers often forget to call __init__() in their subclasses, which
+       would trigger a segfault without this check. */
+    if (self->write == NULL) {
+        PyErr_Format(PicklingError, 
+                     "Pickler.__init__() was not called by %s.__init__()",
+                     Py_TYPE(self)->tp_name);
+        return NULL;
+    }
+
     if (!PyArg_ParseTuple(args, "O:dump", &obj))
         return NULL;