BUG: don't allow users to move from an interned string

pandas/tests/test_util.py

@@ -2,6 +2,9 @@
 import nose
 
 from collections import OrderedDict
+import sys
+import unittest
+from uuid import uuid4
 from pandas.util._move import move_into_mutable_buffer, BadMove
 from pandas.util.decorators import deprecate_kwarg
 from pandas.util.validators import (validate_args, validate_kwargs,
@@ -325,6 +328,46 @@ def test_exactly_one_ref(self):
         # materialize as bytearray to show that it is mutable
         self.assertEqual(bytearray(as_stolen_buf), b'test')
 
+    @unittest.skipIf(
+        sys.version_info[0] > 2,
+        'bytes objects cannot be interned in py3',
+    )
+    def test_interned(self):
+        salt = uuid4().hex
+
+        def make_string():
+            # We need to actually create a new string so that it has refcount
+            # one. We use a uuid so that we know the string could not already
+            # be in the intern table.
+            return ''.join(('testing: ', salt))
+
+        # This should work, the string has one reference on the stack.
+        move_into_mutable_buffer(make_string())
+
+        refcount = [None]  # nonlocal
+
+        def ref_capture(ob):
+            # Subtract two because those are the references owned by this
+            # frame:
+            #   1. The local variables of this stack frame.
+            #   2. The python data stack of this stack frame.
+            refcount[0] = sys.getrefcount(ob) - 2
+            return ob
+
+        with tm.assertRaises(BadMove):
+            # If we intern the string it will still have one reference but now
+            # it is in the intern table so if other people intern the same
+            # string while the mutable buffer holds the first string they will
+            # be the same instance.
+            move_into_mutable_buffer(ref_capture(intern(make_string())))  # noqa
+
+        self.assertEqual(
+            refcount[0],
+            1,
+            msg='The BadMove was probably raised for refcount reasons instead'
+            ' of interning reasons',
+        )
+
 
 def test_numpy_errstate_is_default():
     # The defaults since numpy 1.6.0

pandas/util/move.c

@@ -7,6 +7,9 @@
 #define PyString_CheckExact PyBytes_CheckExact
 #define PyString_AS_STRING PyBytes_AS_STRING
 #define PyString_GET_SIZE PyBytes_GET_SIZE
+
+/* in python 3, we cannot intern bytes objects so this is always false */
+#define PyString_CHECK_INTERNED(cs) 0
 #endif  /* !COMPILING_IN_PY2 */
 
 #ifndef Py_TPFLAGS_HAVE_GETCHARBUFFER
@@ -113,8 +116,9 @@ stolenbuf_new(PyObject *self, PyObject *args, PyObject *kwargs)
         return NULL;
     }
 
-    if (Py_REFCNT(bytes_rvalue) != 1) {
-        /* there is a reference other than the caller's stack */
+    if (Py_REFCNT(bytes_rvalue) != 1 || PyString_CHECK_INTERNED(bytes_rvalue)) {
+        /* there is a reference other than the caller's stack or the string is
+           interned */
         PyErr_SetObject(badmove, bytes_rvalue);
         return NULL;
     }