Patch 1012740: cStringIO's truncate doesn't

truncate() left the stream position unchanged, which meant the
"truncated" data didn't go away:

>>> io.write('abc')
>>> io.truncate(0)
>>> io.write('xyz')
>>> io.getvalue()
'abcxyz'

Patch by Dima Dorfman.

Lib/test/test_StringIO.py

@@ -49,9 +49,10 @@ class TestGenericStringIO(unittest.TestCase):
         f.seek(10)
         f.truncate()
         eq(f.getvalue(), 'abcdefghij')
-        f.seek(0)
         f.truncate(5)
         eq(f.getvalue(), 'abcde')
+        f.write('xyz')
+        eq(f.getvalue(), 'abcdexyz')
         f.close()
         self.assertRaises(ValueError, f.write, 'frobnitz')
 

Misc/NEWS

@@ -41,6 +41,11 @@ Core and builtins
 Extension modules
 -----------------
 
+- Patch 1012740:  truncate() on a writeable cStringIO now resets the
+  position to the end of the stream.  This is consistent with the original
+  StringIO module and avoids inadvertently resurrecting data that was
+  supposed to have been truncated away.
+
 - Added socket.socketpair().
 
 Library

Modules/cStringIO.c

@@ -289,6 +289,7 @@ IO_truncate(IOobject *self, PyObject *args) {
         if (pos < 0) pos = self->pos;
 
         if (self->string_size > pos) self->string_size = pos;
+        self->pos = self->string_size;
 
         Py_INCREF(Py_None);
         return Py_None;