Make cStringIO.truncate raise IOError for negative

arguments (even for -1). Fixes the last bit of
#1359365.

Lib/test/test_StringIO.py

@@ -62,6 +62,7 @@ class TestGenericStringIO(unittest.TestCase):
         eq(f.getvalue(), 'abcde')
         f.write('xyz')
         eq(f.getvalue(), 'abcdexyz')
+        self.assertRaises(IOError, f.truncate, -1)
         f.close()
         self.assertRaises(ValueError, f.write, 'frobnitz')
 

Misc/NEWS

@@ -101,6 +101,9 @@ Core and builtins
 Library
 -------
 
+- cStringIO.truncate(-1) now raises an IOError, like StringIO and
+  regular files.
+
 - Patch #1472877: Fix Tix subwidget name resolution.
 
 - Patch #1594554: Always close a tkSimpleDialog on ok(), even

Modules/cStringIO.c

@@ -289,7 +289,17 @@ IO_truncate(IOobject *self, PyObject *args) {
 	
         if (!IO__opencheck(self)) return NULL;
         if (!PyArg_ParseTuple(args, "|n:truncate", &pos)) return NULL;
-        if (pos < 0) pos = self->pos;
+
+	if (PyTuple_Size(args) == 0) {
+		/* No argument passed, truncate to current position */
+		pos = self->pos;
+	}
+
+        if (pos < 0) {
+		errno = EINVAL;
+		PyErr_SetFromErrno(PyExc_IOError);
+		return NULL;
+	}
 
         if (self->string_size > pos) self->string_size = pos;
         self->pos = self->string_size;