Patch # 1323 by Amaury Forgeot d'Arc.

This patch corrects a problem in test_file.py on Windows:
f.truncate() seeks to the truncation point, but does not empty the
buffers. In the test, f.tell() returns -1...

Lib/io.py

@@ -597,9 +597,24 @@ class _BufferedIOMixin(BufferedIOBase):
         return self.raw.tell()
 
     def truncate(self, pos=None):
+        # On Windows, the truncate operation changes the current position
+        # to the end of the file, which may leave us with desynchronized
+        # buffers.
+        # Since we promise that truncate() won't change the current position,
+        # the easiest thing is to capture current pos now and seek back to
+        # it at the end.
+
+        initialpos = self.tell()
         if pos is None:
-            pos = self.tell()
-        return self.raw.truncate(pos)
+            pos = initialpos
+
+        # Flush the stream.  We're mixing buffered I/O with lower-level I/O,
+        # and a flush may be necessary to synch both views of the current
+        # file state.
+        self.flush()
+        newpos = self.raw.truncate(pos)
+        self.seek(initialpos)
+        return newpos
 
     ### Flush and close ###