Add missing tell() method to GeneratorFile and add tests

util/registry/generatorfile.py

@@ -10,10 +10,16 @@ class GeneratorFile(object):
     self._generator = generator
     self._closed = False
     self._buf = ''
+    self._position = 0
 
   def __iter__(self):
     return self
 
+  def tell(self):
+    """Return the file's current position, like stdio's ftell()."""
+    _complain_ifclosed(self._closed)
+    return self._position
+
   def next(self):
     """A file object is its own iterator, for example iter(f) returns f
     (unless f is closed). When a file is used as an iterator, typically
@@ -62,8 +68,11 @@ class GeneratorFile(object):
       self._buf = ''
       returned = buf
 
+    self._position = self._position + len(returned)
     return returned
 
+  def seek(self):
+    raise NotImplementedError
 
   def close(self):
     self._closed = True