updater: more optimal file length checking

Rather than read to the end of the file in order to determin its size, use
the whence value of seek() to move the file object's position to the end
of the file, then the tell() method of the file object to read the current
position in bytes.

Co-authored-by: Jussi Kukkonen <jkukkonen@vmware.com>
Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

tuf/client/updater.py

@@ -1244,8 +1244,10 @@ def _check_file_length(self, file_object, trusted_file_length):
       None.
     """
 
-    file_object.seek(0)
-    observed_length = len(file_object.read())
+    # seek to the end of the file; that is offset 0 from the end of the file,
+    # represented by a whence value of 2
+    file_object.seek(0, 2)
+    observed_length = file_object.tell()
 
     # Return and log a message if the length 'file_object' is equal to
     # 'trusted_file_length', otherwise raise an exception.  A hard check