Add logging to build_scan.py for url retries

scripts/slave/build_scan.py

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Scans a list of masters and saves information in a build_db."""
 
 from contextlib import closing
 import json
 import logging
@@ skipping 20 matching lines @@
   attempts = 0
   while True:
     try:
       with closing(urllib2.urlopen(url, timeout=URL_TIMEOUT)) as f:
         return json.load(f)
     except (urllib2.URLError, IOError) as f:
       if attempts > MAX_ATTEMPTS:
         raise
 
       attempts += 1
-      time.sleep(2 ** attempts)
+      time_to_sleep = 2 ** attempts
+      logging.info(
+        "url fetch encountered %s, sleeping for %d seconds and retrying..." % (
+            f, time_to_sleep))
+
+      time.sleep(time_to_sleep)
 
 
 def get_root_json(master_url):
   """Pull down root JSON which contains builder and build info."""
   url = master_url + '/json'
   logging.info('opening %s' % url)
   return _url_open_json(url)
 
 
 def find_new_builds(master_url, root_json, build_db):
@@ skipping 168 matching lines @@
     print '%s:%s:%s' % (master_url, builder, buildnum)
 
   if not options.skip_build_db_update:
     build_scan_db.save_build_db(build_db, {}, options.build_db)
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())