Add exponential backoff to bot_update.

scripts/slave/bot_update.py

Index: scripts/slave/bot_update.py
diff --git a/scripts/slave/bot_update.py b/scripts/slave/bot_update.py
index 65a8178e0a959c14e5c0502a0e16dbef3f13d726..b86262f2159c4bb2b7261f87eec3d320bb614311 100755
--- a/scripts/slave/bot_update.py
+++ b/scripts/slave/bot_update.py
@@ -14,6 +14,7 @@ import json
 import optparse
 import os
 import pprint
+import random
 import re
 import socket
 import subprocess
@@ -415,6 +416,11 @@ def call(*args, **kwargs):  # pragma: no cover
       return outval
     if result is FAIL:
       break
+    if result is RETRY:
+      sleep_backoff = 4 ** attempt
+      sleep_time = random.randint(sleep_backoff, int(sleep_backoff * 1.2))
+      print '===backing off, sleeping for %d secs===' % sleep_time
+      time.sleep(sleep_time)
 
   raise SubprocessFailed('%s failed with code %d in %s after %d attempts.' %
                          (' '.join(args), code, cwd, attempt),