Fix evil retry loop on poll errors.

appengine/swarming/swarming_bot/bot_code/remote_client.py

 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import base64
 import logging
 import threading
 import time
 import traceback
 import urllib
 
 from utils import net
 
+from remote_client_errors import BotCodeError
 from remote_client_errors import InitializationError
-from remote_client_errors import BotCodeError
 from remote_client_errors import InternalError
+from remote_client_errors import PollError
 
 
 # RemoteClient will attempt to refresh the authentication headers once they are
 # this close to the expiration.
 AUTH_HEADERS_EXPIRATION_SEC = 3*60
 
 
 # How long to wait for a response from the server. Must not be greater than
 # AUTH_HEADERS_EXPIRATION_SEC, since otherwise there's a chance auth headers
 # will expire while we wait for connection.
@@ skipping 180 matching lines @@
   def do_handshake(self, attributes):
     """Performs the initial handshake. Returns a dict (contents TBD)"""
     return self._url_read_json(
         '/swarming/api/v1/bot/handshake',
         data=attributes)
 
   def poll(self, attributes):
     """Polls for new work or other commands; returns a (cmd, value) pair as
     shown below.
 
-    Note that if the returned dict does not have the correct
-    values set, this method will raise an exception.
+    Raises:
+      PollError if can't contact the server after many attempts, the server
+      replies with an error or the returned dict does not have the correct
+      values set.
     """
     resp = self._url_read_json('/swarming/api/v1/bot/poll', data=attributes)
-    if not resp:
-      return (None, None)
+    if not resp or resp.get('error'):
+      raise PollError(
+          resp.get('error') if resp else 'Failed to contact server')
 
     cmd = resp['cmd']
     if cmd == 'sleep':
       return (cmd, resp['duration'])
     if cmd == 'terminate':
       return (cmd, resp['task_id'])
     if cmd == 'run':
       return (cmd, resp['manifest'])
     if cmd == 'update':
       return (cmd, resp['version'])
     if cmd == 'restart':
       return (cmd, resp['message'])
-    raise ValueError('Unexpected command: %s\n%s' % (cmd, resp))
+    raise PollError('Unexpected command: %s\n%s' % (cmd, resp))
 
   def get_bot_code(self, new_zip_path, bot_version, bot_id):
     """Downloads code into the file specified by new_zip_fn (a string).
 
     Throws BotCodeError on error.
     """
     url_path = '/swarming/api/v1/bot/bot_code/%s?bot_id=%s' % (
         bot_version, urllib.quote_plus(bot_id))
     if not self._url_retrieve(new_zip_path, url_path):
       raise BotCodeError(new_zip_path, self._server + url_path, bot_version)
 
   def ping(self):
     """Unlike all other methods, this one isn't authenticated."""
     resp = net.url_read(self._server + '/swarming/api/v1/bot/server_ping')
     if resp is None:
       logging.error('No response from server_ping')