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#!/usr/bin/python
# Copyright 2015 Google Inc. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import httplib
import time
import urllib
import urllib2
METADATA_URL = 'http://metadata.google.internal/computeMetadata/v1/'
class Error(Exception):
pass
class UnexpectedStatusException(Error):
pass
class MetadataWatcher(object):
"""Watches for changing metadata."""
def __init__(self, httplib_module=httplib, time_module=time,
urllib_module=urllib, urllib2_module=urllib2):
self.httplib = httplib_module
self.time = time_module
self.urllib = urllib_module
self.urllib2 = urllib2_module
def WatchMetadataForever(self, metadata_key, handler, initial_value=None):
"""Watches for a change in the value of metadata.
Args:
metadata_key: The key identifying which metadata to watch for changes.
handler: A callable to call when the metadata value changes. Will be passed
a single parameter, the new value of the metadata.
initial_value: The expected initial value for the metadata. The handler will
not be called on the initial metadata request unless the value differs
from this.
Raises:
UnexpectedStatusException: If the http request is unsuccessful for an
unexpected reason.
"""
params = {
'wait_for_change': 'true',
'last_etag': 0,
}
value = initial_value
while True:
# start a hanging-GET request for metadata key.
url = '{base_url}{key}?{params}'.format(
base_url=METADATA_URL,
key=metadata_key,
params=self.urllib.urlencode(params)
)
req = self.urllib2.Request(url, headers={'Metadata-Flavor': 'Google'})
try:
response = self.urllib2.urlopen(req)
content = response.read()
status = response.getcode()
except self.urllib2.HTTPError as e:
content = None
status = e.code
if status == self.httplib.SERVICE_UNAVAILABLE:
self.time.sleep(1)
continue
elif status == self.httplib.OK:
# Extract new metadata value and latest etag.
new_value = content
headers = response.info()
params['last_etag'] = headers['ETag']
else:
raise UnexpectedStatusException(status)
# If the metadata value changed, call the appropriate handler.
if value == initial_value:
value = new_value
elif value != new_value:
value = new_value
handler(value)
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