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#!/usr/bin/python
# Copyright 2016 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.
"""A library for watching changes in the metadata server."""
import functools
import json
import logging
import os
import socket
import time
from google_compute_engine.compat import httpclient
from google_compute_engine.compat import urlerror
from google_compute_engine.compat import urlparse
from google_compute_engine.compat import urlrequest
METADATA_SERVER = 'http://metadata.google.internal/computeMetadata/v1'
class StatusException(urlerror.HTTPError):
def __init__(self, response):
url = response.geturl()
code = response.getcode()
message = httpclient.responses.get(code)
headers = response.headers
super(StatusException, self).__init__(url, code, message, headers, response)
def RetryOnUnavailable(func):
"""Function decorator to retry on a service unavailable exception."""
@functools.wraps(func)
def Wrapper(*args, **kwargs):
while True:
try:
response = func(*args, **kwargs)
except urlerror.HTTPError as e:
if e.getcode() == httpclient.SERVICE_UNAVAILABLE:
time.sleep(5)
else:
raise
else:
if response.getcode() == httpclient.OK:
return response
else:
raise StatusException(response)
return Wrapper
class MetadataWatcher(object):
"""Watches for changes in metadata."""
def __init__(self, logger=logging, timeout=60):
"""Constructor.
Args:
logger: logger object, used to write to SysLog and serial port.
timeout: int, timeout in seconds for metadata requests.
"""
self.etag = 0
self.logger = logger
self.timeout = timeout
@RetryOnUnavailable
def _GetMetadataRequest(self, metadata_url, params=None):
"""Performs a GET request with the metadata headers.
Args:
metadata_url: string, the URL to perform a GET request on.
params: dictionary, the query parameters in the GET request.
Returns:
HTTP response from the GET request.
Raises:
urlerror.HTTPError: raises when the GET request fails.
"""
headers = {'Metadata-Flavor': 'Google'}
params = urlparse.urlencode(params or {})
url = '%s?%s' % (metadata_url, params)
request = urlrequest.Request(url, headers=headers)
request_opener = urlrequest.build_opener(urlrequest.ProxyHandler({}))
return request_opener.open(request, timeout=self.timeout*1.1)
def _UpdateEtag(self, response):
"""Update the etag from an API response.
Args:
response: HTTP response with a header field.
Returns:
bool, True if the etag in the response header updated.
"""
etag = response.headers.get('etag', self.etag)
etag_updated = self.etag != etag
self.etag = etag
return etag_updated
def _GetMetadataUpdate(self, metadata_key='', recursive=True, wait=True):
"""Request the contents of metadata server and deserialize the response.
Args:
metadata_key: string, the metadata key to watch for changes.
recursive: bool, True if we should recursively watch for metadata changes.
wait: bool, True if we should wait for a metadata change.
Returns:
json, the deserialized contents of the metadata server.
"""
metadata_key = os.path.join(metadata_key, '') if recursive else metadata_key
metadata_url = os.path.join(METADATA_SERVER, metadata_key)
params = {
'alt': 'json',
'last_etag': self.etag,
'recursive': recursive,
'timeout_sec': self.timeout,
'wait_for_change': wait,
}
while True:
response = self._GetMetadataRequest(metadata_url, params=params)
etag_updated = self._UpdateEtag(response)
if wait and not etag_updated:
# Retry until the etag is updated.
continue
else:
# Waiting for change is not required or the etag is updated.
break
return json.loads(response.read().decode('utf-8'))
def _HandleMetadataUpdate(self, metadata_key='', recursive=True, wait=True):
"""Wait for a successful metadata response.
Args:
metadata_key: string, the metadata key to watch for changes.
recursive: bool, True if we should recursively watch for metadata changes.
wait: bool, True if we should wait for a metadata change.
Returns:
json, the deserialized contents of the metadata server.
"""
exception = None
while True:
try:
return self._GetMetadataUpdate(
metadata_key=metadata_key, recursive=recursive, wait=wait)
except (httpclient.HTTPException, socket.error, urlerror.URLError) as e:
if isinstance(e, type(exception)):
continue
else:
exception = e
self.logger.exception('GET request error retrieving metadata.')
def WatchMetadata(self, handler, metadata_key='', recursive=True):
"""Watch for changes to the contents of the metadata server.
Args:
handler: callable, a function to call with the updated metadata contents.
metadata_key: string, the metadata key to watch for changes.
recursive: bool, True if we should recursively watch for metadata changes.
"""
while True:
response = self._HandleMetadataUpdate(
metadata_key=metadata_key, recursive=recursive, wait=True)
try:
handler(response)
except Exception as e:
self.logger.exception('Exception calling the response handler. %s.', e)
def GetMetadata(self, metadata_key='', recursive=True):
"""Retrieve the contents of metadata server for a metadata key.
Args:
metadata_key: string, the metadata key to watch for changes.
recursive: bool, True if we should recursively watch for metadata changes.
Returns:
json, the deserialized contents of the metadata server or None if error.
"""
return self._HandleMetadataUpdate(
metadata_key=metadata_key, recursive=recursive, wait=False)
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