gerrit_util: Add GCE metadata server auth.

gerrit_util.py

Index: gerrit_util.py
diff --git a/gerrit_util.py b/gerrit_util.py
index 62fc1b9ab53cac7ddbf60f3f1dc70fae8ba93afd..691daf9182989a93339e8ed4245b4e8f326713e1 100755
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -15,36 +15,19 @@ import logging
 import netrc
 import os
 import re
+import socket
 import stat
 import sys
 import time
 import urllib
+import urlparse
 from cStringIO import StringIO
 
-_netrc_file = '_netrc' if sys.platform.startswith('win') else '.netrc'
-_netrc_file = os.path.join(os.environ['HOME'], _netrc_file)
-try:
-  NETRC = netrc.netrc(_netrc_file)
-except IOError:
-  print >> sys.stderr, 'WARNING: Could not read netrc file %s' % _netrc_file
-  NETRC = netrc.netrc(os.devnull)
-except netrc.NetrcParseError as e:
-  _netrc_stat = os.stat(e.filename)
-  if _netrc_stat.st_mode & (stat.S_IRWXG | stat.S_IRWXO):
-    print >> sys.stderr, (
-        'WARNING: netrc file %s cannot be used because its file permissions '
-        'are insecure.  netrc file permissions should be 600.' % _netrc_file)
-  else:
-    print >> sys.stderr, ('ERROR: Cannot use netrc file %s due to a parsing '
-                          'error.' % _netrc_file)
-    raise
-  del _netrc_stat
-  NETRC = netrc.netrc(os.devnull)
-del _netrc_file
 
 LOGGER = logging.getLogger()
 TRY_LIMIT = 5
 
+
 # Controls the transport protocol used to communicate with gerrit.
 # This is parameterized primarily to enable GerritTestCase.
 GERRIT_PROTOCOL = 'https'
@@ -84,17 +67,141 @@ def GetConnectionClass(protocol=None):
         "Don't know how to work with protocol '%s'" % protocol)
 
 
+class Authenticator(object):
+  """Base authenticator class for authenticator implementations to subclass."""
+
+  def get_auth_header(self, host):
+    raise NotImplementedError()
+
+  @staticmethod
+  def get():
+    """Returns: (Authenticator) The identified Authenticator to use.
+
+    Probes the local system and its environment and identifies the
+    Authenticator instance to use.
+    """
+    if GceAuthenticator.is_gce():
+      return GceAuthenticator()
+    return NetrcAuthenticator()
+
+
+class NetrcAuthenticator(Authenticator):
+  """Authenticator implementation that uses ".netrc" for token.
+  """
+
+  def __init__(self):
+    self.netrc = self._get_netrc()
+
+  @staticmethod
+  def _get_netrc():
+    path = '_netrc' if sys.platform.startswith('win') else '.netrc'
+    path = os.path.join(os.environ['HOME'], path)
+    try:
+      return netrc.netrc(path)
+    except IOError:
+      print >> sys.stderr, 'WARNING: Could not read netrc file %s' % path
+      return netrc.netrc(os.devnull)
+    except netrc.NetrcParseError as e:
+      st = os.stat(e.path)
+      if st.st_mode & (stat.S_IRWXG | stat.S_IRWXO):
+        print >> sys.stderr, (
+            'WARNING: netrc file %s cannot be used because its file '
+            'permissions are insecure.  netrc file permissions should be '
+            '600.' % path)
+      else:
+        print >> sys.stderr, ('ERROR: Cannot use netrc file %s due to a '
+                              'parsing error.' % path)
+        raise
+      return netrc.netrc(os.devnull)
+
+  def get_auth_header(self, host):
+    auth = self.netrc.authenticators(host)
+    if auth:
+      return 'Basic %s' % (base64.b64encode('%s:%s' % (auth[0], auth[2])))
+    return None
+
+
+class GceAuthenticator(Authenticator):
+  """Authenticator implementation that uses GCE metadata service for token.
+  """
+
+  _INFO_URL = 'http://metadata.google.internal'
+  _ACQUIRE_URL = ('http://metadata/computeMetadata/v1/instance/'
+                  'service-accounts/default/token')
+  _ACQUIRE_HEADERS = {"Metadata-Flavor": "Google"}
+
+  _cache_is_gce = None
+  _token_cache = None
+  _token_expiration = None
+
+  @classmethod
+  def is_gce(cls):
+    if cls._cache_is_gce is None:
+      cls._cache_is_gce = cls._test_is_gce()
+    return cls._cache_is_gce
+
+  @classmethod
+  def _test_is_gce(cls):
+    # Based on https://cloud.google.com/compute/docs/metadata#runninggce
+    try:
+      resp = cls._get(cls._INFO_URL)
+    except socket.error:
+      # Could not resolve URL.
+      return False
+    return resp.getheader('Metadata-Flavor', None) == 'Google'
+
+  @staticmethod
+  def _get(url, **kwargs):
+    next_delay_sec = 1
+    for i in xrange(TRY_LIMIT):
+      if i > 0:
+        # Retry server error status codes.
+        LOGGER.info('Encountered server error; retrying after %d second(s).',
+                    next_delay_sec)
+        time.sleep(next_delay_sec)
+        next_delay_sec *= 2
+
+      p = urlparse.urlparse(url)
+      c = GetConnectionClass(protocol=p.scheme)(p.netloc)
+      c.request('GET', url, **kwargs)
+      resp = c.getresponse()
+      LOGGER.debug('GET [%s] #%d/%d (%d)', url, i+1, TRY_LIMIT, resp.status)
+      if resp.status < httplib.INTERNAL_SERVER_ERROR:
+        return resp
+
+
+  @classmethod
+  def _get_token_dict(cls):
+    if cls._token_cache:
+      # If it expires within 25 seconds, refresh.
+      if cls._token_expiration < time.time() - 25:
+        return cls._token_cache
+
+    resp = cls._get(cls._ACQUIRE_URL, headers=cls._ACQUIRE_HEADERS)
+    if resp.status != httplib.OK:
+      return None
+    cls._token_cache = json.load(resp)
+    cls._token_expiration = cls._token_cache['expires_in'] + time.time()
+    return cls._token_cache
+
+  def get_auth_header(self, _host):
+    token_dict = self._get_token_dict()
+    if not token_dict:
+      return None
+    return '%(token_type)s %(access_token)s' % token_dict
+
+
+
 def CreateHttpConn(host, path, reqtype='GET', headers=None, body=None):
   """Opens an https connection to a gerrit service, and sends a request."""
   headers = headers or {}
   bare_host = host.partition(':')[0]
-  auth = NETRC.authenticators(bare_host)
 
+  auth = Authenticator.get().get_auth_header(bare_host)
   if auth:
-    headers.setdefault('Authorization', 'Basic %s' % (
-        base64.b64encode('%s:%s' % (auth[0], auth[2]))))
+    headers.setdefault('Authorization', auth)
   else:
-    LOGGER.debug('No authorization found in netrc for %s.' % bare_host)
+    LOGGER.debug('No authorization found for %s.' % bare_host)
 
   if 'Authorization' in headers and not path.startswith('a/'):
     url = '/a/%s' % path