Allow customizing the GCE metadata service address via an env var. (#704)

The goal here is to make it possible for a user of a binary that depends on
this library (eg the google cloud SDK) to be able to customize where it looks
for the GCE metadata service. (An adventurous user can already customize the
GCE metadata service location via the existing global vars in this library.)

The only bit of awkwardness here is really the test: since this is a top-level
statement, reloading is the only way to ensure it works.

oauth2client/client.py

@@ -108,7 +108,7 @@ except ValueError:  # pragma: NO COVER
     GCE_METADATA_TIMEOUT = 3
 
 _SERVER_SOFTWARE = 'SERVER_SOFTWARE'
-_GCE_METADATA_URI = 'http://169.254.169.254'
+_GCE_METADATA_URI = 'http://' + os.getenv('GCE_METADATA_IP', '169.254.169.254')
 _METADATA_FLAVOR_HEADER = 'metadata-flavor'  # lowercase header
 _DESIRED_METADATA_FLAVOR = 'Google'
 _GCE_HEADERS = {_METADATA_FLAVOR_HEADER: _DESIRED_METADATA_FLAVOR}

oauth2client/contrib/_metadata.py

@@ -19,6 +19,7 @@ See https://cloud.google.com/compute/docs/metadata
 
 import datetime
 import json
+import os
 
 from six.moves import http_client
 from six.moves.urllib import parse as urlparse
@@ -28,7 +29,8 @@ from oauth2client import client
 from oauth2client import transport
 
 
-METADATA_ROOT = 'http://metadata.google.internal/computeMetadata/v1/'
+METADATA_ROOT = 'http://{}/computeMetadata/v1/'.format(
+    os.getenv('GCE_METADATA_ROOT', 'metadata.google.internal'))
 METADATA_HEADERS = {'Metadata-Flavor': 'Google'}
 
 

tests/contrib/test_gce.py

@@ -16,10 +16,12 @@
 
 import datetime
 import json
+import os
 import unittest
 
 import mock
 from six.moves import http_client
+from six.moves import reload_module
 
 from oauth2client import client
 from oauth2client.contrib import _metadata
@@ -155,3 +157,19 @@ class AppAssertionCredentialsTests(unittest.TestCase):
                 client.save_to_well_known_file(credentials)
         finally:
             os.path.isdir = ORIGINAL_ISDIR
+
+    def test_custom_metadata_root_from_env(self):
+        headers = {'content-type': 'application/json'}
+        http = http_mock.HttpMock(headers=headers, data='{}')
+        fake_metadata_root = 'another.metadata.service'
+        os.environ['GCE_METADATA_ROOT'] = fake_metadata_root
+        reload_module(_metadata)
+        try:
+            _metadata.get(http, '')
+        finally:
+            del os.environ['GCE_METADATA_ROOT']
+            reload_module(_metadata)
+        # Verify mock.
+        self.assertEqual(http.requests, 1)
+        expected_uri = 'http://{}/computeMetadata/v1/'.format(fake_metadata_root)
+        self.assertEqual(http.uri, expected_uri)