Add GCF bearer token sample (GoogleCloudPlatform#3277)

* Add bearer token sample

* Add tests

* Address comments (pt 1)

Author: Ace Nassri

functions/security/main.py

@@ -0,0 +1,42 @@
+# Copyright 2020 Google LLC
+#
+# 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.
+
+# [START functions_bearer_token]
+import requests
+
+# TODO<developer>: set these values
+REGION = 'us-central1'
+PROJECT_ID = 'my-project'
+RECEIVING_FUNCTION = 'my-function'
+
+# Constants for setting up metadata server request
+# See https://cloud.google.com/compute/docs/instances/verifying-instance-identity#request_signature
+function_url = f'https://{REGION}-{PROJECT_ID}.cloudfunctions.net/{RECEIVING_FUNCTION}'
+metadata_server_url = \
+    'http://metadata/computeMetadata/v1/instance/service-accounts/default/identity?audience='
+token_full_url = metadata_server_url + function_url
+token_headers = {'Metadata-Flavor': 'Google'}
+
+
+def calling_function(request):
+    # Fetch the token
+    token_response = requests.get(token_full_url, headers=token_headers)
+    jwt = token_response.content
+
+    # Provide the token in the request to the receiving function
+    function_headers = {'Authorization': f'bearer {jwt}'}
+    function_response = requests.get(function_url, headers=function_headers)
+
+    return function_response.content
+# [END functions_bearer_token]

functions/security/main_test.py

@@ -0,0 +1,37 @@
+# Copyright 2020 Google LLC
+#
+# 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 flask
+import mock
+
+import main
+
+
+class Response(object):
+    def __init__(self, content=u''):
+        self.content = content
+
+
+@mock.patch("main.requests")
+def test_functions_bearer_token_should_run(requestsMock):
+    requestsMock.get.side_effect = [
+        Response(u'some-token'),
+        Response(u'function-done')
+    ]
+
+    res = main.calling_function(flask.request)
+
+    second_headers = requestsMock.get.call_args_list[0][1]
+    assert second_headers == {'headers': {'Metadata-Flavor': 'Google'}}
+    assert res == 'function-done'