fix(google.auth.compute_engine.metadata): add retry to google.auth.co…

…mpute_engine._metadata.get() (#398)

Initial fix of issue #211 was done in CL #323, but only for .ping()
This one is adding same behaviour & tests for .get() method, as the problem still occurres
See the issue for details

Refs: #323
Resolves: #211

google/auth/compute_engine/_metadata.py

@@ -99,7 +99,7 @@ def ping(request, timeout=_METADATA_DEFAULT_TIMEOUT, retry_count=3):
     return False
 
 
-def get(request, path, root=_METADATA_ROOT, recursive=False):
+def get(request, path, root=_METADATA_ROOT, recursive=False, retry_count=5):
     """Fetch a resource from the metadata server.
 
     Args:
@@ -111,6 +111,8 @@ def get(request, path, root=_METADATA_ROOT, recursive=False):
         recursive (bool): Whether to do a recursive query of metadata. See
             https://cloud.google.com/compute/docs/metadata#aggcontents for more
             details.
+        retry_count (int): How many times to attempt connecting to metadata
+            server using above timeout.
 
     Returns:
         Union[Mapping, str]: If the metadata server returns JSON, a mapping of
@@ -129,7 +131,24 @@ def get(request, path, root=_METADATA_ROOT, recursive=False):
 
     url = _helpers.update_query(base_url, query_params)
 
-    response = request(url=url, method="GET", headers=_METADATA_HEADERS)
+    retries = 0
+    while retries < retry_count:
+        try:
+            response = request(url=url, method="GET", headers=_METADATA_HEADERS)
+            break
+
+        except exceptions.TransportError:
+            _LOGGER.info(
+                "Compute Engine Metadata server unavailable on" "attempt %s of %s",
+                retries + 1,
+                retry_count,
+            )
+            retries += 1
+    else:
+        raise exceptions.TransportError(
+            "Failed to retrieve {} from the Google Compute Engine"
+            "metadata service. Compute Engine Metadata server unavailable".format(url)
+        )
 
     if response.status == http_client.OK:
         content = _helpers.from_bytes(response.data)

tests/compute_engine/test__metadata.py

@@ -30,14 +30,17 @@
 PATH = "instance/service-accounts/default"
 
 
-def make_request(data, status=http_client.OK, headers=None):
+def make_request(data, status=http_client.OK, headers=None, retry=False):
     response = mock.create_autospec(transport.Response, instance=True)
     response.status = status
     response.data = _helpers.to_bytes(data)
     response.headers = headers or {}
 
     request = mock.create_autospec(transport.Request)
-    request.return_value = response
+    if retry:
+        request.side_effect = [exceptions.TransportError(), response]
+    else:
+        request.return_value = response
 
     return request
 
@@ -55,6 +58,20 @@ def test_ping_success():
     )
 
 
+def test_ping_success_retry():
+    request = make_request("", headers=_metadata._METADATA_HEADERS, retry=True)
+
+    assert _metadata.ping(request)
+
+    request.assert_called_with(
+        method="GET",
+        url=_metadata._METADATA_IP_ROOT,
+        headers=_metadata._METADATA_HEADERS,
+        timeout=_metadata._METADATA_DEFAULT_TIMEOUT,
+    )
+    assert request.call_count == 2
+
+
 def test_ping_failure_bad_flavor():
     request = make_request("", headers={_metadata._METADATA_FLAVOR_HEADER: "meep"})
 
@@ -105,6 +122,25 @@ def test_get_success_json():
     assert result[key] == value
 
 
+def test_get_success_retry():
+    key, value = "foo", "bar"
+
+    data = json.dumps({key: value})
+    request = make_request(
+        data, headers={"content-type": "application/json"}, retry=True
+    )
+
+    result = _metadata.get(request, PATH)
+
+    request.assert_called_with(
+        method="GET",
+        url=_metadata._METADATA_ROOT + PATH,
+        headers=_metadata._METADATA_HEADERS,
+    )
+    assert request.call_count == 2
+    assert result[key] == value
+
+
 def test_get_success_text():
     data = "foobar"
     request = make_request(data, headers={"content-type": "text/plain"})
@@ -154,6 +190,23 @@ def test_get_failure():
     )
 
 
+def test_get_failure_connection_failed():
+    request = make_request("")
+    request.side_effect = exceptions.TransportError()
+
+    with pytest.raises(exceptions.TransportError) as excinfo:
+        _metadata.get(request, PATH)
+
+    assert excinfo.match(r"Compute Engine Metadata server unavailable")
+
+    request.assert_called_with(
+        method="GET",
+        url=_metadata._METADATA_ROOT + PATH,
+        headers=_metadata._METADATA_HEADERS,
+    )
+    assert request.call_count == 5
+
+
 def test_get_failure_bad_json():
     request = make_request("{", headers={"content-type": "application/json"})