fix(metadata): enhance retry logic for metadata server access in _met…

…adata.py (#1545)

* fix(metadata): Use exponential backoff retry logic for GCE metadata server access

CONTRIBUTING.rst

@@ -16,7 +16,7 @@ A few notes on making changes to ``google-auth-library-python``.
 - If you've added a new feature or modified an existing feature, be sure to
   add or update any applicable documentation in docstrings and in the
   documentation (in ``docs/``). You can re-generate the reference documentation
-  using ``nox -s docgen``.
+  using ``nox -s docs``.
 
 - The change must work fully on the following CPython versions:
   3.7, 3.8, 3.9, 3.10, 3.11 and 3.12 across macOS, Linux, and Windows.

google/auth/compute_engine/_metadata.py

@@ -28,11 +28,12 @@
 from google.auth import environment_vars
 from google.auth import exceptions
 from google.auth import metrics
+from google.auth._exponential_backoff import ExponentialBackoff
 
 _LOGGER = logging.getLogger(__name__)
 
 # Environment variable GCE_METADATA_HOST is originally named
-# GCE_METADATA_ROOT. For compatiblity reasons, here it checks
+# GCE_METADATA_ROOT. For compatibility reasons, here it checks
 # the new variable first; if not set, the system falls back
 # to the old variable.
 _GCE_METADATA_HOST = os.getenv(environment_vars.GCE_METADATA_HOST, None)
@@ -119,11 +120,12 @@ def ping(request, timeout=_METADATA_DEFAULT_TIMEOUT, retry_count=3):
     #       could lead to false negatives in the event that we are on GCE, but
     #       the metadata resolution was particularly slow. The latter case is
     #       "unlikely".
-    retries = 0
     headers = _METADATA_HEADERS.copy()
     headers[metrics.API_CLIENT_HEADER] = metrics.mds_ping()
 
-    while retries < retry_count:
+    backoff = ExponentialBackoff(total_attempts=retry_count)
+
+    for attempt in backoff:
         try:
             response = request(
                 url=_METADATA_IP_ROOT, method="GET", headers=headers, timeout=timeout
@@ -139,11 +141,10 @@ def ping(request, timeout=_METADATA_DEFAULT_TIMEOUT, retry_count=3):
             _LOGGER.warning(
                 "Compute Engine Metadata server unavailable on "
                 "attempt %s of %s. Reason: %s",
-                retries + 1,
+                attempt,
                 retry_count,
                 e,
             )
-            retries += 1
 
     return False
 
@@ -179,7 +180,7 @@ def get(
 
     Returns:
         Union[Mapping, str]: If the metadata server returns JSON, a mapping of
-            the decoded JSON is return. Otherwise, the response content is
+            the decoded JSON is returned. Otherwise, the response content is
             returned as a string.
 
     Raises:
@@ -198,8 +199,9 @@ def get(
 
     url = _helpers.update_query(base_url, query_params)
 
-    retries = 0
-    while retries < retry_count:
+    backoff = ExponentialBackoff(total_attempts=retry_count)
+
+    for attempt in backoff:
         try:
             response = request(url=url, method="GET", headers=headers_to_use)
             break
@@ -208,11 +210,10 @@ def get(
             _LOGGER.warning(
                 "Compute Engine Metadata server unavailable on "
                 "attempt %s of %s. Reason: %s",
-                retries + 1,
+                attempt,
                 retry_count,
                 e,
             )
-            retries += 1
     else:
         raise exceptions.TransportError(
             "Failed to retrieve {} from the Google Compute Engine "

tests/compute_engine/test__metadata.py

@@ -125,13 +125,15 @@ def test_ping_success_retry(mock_metrics_header_value):
     assert request.call_count == 2
 
 
-def test_ping_failure_bad_flavor():
+@mock.patch("time.sleep", return_value=None)
+def test_ping_failure_bad_flavor(mock_sleep):
     request = make_request("", headers={_metadata._METADATA_FLAVOR_HEADER: "meep"})
 
     assert not _metadata.ping(request)
 
 
-def test_ping_failure_connection_failed():
+@mock.patch("time.sleep", return_value=None)
+def test_ping_failure_connection_failed(mock_sleep):
     request = make_request("")
     request.side_effect = exceptions.TransportError()
 
@@ -194,7 +196,8 @@ def test_get_success_json_content_type_charset():
     assert result[key] == value
 
 
-def test_get_success_retry():
+@mock.patch("time.sleep", return_value=None)
+def test_get_success_retry(mock_sleep):
     key, value = "foo", "bar"
 
     data = json.dumps({key: value})
@@ -310,7 +313,8 @@ def test_get_success_custom_root_old_variable():
     )
 
 
-def test_get_failure():
+@mock.patch("time.sleep", return_value=None)
+def test_get_failure(mock_sleep):
     request = make_request("Metadata error", status=http_client.NOT_FOUND)
 
     with pytest.raises(exceptions.TransportError) as excinfo:
@@ -337,7 +341,8 @@ def test_get_return_none_for_not_found_error():
     )
 
 
-def test_get_failure_connection_failed():
+@mock.patch("time.sleep", return_value=None)
+def test_get_failure_connection_failed(mock_sleep):
     request = make_request("")
     request.side_effect = exceptions.TransportError()