tests: harden old instance cleanup against NotFound (#471)

Toward #435, #436, #437.

samples/samples/conftest.py

@@ -41,6 +41,17 @@ def spanner_client():
     return client.Client()
 
 
+def scrub_instance_ignore_not_found(to_scrub):
+    """Helper for func:`cleanup_old_instances`"""
+    try:
+        for backup_pb in to_scrub.list_backups():
+            backup.Backup.from_pb(backup_pb, to_scrub).delete()
+
+        to_scrub.delete()
+    except exceptions.NotFound:
+        pass
+
+
 @pytest.fixture(scope="session")
 def cleanup_old_instances(spanner_client):
     """Delete instances, created by samples, that are older than an hour."""
@@ -54,11 +65,7 @@ def cleanup_old_instances(spanner_client):
             create_time = int(inst.labels["created"])
 
             if create_time <= cutoff:
-
-                for backup_pb in inst.list_backups():
-                    backup.Backup.from_pb(backup_pb, inst).delete()
-
-                inst.delete()
+                scrub_instance_ignore_not_found(inst)
 
 
 @pytest.fixture(scope="module")
@@ -76,19 +83,15 @@ def instance_config(spanner_client):
 
 @pytest.fixture(scope="module")
 def sample_instance(
-    spanner_client,
-    cleanup_old_instances,
-    instance_id,
-    instance_config,
-    sample_name,
+    spanner_client, cleanup_old_instances, instance_id, instance_config, sample_name,
 ):
     sample_instance = spanner_client.instance(
         instance_id,
         instance_config,
         labels={
             "cloud_spanner_samples": "true",
             "sample_name": sample_name,
-            "created": str(int(time.time()))
+            "created": str(int(time.time())),
         },
     )
     retry_429 = retry.RetryErrors(exceptions.ResourceExhausted, delay=15)