Raise errors on unrecoverable problems (#110)

Raise errors for snap install or refresh failure.  Raise an error because it's a fatal failure, and juju will auto-retry the hook, which may succeed on retry if the error was transient.

Improve blocked status message for if it detects snap not installed or service not active in the status hook.
I think Blocked is better than Error status here:
- auto retrying the status hook isn't going to fix anything
- blocked status lets the charm provide a more useful status message to the user
- blocked status will automatically update to active on the next update-status hook once the issue is resolved


Partially fixes: #108

src/charm.py

@@ -16,7 +16,6 @@
 import ops
 import yaml
 from charms.grafana_agent.v0.cos_agent import COSAgentProvider
-from charms.operator_libs_linux.v2.snap import SnapError
 from ops.model import ActiveStatus, BlockedStatus, ModelError, WaitingStatus
 
 from service import SNAP_NAME, UPSTREAM_SNAP, get_installed_snap_service, snap_install_or_refresh
@@ -148,12 +147,10 @@ def get_resource(self) -> Optional[str]:
 
     def install(self) -> None:
         """Install the necessary resources for the charm."""
-        try:
-            snap_install_or_refresh(self.get_resource(), self.model.config["snap_channel"])
-        except SnapError:
-            self.model.unit.status = BlockedStatus(
-                "Failed to remove/install openstack-exporter snap"
-            )
+        # If this fails, it's not recoverable.
+        # So we don't catch the error, instead letting this become a charm error status.
+        # Errored hooks are auto-retried by juju, so the install may work on retry.
+        snap_install_or_refresh(self.get_resource(), self.model.config["snap_channel"])
 
     def _configure(self, _: ops.HookEvent) -> None:
         """Configure the charm.
@@ -228,11 +225,19 @@ def _on_collect_unit_status(self, event: ops.CollectStatusEvent) -> None:
 
         if not snap_service.present:
             event.add_status(
-                BlockedStatus("snap service is not installed, please check snap service")
+                BlockedStatus(
+                    f"{SNAP_NAME} snap is not installed. "
+                    "Please wait for installation to complete, "
+                    "or manually reinstall the snap if the issue persists."
+                )
             )
         elif not snap_service.is_active():
             event.add_status(
-                BlockedStatus("snap service is not running, please check snap service")
+                BlockedStatus(
+                    f"{SNAP_NAME} snap service is not active. "
+                    "Please wait for configuration to complete, "
+                    "or manually start the service the issue persists."
+                )
             )
 
         event.add_status(ActiveStatus())

tests/integration/tests/charm_tests/openstack_exporter.py

@@ -246,9 +246,9 @@ def test_openstack_exporter_snap_down(self):
         model.block_until_unit_wl_status(
             self.leader_unit_entity_id, "blocked", timeout=STATUS_TIMEOUT
         )
-        self.assertEqual(
+        self.assertIn(
+            "snap service is not active",
             self.leader_unit.workload_status_message,
-            "snap service is not running, please check snap service",
         )
 
         # Start the exporter snap

tests/unit/test_charm.py

@@ -8,14 +8,13 @@
 import ops
 import ops.testing
 import pytest
-from ops.model import BlockedStatus
+from charms.operator_libs_linux.v2.snap import SnapError
 
 from charm import (
     CLOUD_NAME,
     OS_CLIENT_CONFIG,
     SNAP_NAME,
     OpenstackExporterOperatorCharm,
-    SnapError,
 )
 from service import SnapService
 
@@ -106,6 +105,5 @@ def test_install_snap(self, mock_install, _):
     def test_install_snap_error(self, mock_install):
         mock_install.side_effect = SnapError("My Error")
         self.harness.begin()
-        self.harness.charm.on.install.emit()
-        expected_msg = "Failed to remove/install openstack-exporter snap"
-        assert self.harness.charm.unit.status == BlockedStatus(expected_msg)
+        with pytest.raises(SnapError, match="My Error"):
+            self.harness.charm.on.install.emit()