test: adopt -Werror in unit tests

pyproject.toml

@@ -4,7 +4,7 @@ version = "0.0.0"
 requires-python = ">=3.10"
 dependencies = [
     "cosl",
-    "ops ~= 2.5",
+    "ops ~= 3.8",
     "pydantic < 2",
     "pyyaml ~= 6.0",
 ]
@@ -34,6 +34,22 @@ lint = [
 reformat = ["ruff"]
 tics = ["coverage[toml]"]
 
+[tool.pytest.ini_options]
+# Turn warnings into errors so deprecations and resource leaks fail the unit
+# suite. Each ignore below is message-anchored and narrow; see the comment on
+# each for why it is allowed and where the eventual fix belongs.
+filterwarnings = [
+    "error",
+    # Harness is deprecated in favour of Scenario. This is the intentional
+    # signal, ignored narrowly until tests/unit/test_charm.py is migrated; it
+    # pairs with a migration nudge in the PR.
+    "ignore:Harness is deprecated:PendingDeprecationWarning",
+    # The vendored grafana_agent/v0/cos_agent.py lib calls cosl's deprecated
+    # GrafanaDashboard._serialize; the fix belongs upstream in
+    # canonical/grafana-agent-operator (lib) / cosl.
+    "ignore:GrafanaDashboard._serialize is deprecated:DeprecationWarning",
+]
+
 [tool.ruff]
 line-length = 99
 preview = true

src/charm.py

@@ -16,6 +16,7 @@
 import ops
 import yaml
 from charms.grafana_agent.v0.cos_agent import COSAgentProvider
+from charms.operator_libs_linux.v2 import snap
 from ops.model import ActiveStatus, BlockedStatus, ModelError, WaitingStatus
 
 from service import SNAP_NAME, UPSTREAM_SNAP, get_installed_snap_service, snap_install_or_refresh
@@ -183,8 +184,7 @@ def _configure(self, _: ops.HookEvent) -> None:
 
         self.install()
 
-        upstream_snap = get_installed_snap_service(UPSTREAM_SNAP)
-        if upstream_snap.present:
+        if self._upstream_snap_present():
             return
 
         snap_service = get_installed_snap_service(SNAP_NAME)
@@ -219,6 +219,13 @@ def _on_upgrade(self, _: ops.UpgradeCharmEvent) -> None:
         """Handle upgrade charm event."""
         self.install()
 
+    def _upstream_snap_present(self) -> bool:
+        """Return True iff the legacy `golang-openstack-exporter` snap is installed and present."""
+        try:
+            return get_installed_snap_service(UPSTREAM_SNAP).present
+        except snap.SnapNotFoundError:
+            return False
+
     def _on_collect_unit_status(self, event: ops.CollectStatusEvent) -> None:
         """Handle collect unit status event (called after every event)."""
         if config_error := self.validate_configs():
@@ -233,10 +240,8 @@ def _on_collect_unit_status(self, event: ops.CollectStatusEvent) -> None:
         if not self.model.relations.get("cos-agent"):
             event.add_status(BlockedStatus("Grafana Agent is not related"))
 
-        upstream_snap = get_installed_snap_service(UPSTREAM_SNAP)
-
         # this is necessary when doing a charm upgrade coming from revision 27
-        if upstream_snap.present:
+        if self._upstream_snap_present():
             event.add_status(
                 BlockedStatus(
                     "golang-openstack-exporter detected. Please see: "

tests/unit/test_charm.py

@@ -258,6 +258,18 @@ def test_on_collect_unit_status(self, config, relations, test_values, expected_s
         self.harness.charm._on_collect_unit_status(mock_event)
         mock_event.add_status.assert_any_call(expected_status)
 
+    def test_upstream_snap_present_returns_false_when_snap_missing(self, mocker):
+        """_upstream_snap_present treats SnapNotFoundError as absent (not crash)."""
+        from charms.operator_libs_linux.v2 import snap as snap_mod
+
+        mock_get_installed_snap_service = mocker.patch("charm.get_installed_snap_service")
+        mock_get_installed_snap_service.side_effect = snap_mod.SnapNotFoundError(
+            "Snap 'golang-openstack-exporter' not found!"
+        )
+        self.harness.begin()
+        assert self.harness.charm._upstream_snap_present() is False
+        mock_get_installed_snap_service.assert_called_once_with(UPSTREAM_SNAP)
+
     @pytest.mark.parametrize(
         "protocol, expected_verify",
         [

tests/unit/test_service.py

@@ -55,9 +55,13 @@ def test_snap_install_or_refresh_snap_store(
 
 
 @mock.patch("service.log_ssdlc_system_event")
+@mock.patch("service.remove_upstream_snap")
+@mock.patch("service.remove_snap_as_resource")
 @mock.patch("service.workaround_bug_268")
 @mock.patch("service.snap.add")
-def test_snap_install_or_refresh_exception_raises(mock_snap, mock_workaround, mock_ssdlc):
+def test_snap_install_or_refresh_exception_raises(
+    mock_snap, mock_workaround, mock_remove_resource, mock_remove_upstream, mock_ssdlc
+):
     """Test that when an exception happens, it will raise an exception to the caller."""
     mock_snap.side_effect = service.snap.SnapError("My Error")
     with pytest.raises(service.snap.SnapError):