feat: drop alert labels when label value is empty

lib/charms/loki_k8s/v1/loki_push_api.py

@@ -1328,11 +1328,24 @@ def __init__(
 
     @staticmethod
     def _inject_extra_labels_to_alert_rules(rules: Dict, extra_alert_labels: Dict) -> Dict:
-        """Return a copy of the rules dict with extra labels injected."""
+        """Return a copy of the rules dict with extra labels injected.
+
+        Labels whose value is None or an empty string are removed from every
+        rule rather than being set.  If removing labels leaves the ``labels``
+        dict empty, the key is dropped from the rule entirely.
+        """
         result = copy.deepcopy(rules)
+        labels_to_drop = {k for k, v in extra_alert_labels.items() if v is None or v == ""}
+        labels_to_set = {k: v for k, v in extra_alert_labels.items() if k not in labels_to_drop}
+
         for group in result.get("groups", []):
             for rule in group.get("rules", []):
-                rule.setdefault("labels", {}).update(extra_alert_labels)
+                rule_labels = rule.setdefault("labels", {})
+                rule_labels.update(labels_to_set)
+                for key in labels_to_drop:
+                    rule_labels.pop(key, None)
+                if not rule_labels:
+                    del rule["labels"]
         return result
 
     def _handle_alert_rules(self, relation):

pyproject.toml

@@ -28,6 +28,7 @@ dev = [
   "pyright",
   # Unit
   "pytest",
+  "pytest-bdd",
   "coverage[toml]",
   "ops[testing]",
   "responses",
@@ -86,3 +87,8 @@ markers = ["setup", "work", "teardown"]
 [tool.codespell]
 skip = ".git,.tox,build,venv*"
 ignore-words-list = "assertIn"
+
+[dependency-groups]
+dev = [
+    "pytest-bdd>=8.1.0",
+]

tests/unit/features/inject_extra_labels.feature

@@ -0,0 +1,123 @@
+Feature: Inject extra labels to alert rules
+
+  Scenario: Extra labels are appended to all rules
+    Given rules
+      """
+      groups:
+        - rules:
+            - labels:
+                severity: critical
+            - labels:
+                severity: warning
+      """
+    When extra labels are injected
+      """
+      environment: production
+      team: observability
+      """
+    Then modified rules match
+      """
+      groups:
+        - rules:
+            - labels:
+                severity: critical
+                environment: production
+                team: observability
+            - labels:
+                severity: warning
+                environment: production
+                team: observability
+      """
+
+  Scenario: Extra labels override existing labels with the same key
+    Given rules
+      """
+      groups:
+        - rules:
+            - labels:
+                severity: warning
+                team: old-team
+      """
+    When extra labels are injected
+      """
+      team: new-team
+      """
+    Then modified rules match
+      """
+      groups:
+        - rules:
+            - labels:
+                severity: warning
+                team: new-team
+      """
+
+  Scenario: Empty extra labels leaves rules unchanged
+    Given rules
+      """
+      groups:
+        - rules:
+            - labels:
+                severity: warning
+                team: some-team
+      """
+    When extra labels are injected
+      """
+      ---
+      """
+    Then modified rules match
+      """
+      groups:
+        - rules:
+            - labels:
+                severity: warning
+                team: some-team
+      """
+
+  Scenario: Rules without a labels key get labels added
+    Given rules
+      """
+      groups:
+        - rules:
+            - alert: NoLabelsAlert
+      """
+    When extra labels are injected
+      """
+      env: staging
+      """
+    Then modified rules match
+      """
+      groups:
+        - rules:
+            - alert: NoLabelsAlert
+              labels:
+                env: staging
+      """
+
+  Scenario: Empty label values removes the label
+    Given rules
+      """
+      groups:
+        - rules:
+            - name: first
+              labels:
+                env: env1
+            - name: second
+              labels:
+                env: env2
+                foo: bar
+                severity: warning
+      """
+    When extra labels are injected
+      """
+      env: null
+      foo: ""
+      """
+    Then modified rules match
+      """
+      groups:
+        - rules:
+            - name: first
+            - name: second
+              labels:
+                severity: warning
+      """

tests/unit/test_inject_extra_labels.py

@@ -0,0 +1,31 @@
+"""Tests for ConsumerBase._inject_extra_labels_to_alert_rules using pytest-bdd."""
+
+
+import yaml
+from charms.loki_k8s.v1.loki_push_api import ConsumerBase
+from pytest_bdd import given, scenarios, then, when
+
+scenarios("features/inject_extra_labels.feature")
+
+
+@given("rules", target_fixture="rules")
+def given_rules(docstring):
+    """Parse the alert rules from the docstring fixture."""
+    return yaml.safe_load(docstring)
+
+
+@when("extra labels are injected", target_fixture="modified_rules")
+def when_extra_labels_injected(docstring, rules):
+    """Inject extra labels using the method under test."""
+    extra_labels = yaml.safe_load(docstring) or {}
+
+    return ConsumerBase._inject_extra_labels_to_alert_rules(
+        rules, extra_labels
+    )
+
+
+@then("modified rules match")
+def then_modified_rules_match(docstring, modified_rules):
+    """Verify every rule has the extra labels."""
+    expected = yaml.safe_load(docstring)
+    assert expected == modified_rules