Fix content-cache-k8s hanging in maintenance when nginx-proxy is consumed via CMR

src/charm.py

@@ -356,13 +356,26 @@ def _make_env_config(self, domain="svc.cluster.local") -> dict | None:
             svc_name = relation.data[relation.app].get("service-name")
             svc_port = relation.data[relation.app].get("service-port")
             backend_site_name = relation.data[relation.app].get("service-hostname")
-            clients = []
-            for peer in relation.units:
-                unit_name = peer.name.replace("/", "-")
-                service_url = f"{unit_name}.{svc_name}-endpoints.{self.model.name}.{domain}"
-                clients.append(f"http://{service_url}:{svc_port}")
-            # XXX: Will need to deal with multiple units at some point
-            backend = clients[0]
+            # In a Cross-Model Relation (CMR), remote unit names follow the pattern
+            # "remote-<uuid>/<n>" and cannot be used to build headless pod DNS entries
+            # (the unit name is a Juju-internal proxy token, not a real K8s pod name).
+            # Detect CMR by checking for the "remote-" prefix, and fall back to
+            # service-level DNS using the namespace provided in the relation data.
+            is_cmr = any(peer.name.startswith("remote-") for peer in relation.units)
+            if is_cmr:
+                svc_namespace = relation.data[relation.app].get(
+                    "service-namespace", self.model.name
+                )
+                service_url = f"{svc_name}.{svc_namespace}.{domain}"
+                backend = f"http://{service_url}:{svc_port}"
+            else:
+                clients = []
+                for peer in relation.units:
+                    unit_name = peer.name.replace("/", "-")
+                    service_url = f"{unit_name}.{svc_name}-endpoints.{self.model.name}.{domain}"
+                    clients.append(f"http://{service_url}:{svc_port}")
+                # XXX: Will need to deal with multiple units at some point
+                backend = clients[0]
         elif relation:
             return None
         else:

tests/unit/test_charm.py

@@ -501,6 +501,61 @@ def test_make_env_config_with_proxy_relation(self):
         new_site = new_env_config["CONTENT_CACHE_SITE"]
         assert new_site == relations_data["service-hostname"]
 
+    def test_make_env_config_with_cmr_proxy_relation(self):
+        """
+        arrange: set nginx-proxy relation with a CMR (Cross-Model Relation) remote unit
+        act: verify env config
+        assert: backend uses service-level DNS with service-namespace, not pod-level DNS
+        """
+        config = self.config
+        harness = self.harness
+        harness.disable_hooks()
+        harness.update_config(config)
+        # CMR remote application names follow the "remote-<uuid>" pattern
+        cmr_app_name = "remote-9e5538f0707b46568cc9b04d9f39f708"
+        relation_id = harness.add_relation("nginx-proxy", cmr_app_name)
+        harness.add_relation_unit(relation_id, f"{cmr_app_name}/0")
+        relations_data = {
+            "service-name": "indico",
+            "service-hostname": "indico.example.com",
+            "service-port": "8080",
+            "service-namespace": "target-model",
+        }
+        harness.update_relation_data(relation_id, cmr_app_name, relations_data)
+        env_config = harness.charm._make_env_config()
+        # The backend must use service-level K8s DNS, not pod-level headless DNS
+        assert env_config["CONTENT_CACHE_BACKEND"] == (
+            "http://indico.target-model.svc.cluster.local:8080"
+        )
+        assert env_config["NGINX_BACKEND"] == "http://indico.target-model.svc.cluster.local:8080"
+        assert env_config["CONTENT_CACHE_SITE"] == "indico.example.com"
+
+    def test_make_env_config_with_cmr_proxy_relation_no_namespace(self):
+        """
+        arrange: set nginx-proxy relation with a CMR remote unit but no service-namespace
+        act: verify env config
+        assert: backend falls back to service-level DNS using the local model name
+        """
+        config = self.config
+        harness = self.harness
+        harness.disable_hooks()
+        harness.update_config(config)
+        cmr_app_name = "remote-9e5538f0707b46568cc9b04d9f39f708"
+        relation_id = harness.add_relation("nginx-proxy", cmr_app_name)
+        harness.add_relation_unit(relation_id, f"{cmr_app_name}/0")
+        relations_data = {
+            "service-name": "indico",
+            "service-hostname": "indico.example.com",
+            "service-port": "8080",
+        }
+        harness.update_relation_data(relation_id, cmr_app_name, relations_data)
+        env_config = harness.charm._make_env_config()
+        # Falls back to local model name when service-namespace is absent
+        local_model = harness.charm.model.name
+        assert env_config["CONTENT_CACHE_BACKEND"] == (
+            f"http://indico.{local_model}.svc.cluster.local:8080"
+        )
+
     def test_make_pebble_config(self):
         """
         arrange: define pebble config