fix(validation): Validate Dask Cluster Names

This commit introduces cluster name validation in order to avoid the
invalid state in which a `DaskCluster` resource with a too-long or
RFC-1123-noncompliant name is created but cannot be
deleted while the operator retries infinitely to create a scheduler service (see #826 for more details on this bug).

Issues fixed:
#870
#826

dask_kubernetes/operator/controller/controller.py

@@ -1,4 +1,5 @@
 import asyncio
+import re
 import time
 from collections import defaultdict
 from contextlib import suppress
@@ -31,6 +32,19 @@
 KUBERNETES_DATETIME_FORMAT = "%Y-%m-%dT%H:%M:%SZ"
 
 DASK_AUTOSCALER_COOLDOWN_UNTIL_ANNOTATION = "kubernetes.dask.org/cooldown-until"
+KUBERNETES_MAX_RESOURCE_NAME_LENGTH = 63
+SCHEDULER_NAME_TEMPLATE = "{cluster_name}-scheduler"
+MAX_CLUSTER_NAME_LEN = KUBERNETES_MAX_RESOURCE_NAME_LENGTH - len(
+    SCHEDULER_NAME_TEMPLATE.format(cluster_name="")
+)
+VALID_CLUSTER_NAME = re.compile(
+    rf"^(?=.{{,{MAX_CLUSTER_NAME_LEN}}}$)[a-z0-9]([-a-z0-9]*[a-z0-9])?$"
+)
+
+
+def _validate_cluster_name(cluster_name: str) -> bool:
+    return bool(VALID_CLUSTER_NAME.match(cluster_name))
+
 
 # Load operator plugins from other packages
 PLUGINS = []
@@ -75,7 +89,7 @@ def build_scheduler_deployment_spec(
         }
     )
     metadata = {
-        "name": f"{cluster_name}-scheduler",
+        "name": SCHEDULER_NAME_TEMPLATE.format(cluster_name=cluster_name),
         "labels": labels,
         "annotations": annotations,
     }
@@ -107,7 +121,7 @@ def build_scheduler_service_spec(cluster_name, spec, annotations, labels):
         "apiVersion": "v1",
         "kind": "Service",
         "metadata": {
-            "name": f"{cluster_name}-scheduler",
+            "name": SCHEDULER_NAME_TEMPLATE.format(cluster_name=cluster_name),
             "labels": labels,
             "annotations": annotations,
         },
@@ -273,6 +287,16 @@ async def daskcluster_create(name, namespace, logger, patch, **kwargs):
     This allows us to track that the operator is running.
     """
     logger.info(f"DaskCluster {name} created in {namespace}.")
+
+    if not _validate_cluster_name(name):
+        patch.status["phase"] = "Failed"
+        raise kopf.PermanentError(
+            f"The DaskCluster {name} is invalid: a lowercase RFC 1123 subdomain must "
+            "consist of lower case alphanumeric characters, '-' or '.', and must start "
+            "and end with an alphanumeric character. DaskCluster name must also be under "
+            f"{MAX_CLUSTER_NAME_LEN} characters."
+        )
+
     patch.status["phase"] = "Created"
 
 
@@ -331,6 +355,12 @@ async def daskcluster_create_components(
     patch.status["phase"] = "Pending"
 
 
+@kopf.on.field("daskcluster.kubernetes.dask.org", field="status.phase", new="Failed")
+async def daskcluster_delete(spec, name, namespace, logger, patch, meta, **kwargs):
+    cluster = await DaskCluster.get(name, namespace=namespace)
+    await cluster.delete()
+
+
 @kopf.on.field("service", field="status", labels={"dask.org/component": "scheduler"})
 async def handle_scheduler_service_status(
     spec, labels, status, namespace, logger, **kwargs
@@ -599,7 +629,7 @@ async def daskworkergroup_replica_update(
         if workers_needed < 0:
             worker_ids = await retire_workers(
                 n_workers=-workers_needed,
-                scheduler_service_name=f"{cluster_name}-scheduler",
+                scheduler_service_name=SCHEDULER_NAME_TEMPLATE.format(cluster_name),
                 worker_group_name=name,
                 namespace=namespace,
                 logger=logger,