juju: Add kubelet-extra-config to kubernetes-worker (#145)

cluster/juju/layers/kubernetes-worker/config.yaml

@@ -89,3 +89,20 @@ options:
       as possible. You may also set a custom string as described in the
       'refresh.timer' section here:
         https://forum.snapcraft.io/t/system-options/87
+  kubelet-extra-config:
+    default: "{}"
+    type: string
+    description: |
+      Extra configuration to be passed to kubelet. Any values specified in this
+      config will be merged into a KubeletConfiguration file that is passed to
+      the kubelet service via the --config flag. This can be used to override
+      values provided by the charm.
+
+      Requires Kubernetes 1.10+.
+
+      The value for this config must be a YAML mapping that can be safely
+      merged with a KubeletConfiguration file. For example:
+        {evictionHard: {memory.available: 200Mi}}
+
+      For more information about KubeletConfiguration, see upstream docs:
+      https://kubernetes.io/docs/tasks/administer-cluster/kubelet-config-file/

cluster/juju/layers/kubernetes-worker/reactive/kubernetes_worker.py

@@ -552,8 +552,9 @@ def apply_node_labels():
 
 
 @when_any('config.changed.kubelet-extra-args',
-          'config.changed.proxy-extra-args')
-def extra_args_changed():
+          'config.changed.proxy-extra-args',
+          'config.changed.kubelet-extra-config')
+def config_changed_requires_restart():
     set_state('kubernetes-worker.restart-needed')
 
 
@@ -674,6 +675,20 @@ def configure_kubernetes_service(service, base_args, extra_args_key):
     db.set(prev_args_key, args)
 
 
+def merge_kubelet_extra_config(config, extra_config):
+    ''' Updates config to include the contents of extra_config. This is done
+    recursively to allow deeply nested dictionaries to be merged.
+
+    This is destructive: it modifies the config dict that is passed in.
+    '''
+    for k, extra_config_value in extra_config.items():
+        if isinstance(extra_config_value, dict):
+            config_value = config.setdefault(k, {})
+            merge_kubelet_extra_config(config_value, extra_config_value)
+        else:
+            config[k] = extra_config_value
+
+
 def configure_kubelet(dns, ingress_ip):
     layer_options = layer.options('tls-client')
     ca_cert_path = layer_options.get('ca_certificate_path')
@@ -727,6 +742,12 @@ def configure_kubelet(dns, ingress_ip):
                 'DevicePlugins': True
             }
 
+        # Add kubelet-extra-config. This needs to happen last so that it
+        # overrides any config provided by the charm.
+        kubelet_extra_config = hookenv.config('kubelet-extra-config')
+        kubelet_extra_config = yaml.load(kubelet_extra_config)
+        merge_kubelet_extra_config(kubelet_config, kubelet_extra_config)
+
         # Render the file and configure Kubelet to use it
         os.makedirs('/root/cdk/kubelet', exist_ok=True)
         with open('/root/cdk/kubelet/config.yaml', 'w') as f: