feat: Make it possible to customize k8s resources

Currently there is no way for plugins to customize Kubernetes resources
defined in Tutor deployment manifests.
This change makes that possible by taking advantage of the strategic
merge patching mechanism in `kustomization.yml`.
Any resource definition in a `k8s-override` patch in a plugin will
override the resource defined by Tutor, provided that their names match.

Reference: overhangio#675

CHANGELOG.md

@@ -17,6 +17,7 @@ Every user-facing change should have an entry in this changelog. Please respect
 -->
 
 ## Unreleased
+- [Improvement] Make it possible to override k8s resources in plugins using `k8s-override` patch. (by @foadlind)
 
 ## v13.3.1 (2022-06-06)
 

docs/k8s.rst

@@ -140,3 +140,27 @@ Updating docker images
 Kubernetes does not provide a single command for updating docker images out of the box. A `commonly used trick <https://github.com/kubernetes/kubernetes/issues/33664>`__ is to modify an innocuous label on all resources::
 
     kubectl patch -k "$(tutor config printroot)/env" --patch "{\"spec\": {\"template\": {\"metadata\": {\"labels\": {\"date\": \"`date +'%Y%m%d-%H%M%S'`\"}}}}}"
+
+Customizing Kubernetes resources
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Plugins can customize any Kubernetes resource in Tutor by overriding the definition of the resource with a ``k8s-override`` patch. For example, to change the volume size for MongoDB from ``5Gi`` to ``10Gi``, add the following to the plugin:
+
+::
+
+    # myplugin/tutormyplugin/patches/k8s-override
+    
+    apiVersion: v1
+    kind: PersistentVolumeClaim
+    metadata:
+      name: mongodb
+      labels:
+        app.kubernetes.io/component: volume
+        app.kubernetes.io/name: mongodb
+    spec:
+      accessModes:
+        - ReadWriteOnce
+      resources:
+        requests:
+          storage: 10Gi
+

docs/reference/patches.rst

@@ -125,6 +125,13 @@ File: ``kustomization.yml``
 
 File: ``kustomization.yml``
 
+.. patch:: kustomization-patches-strategic-merge
+
+``kustomization-patches-strategic-merge``
+=========================================
+
+File: ``kustomization.yml``
+
 .. patch:: kustomization-resources
 
 ``kustomization-resources``

tutor/templates/k8s/override.yml

@@ -0,0 +1 @@
+{{ patch("k8s-override") }}

tutor/templates/kustomization.yml

@@ -59,4 +59,8 @@ configMapGenerator:
         app.kubernetes.io/name: redis
 {{ patch("kustomization-configmapgenerator") }}
 
+patchesStrategicMerge:
+- k8s/override.yml
+{{ patch("kustomization-patches-strategic-merge") }}
+
 {{ patch("kustomization") }}