KDP for allow cloning multiple resources (#31)

* KDP for allow cloning multiple resources

Signed-off-by: prateekpandey14 <prateek.pandey@nirmata.com>

* add kinds to filter the resource based on GVK

Signed-off-by: prateekpandey14 <prateek.pandey@nirmata.com>

* Sync with new changes

Signed-off-by: prateekpandey14 <prateek.pandey@nirmata.com>

Signed-off-by: prateekpandey14 <prateek.pandey@nirmata.com>

proposals/clone_mulitple_resource.md

@@ -0,0 +1,163 @@
+# Meta
+[meta]: #meta
+- Name: Allow cloning multiple resources from a Namespace
+- Start Date: 2022-08-04
+- Update data (optional): 2022-08-04
+- Author: [Prateek Pandey](https://github.com/prateekpandey14)
+- Supersedes:  N/A
+
+# Table of Contents
+[table-of-contents]: #table-of-contents
+- [Meta](#meta)
+- [Table of Contents](#table-of-contents)
+- [Overview](#overview)
+- [Definitions](#definitions)
+- [Motivation](#motivation)
+- [Proposal](#proposal)
+- [Implementation](#implementation)
+- [Migration (OPTIONAL)](#migration-optional)
+- [Drawbacks](#drawbacks)
+- [Alternatives](#alternatives)
+- [Prior Art](#prior-art)
+- [Unresolved Questions](#unresolved-questions)
+- [CRD Changes (OPTIONAL)](#crd-changes-optional)
+
+# Overview
+[overview]: #overview
+
+Enhance kyverno generate rule to clone mulitple resources from a given namespace.
+
+
+# Definitions
+[definitions]: #definitions
+
+N/A
+
+# Motivation
+[motivation]: #motivation
+
+The initial idea for this feature came from issue [282](https://github.com/kyverno/kyverno/issues/282) and received feedback from the kyverno community in slack.
+
+It would be good to have a way to copy/clone multiple resources from a namespace in a single rule, instead of
+writing a new rule or policy.
+
+There are primarily main use cases associated with this.
+
+1. To clone set of resources from a common namespace, using a match selector whenever we create a new namespace
+   to bootstrap the deployment process.
+
+2. Implicit, User has to less number of policy/rule to copy the set of necessary resources.
+
+# Proposal
+
+In this proposal, Kyverno generate rule can be extended to allowing to clone multiple resources from a namespace.
+This could be done using  matching selector to select multiple resources of the given apiVersion/group and allowing
+multiple kinds/name pairs, similar to the match/exclude blocks we use to filter resources in the admission control
+webhooks.
+
+In below extended ClusterPolicy example, multiple resources can be selected using a kinds list to copy/clone from 
+`stagging` namespace to newly created namespace.
+
+```yaml
+    generate:
+      namespace: "{{request.object.metadata.name}}"
+      synchronize : true
+      cloneList:
+      - namespace: stagging
+        kinds:
+          - v1/Secret   // clone resource with matching selector i.e. configmap, secrets etc
+```
+
+### Examples:
+
+1. Clone multiple resource using matching kinds list
+
+```yaml
+
+apiVersion: kyverno.io/v1
+kind: ClusterPolicy
+metadata:
+  name: clone-resource-with-selector
+spec:
+  generateExistingOnPolicyUpdate: true
+  rules:
+  - name: clone-resource
+    match:
+      any:
+      - resources:
+          kinds:
+          - Namespace
+    generate:
+      namespace: "{{request.object.metadata.name}}"
+      synchronize : true
+      cloneList:
+      - namespace: stagging
+        kinds:
+          - v1/Secret   // clone resource with matching selector i.e. configmap, secrets etc
+```
+
+
+# Implementation
+
+- As part of implementation changes,firstly we have to extend the existing APIs, the `generate.cloneList` rule
+will have a `kinds` and `namespace` field to provide the list of resources to be cloned.
+
+```go
+// Generation defines how new resources should be created and managed.
+type Generation struct {
+.
+.
+.
+// CloneList specifies the list of source resource used to populate each generated resource.
+// +optional
+CloneList CloneList `json:"cloneList,omitempty" yaml:"cloneList,omitempty"`
+}
+
+type CloneList struct {
+// Namespace specifies source resource namespace.
+// +optional
+Namespace string `json:"namespace,omitempty" yaml:"namespace,omitempty"`
+
+// Kinds is a list of resource kinds.
+// +optional
+Kinds []string `json:"kinds,omitempty" yaml:"kinds,omitempty"`
+}
+```
+
+- Generate controller will filter out the resources based on kinds list, similar to
+  match and exclude block.
+
+- Once the resources have been cloned, we need to make sure the reference resource and newly generated 
+  resources are always synced.
+
+## Link to the Implementation PR
+
+N/A
+
+# Migration (OPTIONAL)
+
+N/A
+
+# Drawbacks
+
+* Makes Kyverno code base more complex
+* Places additional strain on the Kubernetes API server and Kyverno itself as some rule processing may need to list and filter so many resources in a namespace.
+
+# Alternatives
+
+- Clone multiple resource using list of kind/namespace pairs in `spec.clone`
+- which is not easy to extend/manage new or existing policies due to backward compatibility.
+- If we support cloning specific resources from a namespace, we end up writing
+  more then one policy/rule to clone multiple resources.
+
+# Prior Art
+
+N/A
+
+# Unresolved Questions
+
+N/A
+
+# CRD Changes (OPTIONAL)
+
+This KDP will require introduction of two new fields under `generate.cloneList` rule.