Add node autodiscovery to cluster-autoscaler clusterapi provider

cluster-autoscaler/cloudprovider/clusterapi/README.md

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+# Cluster Autoscaler on Cluster API
+
+The cluster autoscaler on [Cluster API](https://cluster-api.sigs.k8s.io/) uses
+the [cluster-api project](https://github.com/kubernetes-sigs/cluster-api) to
+manage the provisioning and de-provisioning of nodes within a Kubernetes
+cluster.
+
+## Kubernetes Version
+
+The cluster-api provider requires Kubernetes v1.16 or greater to run the
+v1alpha3 version of the API.
+
+## Cluster API Prerequisites
+
+Please be aware that currently the cluster autoscaler only supports CAPI
+clusters that have joined their management and workload clusters into a single
+cluster. For more information about this please see the
+[Cluster API Concepts documentations](https://cluster-api.sigs.k8s.io/user/concepts.html)
+and the [`clusterctl move` command documentation](https://cluster-api.sigs.k8s.io/user/concepts.html).
+
+## Starting the Autoscaler
+
+To enable the Cluster API provider, you must first specify it in the command
+line arguments to the cluster autoscaler binary. For example:
+
+```
+cluster-autoscaler --cloud-provider=clusterapi
+```
+
+Please note, this example only shows the cloud provider options, you will
+most likely need other command line flags. For more information you can invoke
+`cluster-autoscaler --help` to see a full list of options.
+
+## Configuring node group auto discovery
+
+If you do not configure node group auto discovery, cluster autoscaler will attempt
+to match nodes against any scalable resources found in any namespace and belonging
+to any Cluster.
+
+Limiting cluster autoscaler to only match against resources in the blue namespace
+
+```
+--node-group-auto-discovery=clusterapi:namespace=blue
+```
+
+Limiting cluster autoscaler to only match against resources belonging to Cluster test1
+
+```
+--node-group-auto-discovery=clusterapi:clusterName=test1
+```
+
+Limiting cluster autoscaler to only match against resources matching the provided labels
+
+```
+--node-group-auto-discovery=clusterapi:color=green,shape=square
+```
+
+These can be mixed and matched in any combination, for example to only match resources
+in the staging namespace, belonging to the purple cluster, with the label owner=jim:
+
+```
+--node-group-auto-discovery=clusterapi:namespace=staging,clusterName=purple,owner=jim
+```
+
+## Enabling Autoscaling
+
+To enable the automatic scaling of components in your cluster-api managed
+cloud there are a few annotations you need to provide. These annotations
+must be applied to either [MachineSet](https://cluster-api.sigs.k8s.io/developer/architecture/controllers/machine-set.html)
+or [MachineDeployment](https://cluster-api.sigs.k8s.io/developer/architecture/controllers/machine-deployment.html)
+resources depending on the type of cluster-api mechanism that you are using.
+
+There are two annotations that control how a cluster resource should be scaled:
+
+* `cluster.k8s.io/cluster-api-autoscaler-node-group-min-size` - This specifies
+  the minimum number of nodes for the associated resource group. The autoscaler
+  will not scale the group below this number. Please note that currently the
+  cluster-api provider will not scale down to zero nodes.
+
+* `cluster.k8s.io/cluster-api-autoscaler-node-group-max-size` - This specifies
+  the maximum number of nodes for the associated resource group. The autoscaler
+  will not scale the group above this number.
+
+The autoscaler will monitor any `MachineSet` or `MachineDeployment` containing
+both of these annotations.
+
+## Specifying a Custom Resource Group
+
+By default all Kubernetes resources consumed by the Cluster API provider will
+use the group `cluster.x-k8s.io`, with a dynamically acquired version. In
+some situations, such as testing or prototyping, you may wish to change this
+group variable. For these situations you may use the environment variable
+`CAPI_GROUP` to change the group that the provider will use.
+
+## Sample manifest
+
+A sample manifest that will create a deployment running the autoscaler is
+available. It can be deployed by passing it through `envsubst`, providing
+these environment variables to set the namespace to deploy into as well as the image and tag to use:
+
+```
+export AUTOSCALER_NS=kube-system
+export AUTOSCALER_IMAGE=us.gcr.io/k8s-artifacts-prod/autoscaling/cluster-autoscaler:v1.18.1
+envsubst < examples/deployment.yaml | kubectl apply -f-
+```

cluster-autoscaler/cloudprovider/clusterapi/clusterapi_autodiscovery.go

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+/*
+Copyright 2020 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package clusterapi
+
+import (
+	"fmt"
+	"strings"
+
+	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
+	"k8s.io/apimachinery/pkg/labels"
+	"k8s.io/apimachinery/pkg/selection"
+
+	"k8s.io/autoscaler/cluster-autoscaler/utils/errors"
+)
+
+type clusterAPIAutoDiscoveryConfig struct {
+	clusterName   string
+	namespace     string
+	labelSelector labels.Selector
+}
+
+func parseAutoDiscoverySpec(spec string) (*clusterAPIAutoDiscoveryConfig, error) {
+	cfg := &clusterAPIAutoDiscoveryConfig{
+		labelSelector: labels.NewSelector(),
+	}
+
+	tokens := strings.Split(spec, ":")
+	if len(tokens) != 2 {
+		return cfg, errors.NewAutoscalerError(errors.ConfigurationError, fmt.Sprintf("spec \"%s\" should be discoverer:key=value,key=value", spec))
+	}
+	discoverer := tokens[0]
+	if discoverer != autoDiscovererTypeClusterAPI {
+		return cfg, errors.NewAutoscalerError(errors.ConfigurationError, fmt.Sprintf("unsupported discoverer specified: %s", discoverer))
+	}
+
+	for _, arg := range strings.Split(tokens[1], ",") {
+		if len(arg) == 0 {
+			continue
+		}
+		kv := strings.Split(arg, "=")
+		if len(kv) != 2 {
+			return cfg, errors.NewAutoscalerError(errors.ConfigurationError, fmt.Sprintf("invalid key=value pair %s", kv))
+		}
+		k, v := kv[0], kv[1]
+
+		switch k {
+		case autoDiscovererClusterNameKey:
+			cfg.clusterName = v
+		case autoDiscovererNamespaceKey:
+			cfg.namespace = v
+		default:
+			req, err := labels.NewRequirement(k, selection.Equals, []string{v})
+			if err != nil {
+				return cfg, errors.NewAutoscalerError(errors.ConfigurationError, fmt.Sprintf("failed to create label selector; %v", err))
+			}
+			cfg.labelSelector = cfg.labelSelector.Add(*req)
+		}
+	}
+	return cfg, nil
+}
+
+func parseAutoDiscovery(specs []string) ([]*clusterAPIAutoDiscoveryConfig, error) {
+	result := make([]*clusterAPIAutoDiscoveryConfig, 0, len(specs))
+	for _, spec := range specs {
+		autoDiscoverySpec, err := parseAutoDiscoverySpec(spec)
+		if err != nil {
+			return result, err
+		}
+		result = append(result, autoDiscoverySpec)
+	}
+	return result, nil
+}
+
+func allowedByAutoDiscoverySpec(spec *clusterAPIAutoDiscoveryConfig, r *unstructured.Unstructured) bool {
+	switch {
+	case spec.namespace != "" && spec.namespace != r.GetNamespace():
+		return false
+	case spec.clusterName != "" && spec.clusterName != clusterNameFromResource(r):
+		return false
+	case !spec.labelSelector.Matches(labels.Set(r.GetLabels())):
+		return false
+	default:
+		return true
+	}
+}