add IsNodeCandidateForScaleDown interface

This function allows cloud providers to specify when a node is not a
good candidate for scaling down. This will occur before the autoscaler has
begun to cordon, drain, and taint any node for scale down.

Also adds a unit test for the prefiltering node processor.

Author: elmiko

cluster-autoscaler/cloudprovider/alicloud/alicloud_cloud_provider.go

@@ -175,6 +175,11 @@ func (ali *aliCloudProvider) Cleanup() error {
 	return nil
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (ali *aliCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // AliRef contains a reference to ECS instance or .
 type AliRef struct {
 	ID     string

cluster-autoscaler/cloudprovider/aws/aws_cloud_provider.go

@@ -193,6 +193,11 @@ func (aws *awsCloudProvider) Refresh() error {
 	return aws.awsManager.Refresh()
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (aws *awsCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // AwsRef contains a reference to some entity in AWS world.
 type AwsRef struct {
 	Name string

cluster-autoscaler/cloudprovider/azure/azure_cloud_provider.go

@@ -174,6 +174,11 @@ func (azure *AzureCloudProvider) Refresh() error {
 	return azure.azureManager.Refresh()
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (azure *AzureCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // azureRef contains a reference to some entity in Azure world.
 type azureRef struct {
 	Name string

cluster-autoscaler/cloudprovider/baiducloud/baiducloud_cloud_provider.go

@@ -228,6 +228,11 @@ func (baiducloud *baiducloudCloudProvider) Refresh() error {
 	return nil
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (baiducloud *baiducloudCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // BaiducloudRef contains a reference to some entity in baiducloud world.
 type BaiducloudRef struct {
 	Name string

cluster-autoscaler/cloudprovider/bizflycloud/bizflycloud_cloud_provider.go

@@ -172,6 +172,11 @@ func (d *bizflycloudCloudProvider) Refresh() error {
 	return d.manager.Refresh()
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (d *bizflycloudCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // BuildBizflyCloud builds the Bizflycloud cloud provider.
 func BuildBizflyCloud(
 	opts config.AutoscalingOptions,

cluster-autoscaler/cloudprovider/brightbox/brightbox_cloud_provider.go

@@ -219,6 +219,11 @@ func (b *brightboxCloudProvider) Cleanup() error {
 	return nil
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (b *brightboxCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // BuildBrightbox builds the Brightbox provider
 func BuildBrightbox(
 	opts config.AutoscalingOptions,

cluster-autoscaler/cloudprovider/cherryservers/cherry_cloud_provider.go

@@ -170,6 +170,11 @@ func (ccp *cherryCloudProvider) Cleanup() error {
 	return nil
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (ccp *cherryCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // BuildCherry is called by the autoscaler to build a Cherry Servers cloud provider.
 //
 // The cherryManager is created here, and the node groups are created

cluster-autoscaler/cloudprovider/civo/civo_cloud_provider.go

@@ -167,6 +167,11 @@ func (d *civoCloudProvider) Refresh() error {
 	return d.manager.Refresh()
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (d *civoCloudProvider) IsNodeCandidateForScaleDown(node *apiv1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // BuildCivo builds the Civo cloud provider.
 func BuildCivo(
 	opts config.AutoscalingOptions,

cluster-autoscaler/cloudprovider/cloud_provider.go

@@ -154,6 +154,14 @@ type CloudProvider interface {
 	// Refresh is called before every main loop and can be used to dynamically update cloud provider state.
 	// In particular the list of node groups returned by NodeGroups can change as a result of CloudProvider.Refresh().
 	Refresh() error
+
+	// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down. This function
+	// will be called during prefiltering of nodes for scaledown to allow cloud providers the opportunity
+	// to reject a node for scale down. This may be used in cases where nodes are undergoing upgrades or other
+	// cloud-specific behavior where the cluster autoscaler should not begin cordoning, draining, and tainting
+	// the node.
+	// Returns true if the node can be safely scaled down or false otherwise.
+	IsNodeCandidateForScaleDown(*apiv1.Node) (bool, error)
 }
 
 // ErrNotImplemented is returned if a method is not implemented.

cluster-autoscaler/cloudprovider/cloudstack/cloudstack_cloud_provider.go

@@ -115,6 +115,11 @@ func (provider *cloudStackCloudProvider) Pricing() (cloudprovider.PricingModel,
 	return nil, cloudprovider.ErrNotImplemented
 }
 
+// IsNodeCandidateForScaleDown returns whether the node is a good candidate for scaling down.
+func (provider *cloudStackCloudProvider) IsNodeCandidateForScaleDown(node *v1.Node) (bool, error) {
+	return true, cloudprovider.ErrNotImplemented
+}
+
 // NewNodeGroup builds a theoretical node group based on the node definition provided. The node group is not automatically
 // created on the cloud provider side. The node group is not returned by NodeGroups() until it is created.
 func (provider *cloudStackCloudProvider) NewNodeGroup(machineType string, labels map[string]string, systemLabels map[string]string, taints []v1.Taint, extraResources map[string]resource.Quantity) (cloudprovider.NodeGroup, error) {