Prototype for nfs-client alias mode

This might be a solution for kubernetes-retired#1210 and kubernetes-retired#1275

In combination with an existing NFS claim in namespace X, allow the administrator or the users to create new claims for the same NFS tree in namespaces A, B, C, etc.

This change tries as much as possible NOT to disrupt existing setups.

A few things still left:

1. Is "alias" the right term? Previous choices I tossed out: clone (easy to mix with real volume cloning), proxy (there's no middle-man NFS server), passthrough (too nebulous).
1. Is NFS_SERVER=--alias the best way to trigger this?
1. Should the auto-detection of the server host:path happen always, even when not in alias mode?
1. Should the parent's ReadOnly field be propagated to the newly provisioned PV?
1. Should this be made more generic so that one deployment can expose N different claims/trees to the other namespaces? See comment below for more details.

nfs-client/README.md

@@ -22,7 +22,7 @@ $ helm install stable/nfs-client-provisioner --set nfs.server=x.x.x.x --set nfs.
 
 **Step 1: Get connection information for your NFS server**. Make sure your NFS server is accessible from your Kubernetes cluster and get the information you need to connect to it. At a minimum you will need its hostname.
 
-**Step 2: Get the NFS-Client Provisioner files**. To setup the provisioner you will download a set of YAML files, edit them to add your NFS server's connection information and then apply each with the ``kubectl`` / ``oc`` command. 
+**Step 2: Get the NFS-Client Provisioner files**. To setup the provisioner you will download a set of YAML files, edit them to add your NFS server's connection information and then apply each with the ``kubectl`` / ``oc`` command.
 
 Get all of the files in the [deploy](https://github.com/kubernetes-incubator/external-storage/tree/master/nfs-client/deploy) directory of this repository. These instructions assume that you have cloned the [external-storage](https://github.com/kubernetes-incubator/external-storage) repository and have a bash-shell open in the ``nfs-client`` directory.
 
@@ -40,7 +40,7 @@ $ kubectl create -f deploy/rbac.yaml
 
 OpenShift:
 
-On some installations of OpenShift the default admin user does not have cluster-admin permissions. If these commands fail refer to the OpenShift documentation for **User and Role Management** or contact your OpenShift provider to help you grant the right permissions to your admin user. 
+On some installations of OpenShift the default admin user does not have cluster-admin permissions. If these commands fail refer to the OpenShift documentation for **User and Role Management** or contact your OpenShift provider to help you grant the right permissions to your admin user.
 
 ```sh
 # Set the subject of the RBAC objects to the current namespace where the provisioner is being deployed
@@ -94,6 +94,8 @@ spec:
             path: /var/nfs
 ```
 
+**Alias mode:** use the provisioner in this mode to share the same existing NFS claim to multiple namespaces, without propagating manually the server/path in each namespace's claim. For example, first create a `data-original` claim as normal, through any provisioner such as `example.com/efs-aws` or the `fuseim.pri/ifs` example below. In the same namespace of your choice, run a new NFS client provisioner that uses the claim. Set NFS_SERVER to the magic value of `--alias`. Give the new deployment a clearer name, `nfs-alias-provisioner`, and set PROVISIONER_NAME to `foo.com/nfs-alias-provisioner`. Then create a StorageClass `nfs-alias` with its provisioner set to `foo.com/nfs-alias-provisioner`. Now, every new `nfs-alias` claim you create in any namespace will have the same `server:path` as the `data-original` volume.
+
 You may also want to change the PROVISIONER_NAME above from ``fuseim.pri/ifs`` to something more descriptive like ``nfs-storage``, but if you do remember to also change the PROVISIONER_NAME in the storage class definition below:
 
 This is `deploy/class.yaml` which defines the NFS-Client's Kubernetes Storage Class:

nfs-client/cmd/nfs-client-provisioner/provisioner.go

@@ -26,10 +26,11 @@ import (
 	"strings"
 
 	"k8s.io/kubernetes/pkg/apis/core/v1/helper"
+	mnt "k8s.io/kubernetes/pkg/util/mount"
 
 	"github.com/golang/glog"
 	"github.com/kubernetes-sigs/sig-storage-lib-external-provisioner/controller"
-	"k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	storage "k8s.io/api/storage/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/util/wait"
@@ -39,12 +40,14 @@ import (
 
 const (
 	provisionerNameKey = "PROVISIONER_NAME"
+	magicAliasHostname = "--alias"
 )
 
 type nfsProvisioner struct {
 	client kubernetes.Interface
 	server string
 	path   string
+	static bool
 }
 
 const (
@@ -54,24 +57,17 @@ const (
 var _ controller.Provisioner = &nfsProvisioner{}
 
 func (p *nfsProvisioner) Provision(options controller.VolumeOptions) (*v1.PersistentVolume, error) {
+	var path string
+	var err error
+
 	if options.PVC.Spec.Selector != nil {
 		return nil, fmt.Errorf("claim Selector is not supported")
 	}
 	glog.V(4).Infof("nfs provisioner: VolumeOptions %v", options)
 
-	pvcNamespace := options.PVC.Namespace
-	pvcName := options.PVC.Name
-
-	pvName := strings.Join([]string{pvcNamespace, pvcName, options.PVName}, "-")
-
-	fullPath := filepath.Join(mountPath, pvName)
-	glog.V(4).Infof("creating path %s", fullPath)
-	if err := os.MkdirAll(fullPath, 0777); err != nil {
-		return nil, errors.New("unable to create directory to provision new pv: " + err.Error())
+	if path, err = p.getOrMakeDir(options); err != nil {
+		return nil, err
 	}
-	os.Chmod(fullPath, 0777)
-
-	path := filepath.Join(p.path, pvName)
 
 	pv := &v1.PersistentVolume{
 		ObjectMeta: metav1.ObjectMeta{
@@ -88,14 +84,35 @@ func (p *nfsProvisioner) Provision(options controller.VolumeOptions) (*v1.Persis
 				NFS: &v1.NFSVolumeSource{
 					Server:   p.server,
 					Path:     path,
-					ReadOnly: false,
+					ReadOnly: false, // Pass ReadOnly through if in alias mode?
 				},
 			},
 		},
 	}
 	return pv, nil
 }
 
+// If in alias mode, forward the server:path details from the PVC we mounted.
+// If not, create a new directory.
+func (p *nfsProvisioner) getOrMakeDir(options controller.VolumeOptions) (_ string, err error) {
+	if p.static {
+		return p.path, nil
+	}
+
+	pvcNamespace := options.PVC.Namespace
+	pvcName := options.PVC.Name
+	pvName := strings.Join([]string{pvcNamespace, pvcName, options.PVName}, "-")
+
+	fullPath := filepath.Join(mountPath, pvName)
+	glog.V(4).Infof("creating path %s", fullPath)
+	if err = os.MkdirAll(fullPath, 0777); err == nil {
+		os.Chmod(fullPath, 0777)
+		return filepath.Join(p.path, pvName), nil
+	}
+	err = errors.New("unable to create directory to provision new pv: " + err.Error())
+	return
+}
+
 func (p *nfsProvisioner) Delete(volume *v1.PersistentVolume) error {
 	path := volume.Spec.PersistentVolumeSource.NFS.Path
 	pvName := filepath.Base(path)
@@ -145,6 +162,17 @@ func (p *nfsProvisioner) getClassForVolume(pv *v1.PersistentVolume) (*storage.St
 	return class, nil
 }
 
+// Return the server and path parts for the given NFS mount
+func getDetailsForMountPoint(m string) (server, path string, err error) {
+	if path, _, err = mnt.GetDeviceNameFromMount(mnt.New(""), m); err == nil {
+		if parts := strings.Split(path, ":"); len(parts) == 2 {
+			return parts[0], parts[1], err
+		}
+		err = errors.New("Can't parse server:path from device string: " + path)
+	}
+	return
+}
+
 func main() {
 	flag.Parse()
 	flag.Set("logtostderr", "true")
@@ -180,10 +208,23 @@ func main() {
 		glog.Fatalf("Error getting server version: %v", err)
 	}
 
+	// If NFS_SERVER=="--alias", we just pass through the server/path we have
+	// mounted and never make a new directory for each volume we provision.
+	var static bool
+	if server == magicAliasHostname {
+		// Figure just once and store the server/path pair
+		if server, path, err = getDetailsForMountPoint(mountPath); err != nil {
+			glog.Fatalf("Error getting server details for %v: %v", mountPath, err)
+		}
+		glog.Infof("Aliasing all new volumes to %v::%v", server, path)
+		static = true
+	}
+
 	clientNFSProvisioner := &nfsProvisioner{
 		client: clientset,
 		server: server,
 		path:   path,
+		static: static,
 	}
 	// Start the provision controller which will dynamically provision efs NFS
 	// PVs