Add tmpfs support to EmptyDir

# *** ERROR: *** docs are out of sync between cli and markdown
# run hack/run-gendocs.sh > docs/kubectl.md to regenerate

#
# Your commit will be aborted unless you regenerate docs.
    COMMIT_BLOCKED_ON_GENDOCS

pkg/kubelet/volume/empty_dir/empty_dir.go

@@ -24,6 +24,7 @@ import (
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/kubelet/volume"
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/types"
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/util"
+	"github.com/GoogleCloudPlatform/kubernetes/pkg/util/mount"
 )
 
 // This is the primary entrypoint for volume plugins.
@@ -70,26 +71,83 @@ func (plugin *emptyDirPlugin) CanSupport(spec *api.Volume) bool {
 }
 
 func (plugin *emptyDirPlugin) NewBuilder(spec *api.Volume, podRef *api.ObjectReference) (volume.Builder, error) {
+	// Inject real implementations here, test through the internal function.
+	return plugin.newBuilderInternal(spec, podRef, mount.New(), &realMediumer{})
+}
+
+func (plugin *emptyDirPlugin) newBuilderInternal(spec *api.Volume, podRef *api.ObjectReference, mounter mount.Interface, mediumer mediumer) (volume.Builder, error) {
 	if plugin.legacyMode {
 		// Legacy mode instances can be cleaned up but not created anew.
 		return nil, fmt.Errorf("legacy mode: can not create new instances")
 	}
-	return &emptyDir{podRef.UID, spec.Name, plugin, false}, nil
+	medium := api.StorageTypeDefault
+	if spec.EmptyDir != nil { // Support a non-specified source as EmptyDir.
+		medium = spec.EmptyDir.Medium
+	}
+	return &emptyDir{
+		podUID:     podRef.UID,
+		volName:    spec.Name,
+		medium:     medium,
+		mediumer:   mediumer,
+		mounter:    mounter,
+		plugin:     plugin,
+		legacyMode: false,
+	}, nil
 }
 
 func (plugin *emptyDirPlugin) NewCleaner(volName string, podUID types.UID) (volume.Cleaner, error) {
+	// Inject real implementations here, test through the internal function.
+	return plugin.newCleanerInternal(volName, podUID, mount.New(), &realMediumer{})
+}
+
+func (plugin *emptyDirPlugin) newCleanerInternal(volName string, podUID types.UID, mounter mount.Interface, mediumer mediumer) (volume.Cleaner, error) {
 	legacy := false
 	if plugin.legacyMode {
 		legacy = true
 	}
-	return &emptyDir{podUID, volName, plugin, legacy}, nil
+	ed := &emptyDir{
+		podUID:     podUID,
+		volName:    volName,
+		medium:     api.StorageTypeDefault, // might be changed later
+		mounter:    mounter,
+		mediumer:   mediumer,
+		plugin:     plugin,
+		legacyMode: legacy,
+	}
+	// Figure out the medium.
+	if medium, err := mediumer.GetMedium(ed.GetPath()); err != nil {
+		return nil, err
+	} else {
+		switch medium {
+		case mediumMemory:
+			ed.medium = api.StorageTypeMemory
+		default:
+			// assume StorageTypeDefault
+		}
+	}
+	return ed, nil
 }
 
+// mediumer abstracts how to find what storageMedium a path is backed by.
+type mediumer interface {
+	GetMedium(path string) (storageMedium, error)
+}
+
+type storageMedium int
+
+const (
+	mediumUnknown storageMedium = 0 // assume anything we don't explicitly handle is this
+	mediumMemory  storageMedium = 1 // memory (e.g. tmpfs on linux)
+)
+
 // EmptyDir volumes are temporary directories exposed to the pod.
 // These do not persist beyond the lifetime of a pod.
 type emptyDir struct {
 	podUID     types.UID
 	volName    string
+	medium     api.StorageType
+	mounter    mount.Interface
+	mediumer   mediumer
 	plugin     *emptyDirPlugin
 	legacyMode bool
 }
@@ -99,8 +157,34 @@ func (ed *emptyDir) SetUp() error {
 	if ed.legacyMode {
 		return fmt.Errorf("legacy mode: can not create new instances")
 	}
-	path := ed.GetPath()
-	return os.MkdirAll(path, 0750)
+	switch ed.medium {
+	case api.StorageTypeDefault:
+		return ed.setupDefault()
+	case api.StorageTypeMemory:
+		return ed.setupTmpfs()
+	default:
+		return fmt.Errorf("unknown storage medium %q", ed.medium)
+	}
+}
+
+func (ed *emptyDir) setupDefault() error {
+	return os.MkdirAll(ed.GetPath(), 0750)
+}
+
+func (ed *emptyDir) setupTmpfs() error {
+	if ed.mounter == nil {
+		return fmt.Errorf("memory storage requested, but mounter is nil")
+	}
+	if err := os.MkdirAll(ed.GetPath(), 0750); err != nil {
+		return err
+	}
+	// Make SetUp idempotent.
+	if medium, err := ed.mediumer.GetMedium(ed.GetPath()); err != nil {
+		return err
+	} else if medium == mediumMemory {
+		return nil // current state is what we expect
+	}
+	return ed.mounter.Mount("tmpfs", ed.GetPath(), "tmpfs", 0, "")
 }
 
 func (ed *emptyDir) GetPath() string {
@@ -111,8 +195,19 @@ func (ed *emptyDir) GetPath() string {
 	return ed.plugin.host.GetPodVolumeDir(ed.podUID, volume.EscapePluginName(name), ed.volName)
 }
 
-// TearDown simply deletes everything in the directory.
+// TearDown simply discards everything in the directory.
 func (ed *emptyDir) TearDown() error {
+	switch ed.medium {
+	case api.StorageTypeDefault:
+		return ed.teardownDefault()
+	case api.StorageTypeMemory:
+		return ed.teardownTmpfs()
+	default:
+		return fmt.Errorf("unknown storage medium %q", ed.medium)
+	}
+}
+
+func (ed *emptyDir) teardownDefault() error {
 	tmpDir, err := volume.RenameDirectory(ed.GetPath(), ed.volName+".deleting~")
 	if err != nil {
 		return err
@@ -123,3 +218,16 @@ func (ed *emptyDir) TearDown() error {
 	}
 	return nil
 }
+
+func (ed *emptyDir) teardownTmpfs() error {
+	if ed.mounter == nil {
+		return fmt.Errorf("memory storage requested, but mounter is nil")
+	}
+	if err := ed.mounter.Unmount(ed.GetPath(), 0); err != nil {
+		return err
+	}
+	if err := os.RemoveAll(ed.GetPath()); err != nil {
+		return err
+	}
+	return nil
+}