containerkit: layer manipulator overlay poc

A light weight overlay implementation that emits working mounts is
demonstrated. One can prepare and commit changes. The diffs are
correctly held on disk.

The next step from here is to implement the changes methods and ensure
that we can work with the docker registry API.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

layers.go

@@ -1,6 +1,13 @@
 package containerkit
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+)
 
 var (
 	errNotImplemented = errors.New("not implemented")
@@ -96,7 +103,34 @@ var (
 // 	work, err := lm.Prepare(dst, parent)
 //  mountAll(work.Mounts())
 // 	work.Commit() || work.Rollback()
-type LayerManipulator struct{}
+//
+// TODO(stevvooe): LayerManipulator should be an interface with several
+// implementations, similar to graphdriver.
+type LayerManipulator struct {
+	root string // root provides paths for internal storage.
+
+	// just a simple overlay implementation.
+	active  map[string]activeLayer
+	parents map[string]string // diff to parent for all committed
+}
+
+type activeLayer struct {
+	parent   string
+	upperdir string
+	workdir  string
+}
+
+func NewLayerManipulator(root string) (*LayerManipulator, error) {
+	if err := os.MkdirAll(root, 0777); err != nil {
+		return nil, err
+	}
+
+	return &LayerManipulator{
+		root:    root,
+		active:  make(map[string]activeLayer),
+		parents: make(map[string]string),
+	}, nil
+}
 
 // Prepare returns a set of mounts such that dst can be used as a location for
 // reading and writing data. If parent is provided, the dst will be setup to
@@ -111,7 +145,61 @@ type LayerManipulator struct{}
 // Once the writes have completed, LayerManipulator.Commit or
 // LayerManipulator.Rollback should be called on dst.
 func (lm *LayerManipulator) Prepare(dst, parent string) ([]Mount, error) {
-	return nil, errNotImplemented
+	// we want to build up lowerdir, upperdir and workdir options for the
+	// overlay mount.
+	//
+	// lowerdir is a list of parent diffs, ordered from top to bottom (base
+	// layer to the "right").
+	//
+	// upperdir will become the diff location. This will be renamed to the
+	// location provided in commit.
+	//
+	// workdir needs to be there but it is not really clear why.
+	var opts []string
+
+	upperdir, err := ioutil.TempDir(lm.root, "diff-")
+	if err != nil {
+		return nil, err
+	}
+	opts = append(opts, "upperdir="+upperdir)
+
+	workdir, err := ioutil.TempDir(lm.root, "work-")
+	if err != nil {
+		return nil, err
+	}
+	opts = append(opts, "workdir="+workdir)
+
+	empty := filepath.Join(lm.root, "empty")
+	if err := os.MkdirAll(empty, 0777); err != nil {
+		return nil, err
+	}
+
+	lm.active[dst] = activeLayer{
+		parent:   parent,
+		upperdir: upperdir,
+		workdir:  workdir,
+	}
+
+	var parents []string
+	for parent != "" {
+		parents = append(parents, parent)
+		parent = lm.Parent(parent)
+	}
+
+	if len(parents) == 0 {
+		parents = []string{empty}
+	}
+
+	opts = append(opts, "lowerdir="+strings.Join(parents, ","))
+
+	return []Mount{
+		{
+			Type:    "overlay",
+			Source:  "none",
+			Target:  dst,
+			Options: opts,
+		},
+	}, nil
 }
 
 // Commit captures the changes between dst and its parent into the path
@@ -121,17 +209,48 @@ func (lm *LayerManipulator) Prepare(dst, parent string) ([]Mount, error) {
 // The contents of diff are opaque to the caller and may be specific to the
 // implementation of the layer backend.
 func (lm *LayerManipulator) Commit(diff, dst string) error {
-	return errNotImplemented
+	active, ok := lm.active[dst]
+	if !ok {
+		return fmt.Errorf("%q must be an active layer", dst)
+	}
+
+	// move upperdir into the diff dir
+	if err := os.Rename(active.upperdir, diff); err != nil {
+		return err
+	}
+
+	// Clean up the working directory; we may not want to do this if we want to
+	// support re-entrant calls to Commit.
+	if err := os.RemoveAll(active.workdir); err != nil {
+		return err
+	}
+
+	lm.parents[diff] = active.parent
+	delete(lm.active, dst) // remove from active, again, consider not doing this to support multiple commits.
+	// note that allowing multiple commits would require copy for overlay.
+
+	return nil
 }
 
 // Rollback can be called after prepare if the caller would like to abandon the
 // changeset.
 func (lm *LayerManipulator) Rollback(dst string) error {
-	return errNotImplemented
+	active, ok := lm.active[dst]
+	if !ok {
+		return fmt.Errorf("%q must be an active layer", dst)
+	}
+
+	var err error
+	err = os.RemoveAll(active.upperdir)
+	err = os.RemoveAll(active.workdir)
+
+	delete(lm.active, dst)
+	return err
 }
 
+// Parent returns the parent of the layer at diff.
 func (lm *LayerManipulator) Parent(diff string) string {
-	return ""
+	return lm.parents[diff]
 }
 
 type ChangeKind int