snapshot/overlay: implement view

Allow creating actives without an upper directory for
capturing changes. Actives without the upper directory
will always be mounted read only. Read only actives
must have a parent.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

snapshot/overlay/overlay.go

@@ -102,7 +102,7 @@ func (o *Snapshotter) stat(path string) (snapshot.Info, error) {
 }
 
 func (o *Snapshotter) Prepare(ctx context.Context, key, parent string) ([]containerd.Mount, error) {
-	active, err := o.newActiveDir(key)
+	active, err := o.newActiveDir(key, false)
 	if err != nil {
 		return nil, err
 	}
@@ -115,7 +115,16 @@ func (o *Snapshotter) Prepare(ctx context.Context, key, parent string) ([]contai
 }
 
 func (o *Snapshotter) View(ctx context.Context, key, parent string) ([]containerd.Mount, error) {
-	panic("not implemented")
+	active, err := o.newActiveDir(key, true)
+	if err != nil {
+		return nil, err
+	}
+	if parent != "" {
+		if err := active.setParent(parent); err != nil {
+			return nil, err
+		}
+	}
+	return active.mounts(o.links)
 }
 
 // Mounts returns the mounts for the transaction identified by key. Can be
@@ -175,7 +184,7 @@ func (o *Snapshotter) Walk(ctx context.Context, fn func(context.Context, snapsho
 	})
 }
 
-func (o *Snapshotter) newActiveDir(key string) (*activeDir, error) {
+func (o *Snapshotter) newActiveDir(key string, readonly bool) (*activeDir, error) {
 	var (
 		path      = filepath.Join(o.root, "active", hash(key))
 		name      = filepath.Join(path, "name")
@@ -186,6 +195,7 @@ func (o *Snapshotter) newActiveDir(key string) (*activeDir, error) {
 		committedDir: filepath.Join(o.root, "committed"),
 		indexlink:    indexlink,
 	}
+	if !readonly {
 		for _, p := range []string{
 			"work",
 			"fs",
@@ -195,6 +205,12 @@ func (o *Snapshotter) newActiveDir(key string) (*activeDir, error) {
 				return nil, err
 			}
 		}
+	} else {
+		if err := os.MkdirAll(path, 0700); err != nil {
+			a.delete()
+			return nil, err
+		}
+	}
 
 	if err := ioutil.WriteFile(name, []byte(key), 0644); err != nil {
 		a.delete()
@@ -286,7 +302,7 @@ func (a *activeDir) mounts(c *cache) ([]containerd.Mount, error) {
 	}
 	if len(parents) == 0 {
 		// if we only have one layer/no parents then just return a bind mount as overlay
-		// will not work
+		// will not work, readonly always has parent
 		return []containerd.Mount{
 			{
 				Source: filepath.Join(a.path, "fs"),
@@ -298,11 +314,18 @@ func (a *activeDir) mounts(c *cache) ([]containerd.Mount, error) {
 			},
 		}, nil
 	}
-	options := []string{
+	var options []string
+
+	if _, err := os.Stat(filepath.Join(a.path, "fs")); err == nil {
+		options = append(options,
 			fmt.Sprintf("workdir=%s", filepath.Join(a.path, "work")),
 			fmt.Sprintf("upperdir=%s", filepath.Join(a.path, "fs")),
-		fmt.Sprintf("lowerdir=%s", strings.Join(parents, ":")),
+		)
+	} else if !os.IsNotExist(err) {
+		return nil, err
 	}
+
+	options = append(options, fmt.Sprintf("lowerdir=%s", strings.Join(parents, ":")))
 	return []containerd.Mount{
 		{
 			Type:    "overlay",

snapshot/overlay/overlay_test.go

@@ -2,6 +2,7 @@ package overlay
 
 import (
 	"context"
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -196,3 +197,49 @@ func TestOverlayOverlayRead(t *testing.T) {
 		return
 	}
 }
+
+func TestOverlayView(t *testing.T) {
+	ctx := context.TODO()
+	root, err := ioutil.TempDir("", "overlay")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(root)
+	o, err := NewSnapshotter(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	key := "/tmp/test"
+	mounts, err := o.Prepare(ctx, key, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	m := mounts[0]
+	if err := ioutil.WriteFile(filepath.Join(m.Source, "foo"), []byte("hi"), 0660); err != nil {
+		t.Fatal(err)
+	}
+	if err := o.Commit(ctx, "base", key); err != nil {
+		t.Fatal(err)
+	}
+	mounts, err = o.View(ctx, "/tmp/test2", "base")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(mounts) != 1 {
+		t.Fatalf("should only have 1 mount but received %d", len(mounts))
+	}
+	m = mounts[0]
+	if m.Type != "overlay" {
+		t.Errorf("mount type should be overlay but received %q", m.Type)
+	}
+	if m.Source != "overlay" {
+		t.Errorf("mount source should be overlay but received %q", m.Source)
+	}
+	if len(m.Options) != 1 {
+		t.Errorf("expected 1 mount option but got %d", len(m.Options))
+	}
+	expected := fmt.Sprintf("lowerdir=%s", filepath.Join(root, "committed", hash("base"), "fs"))
+	if m.Options[0] != expected {
+		t.Errorf("expected option %q but received %q", expected, m.Options[0])
+	}
+}