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_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])
+	}
+}