snapshot: define the snapshot driver interface

We now define the `snapshot.Driver` interface based on earlier work.
Many details of the model are worked out, such as snapshot lifecycle and
parentage of commits against "Active" snapshots.

The impetus of this change is to provide a snapshot POC that does a
complete push/pull workflow. The beginnings of a test suite for snapshot
drivers is included that we can use to verify the assumptions of
drivers. The intent is to port the existing tests over to this test
suite and start scaling contributions and test to the snapshot driver
subsystem.

There are still some details that need to be worked out, such as listing
and metadata access. We can do this activity as we further integrate
with tooling.

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

testutil/helpers.go

@@ -0,0 +1,57 @@
+package testutil
+
+import (
+	"flag"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+var rootEnabled bool
+
+func init() {
+	flag.BoolVar(&rootEnabled, "test.root", false, "enable tests that require root")
+}
+
+// Unmount unmounts a given mountPoint and sets t.Error if it fails
+func Unmount(t *testing.T, mountPoint string) {
+	t.Log("unmount", mountPoint)
+	if err := syscall.Unmount(mountPoint, 0); err != nil {
+		t.Error("Could not umount", mountPoint, err)
+	}
+}
+
+// RequiresRoot skips tests that require root, unless the test.root flag has
+// been set
+func RequiresRoot(t *testing.T) {
+	if !rootEnabled {
+		t.Skip("skipping test that requires root")
+		return
+	}
+	assert.Equal(t, 0, os.Getuid(), "This test must be run as root.")
+}
+
+// DumpDir will log out all of the contents of the provided directory to
+// testing logger.
+//
+// Use this in a defer statement within tests that may allocate and exercise a
+// temporary directory. Immensely useful for sanity checking and debugging
+// failing tests.
+//
+// One should still test that contents are as expected. This is only a visual
+// tool to assist when things don't go your way.
+func DumpDir(t *testing.T, root string) {
+	if err := filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
+		t.Log(fi.Mode(), path)
+		return nil
+	}); err != nil {
+		t.Fatalf("error dumping directory: %v", err)
+	}
+}