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_test.go

@@ -0,0 +1,111 @@
+package containerkit
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// TestLayerManipulatorBasic implements something similar to the conceptual
+// examples we've discussed thus far. It does perform mounts, so you must run
+// as root.
+func TestLayerManipulatorBasic(t *testing.T) {
+	tmpDir, err := ioutil.TempDir("", "test-layman-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// defer os.RemoveAll(tmpDir)
+
+	root := filepath.Join(tmpDir, "root")
+
+	lm, err := NewLayerManipulator(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	preparing := filepath.Join(tmpDir, "preparing")
+	if err := os.MkdirAll(preparing, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	mounts, err := lm.Prepare(preparing, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(mounts) < 1 {
+		t.Fatal("expected mounts to have entries")
+	}
+
+	for _, mount := range mounts {
+		if !strings.HasPrefix(mount.Target, preparing) {
+			t.Fatalf("expected mount target to be prefixed with tmpDir: %q does not startwith %q", mount.Target, preparing)
+		}
+
+		t.Log(MountCommand(mount))
+	}
+
+	if err := MountAll(mounts...); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := ioutil.WriteFile(filepath.Join(preparing, "foo"), []byte("foo\n"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	os.MkdirAll(preparing+"/a/b/c", 0755)
+
+	// defer os.Remove(filepath.Join(tmpDir, "foo"))
+
+	committed := filepath.Join(lm.root, "committed")
+
+	if err := lm.Commit(committed, preparing); err != nil {
+		t.Fatal(err)
+	}
+
+	if lm.Parent(preparing) != "" {
+		t.Fatalf("parent of new layer should be empty, got lm.Parent(%q) == %q", preparing, lm.Parent(preparing))
+	}
+
+	next := filepath.Join(tmpDir, "nextlayer")
+	if err := os.MkdirAll(next, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	mounts, err = lm.Prepare(next, committed)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := MountAll(mounts...); err != nil {
+		t.Fatal(err)
+	}
+
+	for _, mount := range mounts {
+		if !strings.HasPrefix(mount.Target, next) {
+			t.Fatalf("expected mount target to be prefixed with tmpDir: %q does not startwith %q", mount.Target, next)
+		}
+
+		t.Log(MountCommand(mount))
+	}
+
+	if err := ioutil.WriteFile(filepath.Join(next, "bar"), []byte("bar\n"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	// also, change content of foo to bar
+	if err := ioutil.WriteFile(filepath.Join(next, "foo"), []byte("bar\n"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	os.RemoveAll(next + "/a/b")
+	nextCommitted := filepath.Join(lm.root, "committed-next")
+	if err := lm.Commit(nextCommitted, next); err != nil {
+		t.Fatal(err)
+	}
+
+	if lm.Parent(nextCommitted) != committed {
+		t.Fatalf("parent of new layer should be %q, got lm.Parent(%q) == %q (%#v)", committed, next, lm.Parent(next), lm.parents)
+	}
+}