From eeb88554ac9262eea2b96d0071fec00fc7e269fb Mon Sep 17 00:00:00 2001
From: Derek McGowan <derek@mcgstyle.net>
Date: Mon, 27 Feb 2017 13:21:20 -0800
Subject: [PATCH] snapshot/overlay: use readonly bindmount for single parent
 view

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
---
 snapshot/overlay/overlay.go      | 11 +++++++++
 snapshot/overlay/overlay_test.go | 41 +++++++++++++++++++++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/snapshot/overlay/overlay.go b/snapshot/overlay/overlay.go
index 588aec2..533824a 100644
--- a/snapshot/overlay/overlay.go
+++ b/snapshot/overlay/overlay.go
@@ -330,6 +330,17 @@ func (a *activeDir) mounts(c *cache) ([]containerd.Mount, error) {
 		)
 	} else if !os.IsNotExist(err) {
 		return nil, err
+	} else if len(parents) == 1 {
+		return []containerd.Mount{
+			{
+				Source: parents[0],
+				Type:   "bind",
+				Options: []string{
+					"ro",
+					"rbind",
+				},
+			},
+		}, nil
 	}
 
 	options = append(options, fmt.Sprintf("lowerdir=%s", strings.Join(parents, ":")))
diff --git a/snapshot/overlay/overlay_test.go b/snapshot/overlay/overlay_test.go
index 3a19d68..4bd594a 100644
--- a/snapshot/overlay/overlay_test.go
+++ b/snapshot/overlay/overlay_test.go
@@ -209,7 +209,7 @@ func TestOverlayView(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	key := "/tmp/test"
+	key := "/tmp/base"
 	mounts, err := o.Prepare(ctx, key, "")
 	if err != nil {
 		t.Fatal(err)
@@ -221,7 +221,42 @@ func TestOverlayView(t *testing.T) {
 	if err := o.Commit(ctx, "base", key); err != nil {
 		t.Fatal(err)
 	}
-	mounts, err = o.View(ctx, "/tmp/test2", "base")
+
+	key = "/tmp/top"
+	_, err = o.Prepare(ctx, key, "base")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(root, "active", hash(key), "fs", "foo"), []byte("hi, again"), 0660); err != nil {
+		t.Fatal(err)
+	}
+	if err := o.Commit(ctx, "top", key); err != nil {
+		t.Fatal(err)
+	}
+
+	mounts, err = o.View(ctx, "/tmp/view1", "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 != "bind" {
+		t.Errorf("mount type should be bind but received %q", m.Type)
+	}
+	expected := filepath.Join(root, "committed", hash("base"), "fs")
+	if m.Source != expected {
+		t.Errorf("expected source %q but received %q", expected, m.Source)
+	}
+	if m.Options[0] != "ro" {
+		t.Errorf("expected mount option ro but received %q", m.Options[0])
+	}
+	if m.Options[1] != "rbind" {
+		t.Errorf("expected mount option rbind but received %q", m.Options[1])
+	}
+
+	mounts, err = o.View(ctx, "/tmp/view2", "top")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -238,7 +273,7 @@ func TestOverlayView(t *testing.T) {
 	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"))
+	expected = fmt.Sprintf("lowerdir=%s:%s", filepath.Join(root, "committed", hash("top"), "fs"), filepath.Join(root, "committed", hash("base"), "fs"))
 	if m.Options[0] != expected {
 		t.Errorf("expected option %q but received %q", expected, m.Options[0])
 	}