mount: fix read-only bind (#1368)

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

snapshot/testsuite/testsuite.go

@@ -38,6 +38,8 @@ func SnapshotterSuite(t *testing.T, name string, snapshotterFn func(ctx context.
 
 	// Rename test still fails on some kernels with overlay
 	//t.Run("Rename", makeTest(name, snapshotterFn, checkRename))
+
+	t.Run("ViewReadonly", makeTest(name, snapshotterFn, checkSnapshotterViewReadonly))
 }
 
 func makeTest(name string, snapshotterFn func(ctx context.Context, root string) (snapshot.Snapshotter, func() error, error), fn func(ctx context.Context, t *testing.T, snapshotter snapshot.Snapshotter, work string)) func(t *testing.T) {
@@ -651,3 +653,40 @@ func checkRemove(ctx context.Context, t *testing.T, snapshotter snapshot.Snapsho
 		t.Fatal(err)
 	}
 }
+
+// checkSnapshotterViewReadonly ensures a KindView snapshot to be mounted as a read-only filesystem.
+// This function is called only when WithTestViewReadonly is true.
+func checkSnapshotterViewReadonly(ctx context.Context, t *testing.T, snapshotter snapshot.Snapshotter, work string) {
+	preparing := filepath.Join(work, "preparing")
+	if _, err := snapshotter.Prepare(ctx, preparing, ""); err != nil {
+		t.Fatal(err)
+	}
+	committed := filepath.Join(work, "commited")
+	if err := snapshotter.Commit(ctx, committed, preparing); err != nil {
+		t.Fatal(err)
+	}
+	view := filepath.Join(work, "view")
+	m, err := snapshotter.View(ctx, view, committed)
+	if err != nil {
+		t.Fatal(err)
+	}
+	viewMountPoint := filepath.Join(work, "view-mount")
+	if err := os.MkdirAll(viewMountPoint, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	// Just checking the option string of m is not enough, we need to test real mount. (#1368)
+	if err := mount.MountAll(m, viewMountPoint); err != nil {
+		t.Fatal(err)
+	}
+
+	testfile := filepath.Join(viewMountPoint, "testfile")
+	if err := ioutil.WriteFile(testfile, []byte("testcontent"), 0777); err != nil {
+		t.Logf("write to %q failed with %v (EROFS is expected but can be other error code)", testfile, err)
+	} else {
+		t.Fatalf("write to %q should fail (EROFS) but did not fail", testfile)
+	}
+	testutil.Unmount(t, viewMountPoint)
+	assert.NoError(t, snapshotter.Remove(ctx, view))
+	assert.NoError(t, snapshotter.Remove(ctx, committed))
+}