overlay: add Supported() checker

This function is not called during plugin initialization (#2140),
but should be useful for downstream projects that uses overlayfs
snapshotter as a Go library.

Benchmark result on Ubuntu 17.10, GCE n1-standard-4:

BenchmarkOverlaySupportedOnExt4-4                    100          20490598 ns/op
BenchmarkOverlayUnsupportedOnFType0XFS-4           30000             39316 ns/op
BenchmarkOverlaySupportedOnFType1XFS-4               100          19287083 ns/op
BenchmarkOverlayUnsupportedOnFAT-4                   100          14217772 ns/op

i.e. the overhead is typically about 20 msec on this machine.

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

snapshots/overlay/check_test.go

@@ -0,0 +1,89 @@
+// +build linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package overlay
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"testing"
+
+	"github.com/containerd/containerd/testutil"
+)
+
+func testOverlaySupported(t testing.TB, expected bool, mkfs ...string) {
+	testutil.RequiresRoot(t)
+	mnt, err := ioutil.TempDir("", "containerd-fs-test-supports-overlay")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(mnt)
+
+	deviceName, cleanupDevice, err := testutil.NewLoopback(100 << 20) // 100 MB
+	if err != nil {
+		t.Fatal(err)
+	}
+	if out, err := exec.Command(mkfs[0], append(mkfs[1:], deviceName)...).CombinedOutput(); err != nil {
+		// not fatal
+		t.Skipf("could not mkfs (%v) %s: %v (out: %q)", mkfs, deviceName, err, string(out))
+	}
+	if out, err := exec.Command("mount", deviceName, mnt).CombinedOutput(); err != nil {
+		// not fatal
+		t.Skipf("could not mount %s: %v (out: %q)", deviceName, err, string(out))
+	}
+	defer func() {
+		testutil.Unmount(t, mnt)
+		cleanupDevice()
+	}()
+	workload := func() {
+		err = Supported(mnt)
+		if expected && err != nil {
+			t.Fatal(err)
+		}
+		if !expected && err == nil {
+			t.Fatal("error is expected")
+		}
+	}
+	b, ok := t.(*testing.B)
+	if ok {
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			workload()
+		}
+		b.StopTimer()
+	} else {
+		workload()
+	}
+}
+
+func BenchmarkOverlaySupportedOnExt4(b *testing.B) {
+	testOverlaySupported(b, true, "mkfs.ext4", "-F")
+}
+
+func BenchmarkOverlayUnsupportedOnFType0XFS(b *testing.B) {
+	testOverlaySupported(b, false, "mkfs.xfs", "-m", "crc=0", "-n", "ftype=0")
+}
+
+func BenchmarkOverlaySupportedOnFType1XFS(b *testing.B) {
+	testOverlaySupported(b, true, "mkfs.xfs", "-m", "crc=0", "-n", "ftype=1")
+}
+
+func BenchmarkOverlayUnsupportedOnFAT(b *testing.B) {
+	testOverlaySupported(b, false, "mkfs.fat")
+}