Add blockfile snapshotter

Signed-off-by: Derek McGowan <derek@mcg.dev>

snapshots/blockfile/blockfile_loopsetup_test.go

@@ -0,0 +1,86 @@
+//go:build !windows && !darwin
+
+/*
+   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 blockfile
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+
+	"github.com/containerd/containerd/mount"
+	"github.com/containerd/continuity/fs"
+	"github.com/containerd/continuity/testutil/loopback"
+	"golang.org/x/sys/unix"
+)
+
+func setupSnapshotter(t *testing.T) ([]Opt, error) {
+	mkfs, err := exec.LookPath("mkfs.ext4")
+	if err != nil {
+		t.Skipf("Could not find mkfs.ext4: %v", err)
+	}
+
+	loopbackSize := int64(128 << 20) // 128 MB
+	if os.Getpagesize() > 4096 {
+		loopbackSize = int64(650 << 20) // 650 MB
+	}
+
+	loop, err := loopback.New(loopbackSize)
+	if err != nil {
+		return nil, err
+	}
+	defer loop.Close()
+
+	if out, err := exec.Command(mkfs, loop.Device).CombinedOutput(); err != nil {
+		return nil, fmt.Errorf("failed to make ext4 filesystem (out: %q): %w", out, err)
+	}
+	// sync after a mkfs on the loopback before trying to mount the device
+	unix.Sync()
+
+	if err := testMount(t, loop.Device); err != nil {
+		return nil, err
+	}
+
+	scratch := filepath.Join(t.TempDir(), "scratch")
+	err = fs.CopyFile(scratch, loop.File)
+	if err != nil {
+		return nil, err
+	}
+
+	return []Opt{
+		WithScratchFile(scratch),
+	}, nil
+}
+
+func testMount(t *testing.T, device string) error {
+	root, err := os.MkdirTemp(t.TempDir(), "")
+	if err != nil {
+		return err
+	}
+	defer os.RemoveAll(root)
+
+	if out, err := exec.Command("mount", device, root).CombinedOutput(); err != nil {
+		return fmt.Errorf("failed to mount device %s (out: %q): %w", device, out, err)
+	}
+	if err := os.Remove(filepath.Join(root, "lost+found")); err != nil {
+		return err
+	}
+	return mount.UnmountAll(root, unix.MNT_DETACH)
+}