btrfs.go

// +build linux

package btrfs

/*
#include <stdlib.h>
#include <dirent.h>
#include <btrfs/ioctl.h>
#include <btrfs/ctree.h>
*/
import "C"

import (
	"fmt"
	"os"
	"path"
	"path/filepath"
	"syscall"
	"unsafe"

	"github.com/docker/docker/daemon/graphdriver"
	"github.com/docker/docker/pkg/mount"
)

func init() {
	graphdriver.Register("btrfs", Init)
}

// Init returns a new BTRFS driver.
// An error is returned if BTRFS is not supported.
func Init(home string, options []string) (graphdriver.Driver, error) {
	rootdir := path.Dir(home)

	var buf syscall.Statfs_t
	if err := syscall.Statfs(rootdir, &buf); err != nil {
		return nil, err
	}

	if graphdriver.FsMagic(buf.Type) != graphdriver.FsMagicBtrfs {
		return nil, graphdriver.ErrPrerequisites
	}

	if err := os.MkdirAll(home, 0700); err != nil {
		return nil, err
	}

	if err := mount.MakePrivate(home); err != nil {
		return nil, err
	}

	driver := &Driver{
		home: home,
	}

	return graphdriver.NewNaiveDiffDriver(driver), nil
}

// Driver contains information about the filesystem mounted.
type Driver struct {
	//root of the file system
	home string
}

// String prints the name of the driver (btrfs).
func (d *Driver) String() string {
	return "btrfs"
}

// Status returns current driver information in a two dimensional string array.
// Output contains "Build Version" and "Library Version" of the btrfs libraries used.
// Version information can be used to check compatibility with your kernel.
func (d *Driver) Status() [][2]string {
	status := [][2]string{}
	if bv := btrfsBuildVersion(); bv != "-" {
		status = append(status, [2]string{"Build Version", bv})
	}
	if lv := btrfsLibVersion(); lv != -1 {
		status = append(status, [2]string{"Library Version", fmt.Sprintf("%d", lv)})
	}
	return status
}

// GetMetadata returns empty metadata for this driver.
func (d *Driver) GetMetadata(id string) (map[string]string, error) {
	return nil, nil
}

// Cleanup unmounts the home directory.
func (d *Driver) Cleanup() error {
	return mount.Unmount(d.home)
}

func free(p *C.char) {
	C.free(unsafe.Pointer(p))
}

func openDir(path string) (*C.DIR, error) {
	Cpath := C.CString(path)
	defer free(Cpath)

	dir := C.opendir(Cpath)
	if dir == nil {
		return nil, fmt.Errorf("Can't open dir")
	}
	return dir, nil
}

func closeDir(dir *C.DIR) {
	if dir != nil {
		C.closedir(dir)
	}
}

func getDirFd(dir *C.DIR) uintptr {
	return uintptr(C.dirfd(dir))
}

func subvolCreate(path, name string) error {
	dir, err := openDir(path)
	if err != nil {
		return err
	}
	defer closeDir(dir)

	var args C.struct_btrfs_ioctl_vol_args
	for i, c := range []byte(name) {
		args.name[i] = C.char(c)
	}

	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, getDirFd(dir), C.BTRFS_IOC_SUBVOL_CREATE,
		uintptr(unsafe.Pointer(&args)))
	if errno != 0 {
		return fmt.Errorf("Failed to create btrfs subvolume: %v", errno.Error())
	}
	return nil
}

func subvolSnapshot(src, dest, name string) error {
	srcDir, err := openDir(src)
	if err != nil {
		return err
	}
	defer closeDir(srcDir)

	destDir, err := openDir(dest)
	if err != nil {
		return err
	}
	defer closeDir(destDir)

	var args C.struct_btrfs_ioctl_vol_args_v2
	args.fd = C.__s64(getDirFd(srcDir))
	for i, c := range []byte(name) {
		args.name[i] = C.char(c)
	}

	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, getDirFd(destDir), C.BTRFS_IOC_SNAP_CREATE_V2,
		uintptr(unsafe.Pointer(&args)))
	if errno != 0 {
		return fmt.Errorf("Failed to create btrfs snapshot: %v", errno.Error())
	}
	return nil
}

func isSubvolume(p string) (bool, error) {
	var bufStat syscall.Stat_t
	if err := syscall.Lstat(p, &bufStat); err != nil {
		return false, err
	}

	// return true if it is a btrfs subvolume
	return bufStat.Ino == C.BTRFS_FIRST_FREE_OBJECTID, nil
}

func subvolDelete(dirpath, name string) error {
	dir, err := openDir(dirpath)
	if err != nil {
		return err
	}
	defer closeDir(dir)

	var args C.struct_btrfs_ioctl_vol_args

	// walk the btrfs subvolumes
	walkSubvolumes := func(p string, f os.FileInfo, err error) error {
		// we want to check children only so skip itself
		// it will be removed after the filepath walk anyways
		if f.IsDir() && p != path.Join(dirpath, name) {
			sv, err := isSubvolume(p)
			if err != nil {
				return fmt.Errorf("Failed to test if %s is a btrfs subvolume: %v", p, err)
			}
			if sv {
				if err := subvolDelete(p, f.Name()); err != nil {
					return fmt.Errorf("Failed to destroy btrfs child subvolume (%s) of parent (%s): %v", p, dirpath, err)
				}
			}
		}
		return nil
	}
	if err := filepath.Walk(path.Join(dirpath, name), walkSubvolumes); err != nil {
		return fmt.Errorf("Recursively walking subvolumes for %s failed: %v", dirpath, err)
	}

	// all subvolumes have been removed
	// now remove the one originally passed in
	for i, c := range []byte(name) {
		args.name[i] = C.char(c)
	}
	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, getDirFd(dir), C.BTRFS_IOC_SNAP_DESTROY,
		uintptr(unsafe.Pointer(&args)))
	if errno != 0 {
		return fmt.Errorf("Failed to destroy btrfs snapshot %s for %s: %v", dirpath, name, errno.Error())
	}
	return nil
}

func (d *Driver) subvolumesDir() string {
	return path.Join(d.home, "subvolumes")
}

func (d *Driver) subvolumesDirID(id string) string {
	return path.Join(d.subvolumesDir(), id)
}

// Create the filesystem with given id.
func (d *Driver) Create(id string, parent string) error {
	subvolumes := path.Join(d.home, "subvolumes")
	if err := os.MkdirAll(subvolumes, 0700); err != nil {
		return err
	}
	if parent == "" {
		if err := subvolCreate(subvolumes, id); err != nil {
			return err
		}
	} else {
		parentDir, err := d.Get(parent, "")
		if err != nil {
			return err
		}
		if err := subvolSnapshot(parentDir, subvolumes, id); err != nil {
			return err
		}
	}
	return nil
}

// Remove the filesystem with given id.
func (d *Driver) Remove(id string) error {
	dir := d.subvolumesDirID(id)
	if _, err := os.Stat(dir); err != nil {
		return err
	}
	if err := subvolDelete(d.subvolumesDir(), id); err != nil {
		return err
	}
	return os.RemoveAll(dir)
}

// Get the requested filesystem id.
func (d *Driver) Get(id, mountLabel string) (string, error) {
	dir := d.subvolumesDirID(id)
	st, err := os.Stat(dir)
	if err != nil {
		return "", err
	}

	if !st.IsDir() {
		return "", fmt.Errorf("%s: not a directory", dir)
	}

	return dir, nil
}

// Put is not implemented for BTRFS as there is no cleanup required for the id.
func (d *Driver) Put(id string) error {
	// Get() creates no runtime resources (like e.g. mounts)
	// so this doesn't need to do anything.
	return nil
}

// Exists checks if the id exists in the filesystem.
func (d *Driver) Exists(id string) bool {
	dir := d.subvolumesDirID(id)
	_, err := os.Stat(dir)
	return err == nil
}