Add experimenta btrfs driver

This is an experimental btrfs driver. To use it you must have
/var/lib/docker mounted on a btrfs filesystem and explicitly
specify DOCKER_DRIVER=btrfs in the docker daemon environment.

It works by using subvolumes for the docker image/container layers.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

graphdriver/btrfs/btrfs.go

@@ -0,0 +1,215 @@
+// +build linux
+
+package btrfs
+
+/*
+#include <stdlib.h>
+#include <sys/ioctl.h>
+#include <linux/fs.h>
+#include <errno.h>
+#include <sys/types.h>
+#include <dirent.h>
+#include <linux/btrfs.h>
+
+*/
+import "C"
+import (
+	"fmt"
+	"github.com/dotcloud/docker/graphdriver"
+	"os"
+	"path"
+	"syscall"
+	"unsafe"
+)
+
+func init() {
+	graphdriver.Register("btrfs", Init)
+}
+
+func Init(home 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 buf.Type != 0x9123683E {
+		return nil, fmt.Errorf("%s is not a btrfs filesystem", rootdir)
+	}
+
+	return &Driver{
+		home: home,
+	}, nil
+}
+
+type Driver struct {
+	home string
+}
+
+func (d *Driver) String() string {
+	return "btrfs"
+}
+
+func (d *Driver) Status() [][2]string {
+	return nil
+}
+
+func (d *Driver) Cleanup() error {
+	return nil
+}
+
+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("Can't create subvolume")
+	}
+	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("Can't create subvolume")
+	}
+	return nil
+}
+
+func subvolDelete(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_SNAP_DESTROY,
+		uintptr(unsafe.Pointer(&args)))
+	if errno != 0 {
+		return fmt.Errorf("Can't create subvolume")
+	}
+	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)
+}
+
+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
+}
+
+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)
+}
+
+func (d *Driver) Get(id 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
+}
+
+func (d *Driver) Put(id string) {
+}
+
+func (d *Driver) Exists(id string) bool {
+	dir := d.subvolumesDirId(id)
+	_, err := os.Stat(dir)
+	return err == nil
+}

graphdriver/btrfs/dummy_unsupported.go

@@ -0,0 +1,3 @@
+// +build !linux
+
+package btrfs

graphdriver/driver.go

@@ -41,6 +41,8 @@ var (
 		"aufs",
 		"devicemapper",
 		"vfs",
+		// experimental, has to be enabled manually for now
+		"btrfs",
 	}
 )
 

runtime.go

@@ -9,6 +9,7 @@ import (
 	"github.com/dotcloud/docker/execdriver/lxc"
 	"github.com/dotcloud/docker/graphdriver"
 	"github.com/dotcloud/docker/graphdriver/aufs"
+	_ "github.com/dotcloud/docker/graphdriver/btrfs"
 	_ "github.com/dotcloud/docker/graphdriver/devmapper"
 	_ "github.com/dotcloud/docker/graphdriver/vfs"
 	"github.com/dotcloud/docker/pkg/graphdb"