disk: new types Btrfs and BtrfsSubvolume

Structs to represent a Btrfs file system with subvolumes.

Btrfs implements the following interfaces:
- Entity
- Container
- VolumeContainer

BtrfsSubvolume implements the following interfaces:
- Entity
- Sizeable
- Mountable

Co-Authored-By: Christian Kellner <christian@kellner.me>

internal/disk/btrfs.go

@@ -0,0 +1,94 @@
+package disk
+
+import (
+	"fmt"
+	"strings"
+)
+
+type Btrfs struct {
+	UUID       string
+	Label      string
+	Mountpoint string
+	Subvolumes []BtrfsSubvolume
+}
+
+func (b *Btrfs) IsContainer() bool {
+	return true
+}
+
+func (b *Btrfs) GetItemCount() uint {
+	return uint(len(b.Subvolumes))
+}
+
+func (b *Btrfs) GetChild(n uint) Entity {
+	return &b.Subvolumes[n]
+}
+func (b *Btrfs) CreateVolume(mountpoint string, size uint64) (Entity, error) {
+	name := mountpoint
+	if name == "/" {
+		name = "root"
+	}
+	subvolume := BtrfsSubvolume{
+		Size:       size,
+		Mountpoint: mountpoint,
+		GroupID:    0,
+		UUID:       b.UUID, // subvolumes inherit UUID of main volume
+		Name:       name,
+	}
+
+	b.Subvolumes = append(b.Subvolumes, subvolume)
+	return &b.Subvolumes[len(b.Subvolumes)-1], nil
+}
+
+type BtrfsSubvolume struct {
+	Name       string
+	Size       uint64
+	Mountpoint string
+	GroupID    uint64
+
+	MntOps string
+
+	// UUID of the parent volume
+	UUID string
+}
+
+func (subvol *BtrfsSubvolume) IsContainer() bool {
+	return false
+}
+
+func (bs *BtrfsSubvolume) GetSize() uint64 {
+	return bs.Size
+}
+
+func (bs *BtrfsSubvolume) EnsureSize(s uint64) bool {
+	if s > bs.Size {
+		bs.Size = s
+		return true
+	}
+	return false
+}
+
+func (bs *BtrfsSubvolume) GetMountpoint() string {
+	return bs.Mountpoint
+}
+
+func (bs *BtrfsSubvolume) GetFSType() string {
+	return "btrfs"
+}
+
+func (bs *BtrfsSubvolume) GetFSSpec() FSSpec {
+	return FSSpec{
+		UUID:  bs.UUID,
+		Label: bs.Name,
+	}
+}
+
+func (bs *BtrfsSubvolume) GetFSTabOptions() FSTabOptions {
+	ops := strings.Join([]string{bs.MntOps, fmt.Sprintf("subvol=%s", bs.Name)}, ",")
+
+	return FSTabOptions{
+		MntOps: ops,
+		Freq:   0,
+		PassNo: 0,
+	}
+}