disk: new interface UniqueEntity for Entities with UUID

This is for all entities that have a uniquely identifiable via a UUID.
Currently the interface only contains a single function, GenUUID(),
that should generate a new UUID via a random source (`rand.Rand`).
NB: Partitions are uniquely identifiable if and only if the layout is
GPT.

Co-Authored-By: Achilleas Koutsou <achilleas@koutsou.net>

internal/disk/btrfs.go

@@ -2,7 +2,10 @@ package disk
 
 import (
 	"fmt"
+	"math/rand"
 	"strings"
+
+	"github.com/google/uuid"
 )
 
 type Btrfs struct {
@@ -64,6 +67,12 @@ func (b *Btrfs) CreateVolume(mountpoint string, size uint64) (Entity, error) {
 	return &b.Subvolumes[len(b.Subvolumes)-1], nil
 }
 
+func (b *Btrfs) GenUUID(rng *rand.Rand) {
+	if b.UUID == "" {
+		b.UUID = uuid.Must(newRandomUUIDFromReader(rng)).String()
+	}
+}
+
 type BtrfsSubvolume struct {
 	Name       string
 	Size       uint64