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/luks.go

@@ -1,6 +1,11 @@
 package disk
 
-import "fmt"
+import (
+	"fmt"
+	"math/rand"
+
+	"github.com/google/uuid"
+)
 
 type Argon2id struct {
 	Iterations  uint
@@ -59,3 +64,9 @@ func (lc *LUKSContainer) Clone() Entity {
 		Payload: lc.Payload,
 	}
 }
+
+func (lc *LUKSContainer) GenUUID(rng *rand.Rand) {
+	if lc.UUID == "" {
+		lc.UUID = uuid.Must(newRandomUUIDFromReader(rng)).String()
+	}
+}