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/filesystem.go

@@ -1,5 +1,11 @@
 package disk
 
+import (
+	"math/rand"
+
+	"github.com/google/uuid"
+)
+
 // Filesystem related functions
 type Filesystem struct {
 	Type string
@@ -59,3 +65,9 @@ func (fs *Filesystem) GetFSTabOptions() FSTabOptions {
 		PassNo: fs.FSTabPassNo,
 	}
 }
+
+func (fs *Filesystem) GenUUID(rng *rand.Rand) {
+	if fs.UUID == "" {
+		fs.UUID = uuid.Must(newRandomUUIDFromReader(rng)).String()
+	}
+}