disk: implement Container interface for PartitionTable and Partition

A `PartitionTable` is a container for one or more instances of
`Partition`, which itself might contain a `Filesystem`.

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

internal/disk/partition.go

@@ -1,6 +1,10 @@
 package disk
 
-import osbuild "github.com/osbuild/osbuild-composer/internal/osbuild1"
+import (
+	"fmt"
+
+	osbuild "github.com/osbuild/osbuild-composer/internal/osbuild1"
+)
 
 type Partition struct {
 	Start    uint64 // Start of the partition in bytes
@@ -44,3 +48,17 @@ func (p *Partition) QEMUPartition() osbuild.QEMUPartition {
 		Filesystem: fs,
 	}
 }
+
+func (pt *Partition) GetItemCount() uint {
+	if pt.Filesystem == nil {
+		return 0
+	}
+	return 1
+}
+
+func (p *Partition) GetChild(n uint) Entity {
+	if n != 0 {
+		panic(fmt.Sprintf("invalid child index for Partition: %d != 0", n))
+	}
+	return p.Filesystem
+}

internal/disk/partition_table.go

@@ -305,6 +305,14 @@ func (pt *PartitionTable) GenerateUUIDs(rng *rand.Rand) {
 	}
 }
 
+func (pt *PartitionTable) GetItemCount() uint {
+	return uint(len(pt.Partitions))
+}
+
+func (pt *PartitionTable) GetChild(n uint) Entity {
+	return &pt.Partitions[n]
+}
+
 // Dynamically calculate and update the start point for each of the existing
 // partitions. Adjusts the overall size of image to either the supplied
 // value in `size` or to the sum of all partitions if that is lager.