disk: add alignment helper

Add a utility method that can be used to align data to the next "grain"
which is currently defined as 1 MiB -- the default used by sfdisk.

Add corresponding tests.

internal/disk/disk.go

@@ -19,6 +19,8 @@ import (
 const (
 	// Default sector size in bytes
 	DefaultSectorSize = 512
+
+	DefaultGrainBytes = uint64(1024 * 1024) // 1 MiB
 )
 
 type PartitionTable struct {
@@ -58,6 +60,14 @@ type Filesystem struct {
 	FSTabPassNo uint64
 }
 
+// AlignUp will align the given sectors to next aligned sector
+func (pt *PartitionTable) AlignUp(sectors uint64) uint64 {
+
+	grain := pt.BytesToSectors(DefaultGrainBytes)
+
+	return ((sectors + grain) / grain) * grain
+}
+
 // Convert the given bytes to the number of sectors.
 func (pt *PartitionTable) BytesToSectors(size uint64) uint64 {
 	sectorSize := pt.SectorSize

internal/disk/disk_test.go

@@ -9,6 +9,26 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestDisk_AlignUp(t *testing.T) {
+
+	pt := disk.PartitionTable{}
+	firstAligned := pt.BytesToSectors(disk.DefaultGrainBytes)
+
+	tests := []struct {
+		size uint64
+		want uint64
+	}{
+		{0, firstAligned},
+		{1, firstAligned},
+		{firstAligned - 1, firstAligned},
+	}
+
+	for _, tt := range tests {
+		got := pt.AlignUp(tt.size)
+		assert.Equal(t, tt.want, got)
+	}
+}
+
 func TestDisk_DynamicallyResizePartitionTable(t *testing.T) {
 	mountpoints := []blueprint.FilesystemCustomization{
 		{