disk: error on PTs with type dos and len(partitions) > 4

Return an error when a partition table is created with type dos and more
than 4 partitions.

This check can only happen after the partition table is created,
otherwise we would have to predict all the partitions that get created
automatically, which would duplicate a lot of the logic in
NewCustomPartitionTable().

Update tests:
- Convert the plain+swap (valid) test to GPT since now it's invalid as
  DOS.
- Add a fail test for a DOS partition table with too many partitions.

pkg/disk/partition_table.go

@@ -1292,6 +1292,16 @@ func NewCustomPartitionTable(customizations *blueprint.DiskCustomization, option
 	pt.relayout(customizations.MinSize)
 	pt.GenerateUUIDs(rng)
 
+	// One thing not caught by the customization validation is if a final "dos"
+	// partition table has more than 4 partitions. This is not possible to
+	// predict with customizations alone because it depends on the boot type
+	// (which comes from the image type) which controls automatic partition
+	// creation. We should therefore always check the final partition table for
+	// this rule.
+	if pt.Type == PT_DOS && len(pt.Partitions) > 4 {
+		return nil, fmt.Errorf("%s invalid partition table: \"dos\" partition table type only supports up to 4 partitions: got %d after creating the partition table with all necessary partitions", errPrefix, len(pt.Partitions))
+	}
+
 	return pt, nil
 }
 

pkg/disk/partition_table_test.go

@@ -1285,24 +1285,24 @@ func TestNewCustomPartitionTable(t *testing.T) {
 			options: &disk.CustomPartitionTableOptions{
 				DefaultFSType:      disk.FS_XFS,
 				BootMode:           platform.BOOT_HYBRID,
-				PartitionTableType: disk.PT_DOS,
+				PartitionTableType: disk.PT_GPT,
 			},
 			expected: &disk.PartitionTable{
-				Type: disk.PT_DOS,
-				Size: 227 * datasizes.MiB,
+				Type: disk.PT_GPT,
+				Size: 227*datasizes.MiB + datasizes.MiB, // last part + footer
 				UUID: "0194fdc2-fa2f-4cc0-81d3-ff12045b73c8",
 				Partitions: []disk.Partition{
 					{
 						Start:    1 * datasizes.MiB, // header
 						Size:     1 * datasizes.MiB,
 						Bootable: true,
-						Type:     disk.DosBIOSBootID,
+						Type:     disk.BIOSBootPartitionGUID,
 						UUID:     disk.BIOSBootPartitionUUID,
 					},
 					{
 						Start: 2 * datasizes.MiB,
 						Size:  200 * datasizes.MiB,
-						Type:  disk.DosESPID,
+						Type:  disk.EFISystemPartitionGUID,
 						UUID:  disk.EFISystemPartitionUUID,
 						Payload: &disk.Filesystem{
 							Type:         "vfat",
@@ -1317,9 +1317,9 @@ func TestNewCustomPartitionTable(t *testing.T) {
 					{
 						Start:    202 * datasizes.MiB,
 						Size:     20 * datasizes.MiB,
-						Type:     disk.DosLinuxTypeID,
+						Type:     disk.FilesystemDataGUID,
 						Bootable: false,
-						UUID:     "", // partitions on dos PTs don't have UUIDs
+						UUID:     "e2d3d0d0-de6b-48f9-b44c-e85ff044c6b1",
 						Payload: &disk.Filesystem{
 							Type:         "ext4",
 							Label:        "data",
@@ -1333,8 +1333,8 @@ func TestNewCustomPartitionTable(t *testing.T) {
 					{
 						Start:    222 * datasizes.MiB,
 						Size:     5 * datasizes.MiB,
-						Type:     disk.DosSwapID,
-						UUID:     "", // partitions on dos PTs don't have UUIDs
+						Type:     disk.SwapPartitionGUID,
+						UUID:     "f83b8e88-3bbf-457a-ab99-c5b252c7429c",
 						Bootable: false,
 						Payload: &disk.Swap{
 							Label:        "swap",
@@ -1344,9 +1344,9 @@ func TestNewCustomPartitionTable(t *testing.T) {
 					},
 					{
 						Start:    227 * datasizes.MiB,
-						Size:     0,
-						Type:     disk.DosLinuxTypeID,
-						UUID:     "", // partitions on dos PTs don't have UUIDs
+						Size:     1*datasizes.MiB - (disk.DefaultSectorSize + (128 * 128)), // grows by 1 grain size (1 MiB) minus the unaligned size of the header to fit the gpt footer
+						Type:     disk.FilesystemDataGUID,
+						UUID:     "32f3a8ae-b79e-4856-b659-c18f0dcecc77",
 						Bootable: false,
 						Payload: &disk.Filesystem{
 							Type:         "xfs",
@@ -2276,6 +2276,33 @@ func TestNewCustomPartitionTableErrors(t *testing.T) {
 			},
 			errmsg: `error generating partition table: unknown partition table type: toucan (valid: gpt, dos)`,
 		},
+		"dos-too-many-parts": {
+			customizations: &blueprint.DiskCustomization{
+				Partitions: []blueprint.PartitionCustomization{
+					{
+						MinSize: 20 * datasizes.MiB,
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							Mountpoint: "/data",
+							Label:      "data",
+							FSType:     "ext4",
+						},
+					},
+					{
+						MinSize: 5 * datasizes.MiB,
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							Label:  "swap",
+							FSType: "swap",
+						},
+					},
+				},
+			},
+			options: &disk.CustomPartitionTableOptions{
+				DefaultFSType:      disk.FS_XFS,
+				BootMode:           platform.BOOT_HYBRID,
+				PartitionTableType: disk.PT_DOS,
+			},
+			errmsg: `error generating partition table: invalid partition table: "dos" partition table type only supports up to 4 partitions: got 5 after creating the partition table with all necessary partitions`,
+		},
 	}
 
 	// we don't care about the rng for error tests