blueprint: catch obviously invalid dos PTs early

We can't catch all cases of dos partition table with too many
partitions, but catching the obvious ones early can help.

pkg/blueprint/disk_customizations.go

@@ -349,7 +349,16 @@ func (p *DiskCustomization) Validate() error {
 	}
 
 	switch p.Type {
-	case "gpt", "dos", "":
+	case "gpt", "":
+	case "dos":
+		// dos/mbr only supports 4 partitions
+		// Unfortunately, at this stage it's unknown whether we will need extra
+		// partitions (bios boot, root, esp), so this check is just to catch
+		// obvious invalid customizations early. The final partition table is
+		// checked after it's created.
+		if len(p.Partitions) > 4 {
+			return fmt.Errorf("invalid partitioning customizations: \"dos\" partition table type only supports up to 4 partitions: got %d", len(p.Partitions))
+		}
 	default:
 		return fmt.Errorf("unknown partition table type: %s (valid: gpt, dos)", p.Type)
 	}

pkg/blueprint/disk_customizations_test.go

@@ -920,6 +920,56 @@ func TestPartitioningValidation(t *testing.T) {
 			},
 			expectedMsg: "unknown partition table type: toucan (valid: gpt, dos)",
 		},
+		"unhappy-too-many-parts": {
+			partitioning: &blueprint.DiskCustomization{
+				Type: "dos",
+				Partitions: []blueprint.PartitionCustomization{
+					{
+						Type: "plain",
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							FSType:     "xfs",
+							Mountpoint: "/1",
+						},
+					},
+					{
+						Type: "plain",
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							FSType:     "xfs",
+							Mountpoint: "/2",
+						},
+					},
+					{
+						Type: "plain",
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							FSType:     "xfs",
+							Mountpoint: "/3",
+						},
+					},
+					{
+						Type: "plain",
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							FSType:     "ext4",
+							Mountpoint: "/4",
+						},
+					},
+					{
+						Type: "plain",
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							FSType:     "ext4",
+							Mountpoint: "/5",
+						},
+					},
+					{
+						Type: "plain",
+						FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
+							FSType:     "ext4",
+							Mountpoint: "/6",
+						},
+					},
+				},
+			},
+			expectedMsg: `invalid partitioning customizations: "dos" partition table type only supports up to 4 partitions: got 6`,
+		},
 	}
 
 	for name := range testCases {