osbuild: validate sfdisk stage options

DOS partition tables only support up to 4 partitions.  Don't create the
sfdisk stage options if this rule is violated.  Stage validation
failures panic, so we panic on the stage creation.  This might change in
the future (to return errors) but for now we use panics and put the
responsibility on the callers to validate their inputs if they want to
have better error control.

Test sfdisk stage validation failure

pkg/osbuild/sfdisk_stage.go

@@ -1,5 +1,11 @@
 package osbuild
 
+import (
+	"fmt"
+
+	"github.com/osbuild/images/pkg/disk"
+)
+
 // Partition a target using sfdisk(8)
 
 type SfdiskStageOptions struct {
@@ -36,7 +42,18 @@ type SfdiskPartition struct {
 	UUID string `json:"uuid,omitempty"`
 }
 
+func (o SfdiskStageOptions) validate() error {
+	if o.Label == disk.PT_DOS.String() && len(o.Partitions) > 4 {
+		return fmt.Errorf("sfdisk stage creation failed: \"dos\" partition table only supports up to 4 partitions: got %d", len(o.Partitions))
+	}
+	return nil
+}
+
 func NewSfdiskStage(options *SfdiskStageOptions, device *Device) *Stage {
+	if err := options.validate(); err != nil {
+		panic(err)
+	}
+
 	return &Stage{
 		Type:    "org.osbuild.sfdisk",
 		Options: options,

pkg/osbuild/sfdisk_stage_test.go

@@ -41,3 +41,22 @@ func TestNewSfdiskStage(t *testing.T) {
 	actualStageDOS := NewSfdiskStage(&options, device)
 	assert.Equal(t, expectedStage, actualStageDOS)
 }
+
+func TestNewSfdiskStageInvalid(t *testing.T) {
+
+	partition := SfdiskPartition{
+		// doesn't really matter
+	}
+
+	options := SfdiskStageOptions{
+		Label:      "dos",
+		UUID:       "D209C89E-EA5E-4FBD-B161-B461CCE297E0",
+		Partitions: []SfdiskPartition{partition, partition, partition, partition, partition}, // 5 partitions
+	}
+
+	device := NewLoopbackDevice(&LoopbackDeviceOptions{Filename: "disk.raw"})
+
+	assert.Panics(t, func() {
+		NewSfdiskStage(&options, device)
+	})
+}