osbuild2/bootiso: arch-based xz compression option

Added a helper function to the bootiso stage for setting the BCJ option
for xz compression.
The FSCompression struct is changed to use a pointer for the Options
substruct so it can be omitted when nil (omitempty).

internal/distro/rhel84/distro_v2.go

@@ -732,7 +732,7 @@ func (t *imageTypeS2) bootISOMonoStageOptions(kernelVer string) *osbuild.BootISO
 			Size: 4096,
 			Compression: osbuild.FSCompression{
 				Method: "xz",
-				Options: osbuild.FSCompressionOptions{
+				Options: &osbuild.FSCompressionOptions{
 					// TODO: based on image arch
 					BCJ: "x86",
 				},

internal/distro/rhel85/distro.go

@@ -752,6 +752,10 @@ func (t *imageType) kickstartStageOptions(ostreeURL, ostreeRef string) *osbuild.
 }
 
 func (t *imageType) bootISOMonoStageOptions(kernelVer string) *osbuild.BootISOMonoStageOptions {
+	comprOptions := new(osbuild.FSCompressionOptions)
+	if bcj := osbuild.BCJOption(t.arch.Name()); bcj != "" {
+		comprOptions.BCJ = bcj
+	}
 	return &osbuild.BootISOMonoStageOptions{
 		Product: osbuild.Product{
 			Name:    "Red Hat Enterprise Linux",
@@ -774,11 +778,8 @@ func (t *imageType) bootISOMonoStageOptions(kernelVer string) *osbuild.BootISOMo
 		RootFS: osbuild.RootFS{
 			Size: 4096,
 			Compression: osbuild.FSCompression{
-				Method: "xz",
-				Options: osbuild.FSCompressionOptions{
-					// TODO: based on image arch
-					BCJ: "x86",
-				},
+				Method:  "xz",
+				Options: comprOptions,
 			},
 		},
 	}

internal/osbuild2/bootiso_stage.go

@@ -37,14 +37,28 @@ type RootFS struct {
 }
 
 type FSCompression struct {
-	Method  string               `json:"method"`
-	Options FSCompressionOptions `json:"options,omitempty"`
+	Method  string                `json:"method"`
+	Options *FSCompressionOptions `json:"options,omitempty"`
 }
 
 type FSCompressionOptions struct {
 	BCJ string `json:"bcj"`
 }
 
+// BCJOption returns the appropriate xz branch/call/jump (BCJ) filter for the
+// given architecture
+func BCJOption(arch string) string {
+	switch arch {
+	case "x86_64":
+		return "x86"
+	case "aarch64":
+		return "arm"
+	case "ppc64le":
+		return "powerpc"
+	}
+	return ""
+}
+
 func (BootISOMonoStageOptions) isStageOptions() {}
 
 type BootISOMonoStageInputs struct {