fix: generate of modules.dep when on the machine

When running on the machine, the extensionTreePath is not writeable, so
create and clean up a temporary directory to host `modules.dep`
extension.

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

internal/pkg/extensions/kernel_modules.go

@@ -39,7 +39,7 @@ func (ext *Extension) KernelModuleDirectory() string {
 // GenerateKernelModuleDependencyTreeExtension generates a kernel module dependency tree extension.
 //
 //nolint:gocyclo
-func GenerateKernelModuleDependencyTreeExtension(extensionPathsWithKernelModules []string, initramfsPath, extensionTreePath string, printFunc func(format string, v ...any)) (*Extension, error) {
+func GenerateKernelModuleDependencyTreeExtension(extensionPathsWithKernelModules []string, initramfsPath, scratchPath string, printFunc func(format string, v ...any)) (*Extension, error) {
 	printFunc("preparing to run depmod to generate kernel modules dependency tree")
 
 	tempDir, err := os.MkdirTemp("", "ext-modules")
@@ -106,7 +106,7 @@ func GenerateKernelModuleDependencyTreeExtension(extensionPathsWithKernelModules
 	}
 
 	// we want the extension to survive this function, so not storing in a temporary directory
-	kernelModulesDependencyTreeStagingDir := filepath.Join(extensionTreePath, "modules.dep")
+	kernelModulesDependencyTreeStagingDir := filepath.Join(scratchPath, "modules.dep")
 
 	// we want to make sure the root directory has the right permissions.
 	if err := os.MkdirAll(kernelModulesDependencyTreeStagingDir, 0o755); err != nil {

pkg/imager/extensions/extensions.go

@@ -28,6 +28,8 @@ type Builder struct {
 }
 
 // Build rebuilds the initramfs.xz with extensions.
+//
+//nolint:gocyclo
 func (builder *Builder) Build() error {
 	extensionsList, err := extensions.List(builder.ExtensionTreePath)
 	if err != nil {
@@ -49,7 +51,17 @@ func (builder *Builder) Build() error {
 	extensionPathsWithKernelModules := findExtensionsWithKernelModules(extensionsList)
 
 	if len(extensionPathsWithKernelModules) > 0 {
-		kernelModuleDepExtension, genErr := extensions.GenerateKernelModuleDependencyTreeExtension(extensionPathsWithKernelModules, builder.InitramfsPath, builder.ExtensionTreePath, builder.Printf)
+		var scratchPath string
+
+		// create a temporary directory to store 'modules.dep' extension
+		scratchPath, err = os.MkdirTemp("", "ext-modules")
+		if err != nil {
+			return err
+		}
+
+		defer os.RemoveAll(scratchPath) //nolint:errcheck
+
+		kernelModuleDepExtension, genErr := extensions.GenerateKernelModuleDependencyTreeExtension(extensionPathsWithKernelModules, builder.InitramfsPath, scratchPath, builder.Printf)
 		if genErr != nil {
 			return genErr
 		}