provide an option to use shellbang, not symlinks, for busybox

Not all kernels support symlinks, and not all file systems
on all kernels do. Symlinks are not as portable as we want.
For example, a simple u-root on a VFAT file system can not
be created.

It turns out there is a way to use shellbang files that is as
efficient as symlinks and much more portable.

Add an option to the busybox builder to generate shellbang files,
not symlinks.

TODO: update the tests. But this change is known to work on Plan 9.

Question: should we just move to shellbang files
only, given their more universal nature, or keep it as an option?

Signed-off-by: Ronald G. Minnich <[email protected]>

pkg/uroot/builder/bb.go

@@ -31,7 +31,12 @@ var skip = map[string]struct{}{
 //
 // See bb/README.md for a detailed explanation of the implementation of busybox
 // mode.
-type BBBuilder struct{}
+type BBBuilder struct {
+
+	// ShellBang means generate #! files instead of symlinks.
+	// ShellBang are more portable and just as efficient.
+	ShellBang bool
+}
 
 // DefaultBinaryDir implements Builder.DefaultBinaryDir.
 //
@@ -41,7 +46,7 @@ func (BBBuilder) DefaultBinaryDir() string {
 }
 
 // Build is an implementation of Builder.Build for a busybox-like initramfs.
-func (BBBuilder) Build(af *initramfs.Files, opts Opts) error {
+func (b BBBuilder) Build(af *initramfs.Files, opts Opts) error {
 	// Build the busybox binary.
 	bbPath := filepath.Join(opts.TempDir, "bb")
 	if err := bb.BuildBusybox(opts.Env, opts.Packages, opts.NoStrip, bbPath); err != nil {
@@ -63,7 +68,13 @@ func (BBBuilder) Build(af *initramfs.Files, opts Opts) error {
 		}
 
 		// Add a symlink /bbin/{cmd} -> /bbin/bb to our initramfs.
-		if err := af.AddRecord(cpio.Symlink(filepath.Join(opts.BinaryDir, path.Base(pkg)), "bb")); err != nil {
+		// Or add a #! file if b.ShellBang is set ...
+		if b.ShellBang {
+			b := path.Base(pkg)
+			if err := af.AddRecord(cpio.StaticFile(filepath.Join(opts.BinaryDir, b), "#!/bbin/bb #!"+b+"\n", 0755)); err != nil {
+				return err
+			}
+		} else if err := af.AddRecord(cpio.Symlink(filepath.Join(opts.BinaryDir, path.Base(pkg)), "bb")); err != nil {
 			return err
 		}
 	}

u-root.go

@@ -48,6 +48,7 @@ var (
 	noStrip                                 *bool
 	statsOutputPath                         *string
 	statsLabel                              *string
+	shellbang                               *bool
 )
 
 func init() {
@@ -78,6 +79,7 @@ func init() {
 	flag.Var(&extraFiles, "files", "Additional files, directories, and binaries (with their ldd dependencies) to add to archive. Can be speficified multiple times.")
 
 	noStrip = flag.Bool("no-strip", false, "Build unstripped binaries")
+	shellbang = flag.Bool("shellbang", false, "Use #! instead of symlinks for busybox")
 
 	statsOutputPath = flag.String("stats-output-path", "", "Write build stats to this file (JSON)")
 
@@ -263,7 +265,7 @@ func Main() error {
 		var b builder.Builder
 		switch *build {
 		case "bb":
-			b = builder.BBBuilder{}
+			b = builder.BBBuilder{ShellBang: *shellbang}
 		case "binary":
 			b = builder.BinaryBuilder{}
 		case "source":