lm.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"

	"golang.org/x/sys/unix"
)

// Layer encapsulates a specific mount
type Layer struct {
	lower   string
	upper   string
	rootfs  string
	workdir string
}

// FS is a representation of a fully mounted system
type FS struct {
	Mountpoint string
	Layers     []*Layer
}

func main() {
	if len(os.Args) < 3 {
		panic("i pity the fool who doesn't pass arguments")
	}

	args := os.Args[1:]
	fs := &FS{Layers: []*Layer{}}

	for i := 0; i < len(args); i++ {
		var upper, lower, workdir string
		var err error

		if i != 0 {
			lower = args[i-1]
		}

		upper, err = ioutil.TempDir("", "layer-")
		if err != nil {
			panic(err)
		}
		workdir, err = ioutil.TempDir("", "workdir-")
		if err != nil {
			panic(err)
		}

		fs.Layers = append(fs.Layers, &Layer{
			lower:   lower,
			upper:   upper,
			workdir: workdir,
			rootfs:  args[i],
		})
	}

	fs.Layers[1].lower = fs.Layers[0].rootfs

	for i, layer := range fs.Layers {
		os.Mkdir(layer.rootfs, 0777)

		if i == 0 {
			continue
		}

		err := unix.Mount("overlay", layer.rootfs, "overlay", 0, fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", layer.lower, layer.upper, layer.workdir))

		if err != nil {
			panic(err)
		}
	}
}