package main

import (
	"encoding/binary"
	"errors"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"
	"strings"
	"syscall"
)

var (
	perm        = flag.String("perm", "/perm", "new val for perm")
	gloaderHome = "/perm/gloader"
)

func main() {
	flag.Parse()
	gloaderHome = filepath.Join(*perm, "gloader")
	os.MkdirAll(gloaderHome, 0777)
	err := mount()
	if err != nil {
		log.Println(err)
		os.Exit(1)
	}
	d := newDownloader()
	d.DataDir = filepath.Join(gloaderHome, "data")
	d.Start("tcp", ":8844")
}

func mount() error {
	var (
		partUUIDb []byte
		partUUID  string
		dev       string
		err       error
		dataStat  os.FileInfo
		dataDir   = filepath.Join(gloaderHome, "data")
	)
	partUUIDb, err = ioutil.ReadFile(filepath.Join(gloaderHome, "partition"))
	partUUID = strings.TrimSpace(string(partUUIDb))
	if err != nil || partUUID == "" {
		return nil
	}
	dataStat, err = os.Stat(dataDir)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			err = os.MkdirAll(dataDir, 0o755)
			if err != nil {
				return fmt.Errorf("error creating datadir: %w", err)
			}
			dataStat, err = os.Stat(dataDir)
			if err != nil {
				return fmt.Errorf("error mounting datadir: %w", err)
			}
		} else {
			return fmt.Errorf("error mounting datadir: %w", err)
		}
	}
	if dataStat.IsDir() {
		var folder *os.File
		folder, err = os.Open(dataDir)
		if err != nil {
			return fmt.Errorf("error mounting datadir: %w", err)
		}
		_, err = folder.Readdir(1)
		if errors.Is(err, io.EOF) {
			fmt.Printf("mount %s %s", partUUID, dataDir)
			dev, err = findPartUUID(partUUID)
			if err != nil {
				return fmt.Errorf("error mounting datadir: %w", err)
			}
			err = syscall.Mount(dev, dataDir, "ext4", 0, "")
			if err != nil {
				return fmt.Errorf("error mounting datadir: %w", err)
			}
			return nil
		}
		return fmt.Errorf("error mounting datadir: %w", err)
	}
	return fmt.Errorf("error mounting datadir: data dir %s is not a directory", dataDir)
}

func findPartUUID(uuid string) (string, error) {
	var dev string
	err := filepath.Walk("/sys/block", func(path string, info os.FileInfo, err error) error {
		if err != nil {
			log.Printf("findPartUUID: %v", err)
			return nil
		}
		if info.Mode()&os.ModeSymlink == 0 {
			return nil
		}
		devname := "/dev/" + filepath.Base(path)
		f, err := os.Open(devname)
		if err != nil {
			log.Printf("findPartUUID: %v", err)
			return nil
		}
		defer f.Close()
		if _, err := f.Seek(440, io.SeekStart); err != nil {
			var se syscall.Errno
			if errors.As(err, &se) && se == syscall.EINVAL {
				// Seek()ing empty loop devices results in EINVAL.
				return nil
			}
			log.Printf("findPartUUID: %v(%T)", err, err.(*os.PathError).Err)
			return nil
		}
		var diskSig struct {
			ID      uint32
			Trailer uint16
		}
		if err := binary.Read(f, binary.LittleEndian, &diskSig); err != nil {
			log.Printf("findPartUUID: %v", err)
			return nil
		}
		if fmt.Sprintf("%08x", diskSig.ID) == uuid && diskSig.Trailer == 0 {
			dev = devname
			// TODO: abort early with sentinel error code
			return nil
		}
		return nil
	})
	if err != nil {
		return "", err
	}
	if dev == "" {
		return "", fmt.Errorf("PARTUUID=%s not found", uuid)
	}
	return dev, nil
}