// Command xf (xfer) is a single-file HTTP file transfer server. Every
// directory under the served root renders as one embedded web page that lists
// its entries and accepts uploads into that directory; every file is served
// for download.
//
// Usage:
//
//	xf [-addr address] [-dir directory] [-max-upload bytes]
package main

import (
	"crypto/rand"
	"encoding/hex"
	"errors"
	"flag"
	"fmt"
	"html/template"
	"io"
	"log"
	"net"
	"net/http"
	"net/url"
	"os"
	"path"
	"path/filepath"
	"sort"
	"strings"
	"syscall"
	"time"
)

const (
	defaultMaxUpload = 1 << 30
	maxUploadParts   = 1000
	tempPrefix       = ".xf-upload-"

	// readHeaderTimeout bounds a slow client without bounding the time it
	// takes to stream a large upload body.
	readHeaderTimeout = 10 * time.Second
)

var page = template.Must(template.New("index").Parse(indexHTML))

// access logs downloads and uploads to standard output, kept separate from the
// startup and error diagnostics the default logger writes to standard error.
var access = log.New(os.Stdout, "", log.LstdFlags)

type server struct {
	root      *os.Root
	maxUpload int64
}

type stagedFile struct {
	name string
	temp string
	size int64
}

// countedBody counts the bytes read from the request body. multipart can turn
// a MaxBytesReader error into a parse error, so the count, not the error,
// decides whether an upload was refused for its size.
type countedBody struct {
	io.ReadCloser
	bytes int64
}

func (body *countedBody) Read(p []byte) (int, error) {
	n, err := body.ReadCloser.Read(p)
	body.bytes += int64(n)
	return n, err
}

type entry struct {
	Name  string
	Href  string
	Size  string
	IsDir bool
}

type pageData struct {
	Dir     string // URL path of the listed directory
	Parent  bool   // whether to link to the parent directory
	Entries []entry
}

func main() {
	addr := flag.String("addr", "127.0.0.1:8080", "listen address")
	dir := flag.String("dir", ".", "directory to serve")
	maxUpload := flag.Int64("max-upload", defaultMaxUpload,
		"maximum bytes in one upload request")
	flag.Parse()
	if *maxUpload <= 0 {
		log.Fatal("xf: -max-upload must be greater than zero")
	}

	rootName, err := filepath.Abs(*dir)
	if err != nil {
		log.Fatalf("xf: %v", err)
	}
	root, err := os.OpenRoot(rootName)
	if err != nil {
		log.Fatalf("xf: %v", err)
	}

	s := &server{root: root, maxUpload: *maxUpload}

	mux := http.NewServeMux()
	mux.HandleFunc("GET /", s.handleGet)
	mux.HandleFunc("POST /", s.handleUpload)

	srv := &http.Server{
		Addr:              *addr,
		Handler:           mux,
		ReadHeaderTimeout: readHeaderTimeout,
	}

	log.Printf("xf: serving %s on %s", rootName, *addr)
	log.Fatalf("xf: %v", srv.ListenAndServe())
}

// resolve maps a request path to a path relative to the server root.
func (s *server) resolve(urlPath string) string {
	name := strings.TrimPrefix(path.Clean("/"+urlPath), "/")
	if name == "" {
		return "."
	}
	return filepath.FromSlash(name)
}

func (s *server) handleGet(w http.ResponseWriter, r *http.Request) {
	rec := &recorder{ResponseWriter: w, status: http.StatusOK}
	s.serve(rec, r)
	access.Printf("get %q -> %d (%d bytes) from %s",
		r.URL.Path, rec.status, rec.bytes, clientIP(r))
}

func (s *server) serve(w http.ResponseWriter, r *http.Request) {
	name := s.resolve(r.URL.Path)
	if isTempName(filepath.Base(name)) {
		http.NotFound(w, r)
		return
	}
	// O_NONBLOCK so that opening a FIFO cannot hang the handler; non-regular
	// files are refused below.
	f, err := s.root.OpenFile(name, os.O_RDONLY|syscall.O_NONBLOCK, 0)
	switch {
	case os.IsNotExist(err):
		http.NotFound(w, r)
		return
	case os.IsPermission(err):
		http.Error(w, "forbidden", http.StatusForbidden)
		return
	case err != nil:
		// A path the root refuses, such as an escaping symlink or a
		// component that is not a directory, is not a server fault.
		log.Printf("xf: open %q: %v", r.URL.Path, err)
		http.NotFound(w, r)
		return
	}
	defer f.Close()
	fi, err := f.Stat()
	if err != nil {
		log.Printf("xf: stat %q: %v", r.URL.Path, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	if fi.IsDir() {
		// Directory pages need a trailing slash so relative links work.
		if !strings.HasSuffix(r.URL.Path, "/") {
			http.Redirect(w, r, path.Clean(r.URL.Path)+"/", http.StatusMovedPermanently)
			return
		}
		s.list(w, f, r.URL.Path)
		return
	}
	if !fi.Mode().IsRegular() {
		http.Error(w, "unsupported file type", http.StatusForbidden)
		return
	}
	http.ServeContent(w, r, fi.Name(), fi.ModTime(), f)
}

func (s *server) list(w http.ResponseWriter, dir *os.File, urlPath string) {
	dirents, err := dir.ReadDir(-1)
	if err != nil {
		log.Printf("xf: list %q: %v", urlPath, err)
		http.Error(w, "internal server error", http.StatusInternalServerError)
		return
	}
	sort.Slice(dirents, func(i, j int) bool {
		if dirents[i].IsDir() != dirents[j].IsDir() {
			return dirents[i].IsDir()
		}
		return dirents[i].Name() < dirents[j].Name()
	})

	entries := make([]entry, 0, len(dirents))
	for _, de := range dirents {
		if isTempName(de.Name()) {
			continue
		}
		e := entry{
			Name:  de.Name(),
			Href:  (&url.URL{Path: de.Name()}).String(),
			IsDir: de.IsDir(),
		}
		if de.IsDir() {
			e.Href += "/"
			e.Size = "-"
		} else if fi, err := de.Info(); err == nil {
			e.Size = humanSize(fi.Size())
		} else {
			e.Size = "?"
		}
		entries = append(entries, e)
	}

	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	data := pageData{Dir: urlPath, Parent: urlPath != "/", Entries: entries}
	if err := page.Execute(w, data); err != nil {
		log.Printf("xf: render: %v", err)
	}
}

// handleUpload stages every file in the request before renaming any into
// place, so a refused or interrupted upload leaves the directory untouched.
func (s *server) handleUpload(w http.ResponseWriter, r *http.Request) {
	dir, err := s.root.OpenRoot(s.resolve(r.URL.Path))
	if err != nil {
		http.Error(w, "not a directory", http.StatusNotFound)
		return
	}
	defer dir.Close()
	if r.ContentLength > s.maxUpload {
		http.Error(w, "upload too large", http.StatusRequestEntityTooLarge)
		return
	}
	body := &countedBody{ReadCloser: r.Body}
	r.Body = http.MaxBytesReader(w, body, s.maxUpload)
	mr, err := r.MultipartReader()
	if err != nil {
		http.Error(w, "bad upload: "+err.Error(), http.StatusBadRequest)
		return
	}
	var files []stagedFile
	defer func() {
		for _, file := range files {
			dir.Remove(file.temp)
		}
	}()
	names := make(map[string]bool)
	parts := 0
	for {
		part, err := mr.NextPart()
		if err == io.EOF {
			break
		}
		if err != nil {
			writeUploadError(w, err, http.StatusBadRequest, body.bytes > s.maxUpload)
			return
		}
		parts++
		if parts > maxUploadParts {
			http.Error(w, "too many upload parts", http.StatusBadRequest)
			return
		}
		if part.FileName() == "" {
			continue // not a file part
		}
		// The client supplied name is reduced to its base component so an
		// upload can never escape the directory.
		name := filepath.Base(part.FileName())
		if name == "/" || name == "." || name == ".." || isTempName(name) {
			http.Error(w, fmt.Sprintf("invalid filename: %q", part.FileName()),
				http.StatusBadRequest)
			return
		}
		if names[name] {
			http.Error(w, fmt.Sprintf("duplicate filename: %q", name),
				http.StatusBadRequest)
			return
		}
		names[name] = true
		file, err := stage(dir, name, part)
		if err != nil {
			status := http.StatusInternalServerError
			if errors.Is(err, io.ErrUnexpectedEOF) {
				status = http.StatusBadRequest
			}
			writeUploadError(w, err, status, body.bytes > s.maxUpload)
			return
		}
		files = append(files, file)
	}
	if len(files) == 0 {
		http.Error(w, "no files in upload", http.StatusBadRequest)
		return
	}
	for _, file := range files {
		if err := dir.Rename(file.temp, file.name); err != nil {
			log.Printf("xf: commit upload %q: %v", file.name, err)
			http.Error(w, "upload failed", http.StatusInternalServerError)
			return
		}
		access.Printf("upload %q (%d bytes) from %s",
			path.Join(r.URL.Path, file.name), file.size, clientIP(r))
	}
	http.Redirect(w, r, r.URL.Path, http.StatusSeeOther)
}

// stage streams one upload into a temporary file in its target directory, so
// that the commit is an atomic rename.
func stage(dir *os.Root, name string, src io.Reader) (stagedFile, error) {
	mode := os.FileMode(0o600)
	fi, err := dir.Lstat(name)
	switch {
	case os.IsNotExist(err):
	case err != nil:
		return stagedFile{}, err
	case !fi.Mode().IsRegular():
		return stagedFile{}, fmt.Errorf("refusing to replace non-regular file %q", name)
	default:
		mode = fi.Mode().Perm()
	}

	var random [16]byte
	if _, err := rand.Read(random[:]); err != nil {
		return stagedFile{}, err
	}
	tmpName := tempPrefix + hex.EncodeToString(random[:])
	tmp, err := dir.OpenFile(tmpName, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o600)
	if err != nil {
		return stagedFile{}, err
	}

	n, err := io.Copy(tmp, src)
	if err == nil {
		err = tmp.Chmod(mode)
	}
	if cerr := tmp.Close(); err == nil {
		err = cerr
	}
	if err != nil {
		dir.Remove(tmpName)
		return stagedFile{}, err
	}
	return stagedFile{name: name, temp: tmpName, size: n}, nil
}

func isTempName(name string) bool {
	if len(name) != len(tempPrefix)+32 || !strings.HasPrefix(name, tempPrefix) {
		return false
	}
	_, err := hex.DecodeString(strings.TrimPrefix(name, tempPrefix))
	return err == nil
}

func writeUploadError(w http.ResponseWriter, err error, status int, tooLarge bool) {
	switch {
	case tooLarge:
		http.Error(w, "upload too large", http.StatusRequestEntityTooLarge)
	case status == http.StatusInternalServerError:
		log.Printf("xf: upload: %v", err)
		http.Error(w, "upload failed", status)
	default:
		http.Error(w, "bad upload: "+err.Error(), status)
	}
}

// recorder captures the status and size of a response for the access log.
type recorder struct {
	http.ResponseWriter
	status int
	bytes  int64
}

func (rec *recorder) WriteHeader(code int) {
	rec.status = code
	rec.ResponseWriter.WriteHeader(code)
}

func (rec *recorder) Write(p []byte) (int, error) {
	n, err := rec.ResponseWriter.Write(p)
	rec.bytes += int64(n)
	return n, err
}

func clientIP(r *http.Request) string {
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return r.RemoteAddr
	}
	return host
}

func humanSize(n int64) string {
	const unit = 1024
	if n < unit {
		return fmt.Sprintf("%d B", n)
	}
	div, exp := int64(unit), 0
	for x := n / unit; x >= unit; x /= unit {
		div *= unit
		exp++
	}
	return fmt.Sprintf("%.1f %ciB", float64(n)/float64(div), "KMGTPE"[exp])
}

const indexHTML = `<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>xf</title>
<style>
body { font-family: ui-monospace, monospace; max-width: 60rem; margin: 2rem auto; padding: 0 1rem; color: #111; }
h1 { font-size: 1rem; font-weight: normal; color: #555; word-break: break-all; }
form { margin: 1.5rem 0; padding: 1rem; border: 1px solid #ccc; }
table { width: 100%; border-collapse: collapse; }
td { text-align: left; padding: 0.3rem 0.5rem; border-bottom: 1px solid #eee; }
td.size { text-align: right; white-space: nowrap; color: #555; }
a { text-decoration: none; }
a:hover { text-decoration: underline; }
.empty { color: #999; }
</style>
</head>
<body>
<h1>xf: {{.Dir}}</h1>
<form method="post" enctype="multipart/form-data">
<input type="file" name="files" multiple required>
<button type="submit">upload</button>
</form>
<table>
<tbody>
{{if .Parent}}
<tr>
<td><a href="../">../</a></td>
<td class="size">-</td>
</tr>
{{end}}
{{range .Entries}}
<tr>
<td><a href="{{.Href}}">{{.Name}}{{if .IsDir}}/{{end}}</a></td>
<td class="size">{{.Size}}</td>
</tr>
{{else}}
<tr><td class="empty" colspan="2">empty</td></tr>
{{end}}
</tbody>
</table>
</body>
</html>
`