Tiny web server in Go for sharing a folder

webserver.go

/* Tiny web server in Golang for sharing a folder
Copyright (c) 2010 Alexis ROBERT <alexis.robert@gmail.com>

Contains some code from Golang's http.ServeFile method, and
uses lighttpd's directory listing HTML template. */

package main

import (
	"compress/gzip"
	"compress/zlib"
	"container/list"
	"flag"
	"fmt"
	"html/template"
	"io"
	"io/fs"
	"mime"
	"net/http"
	"net/url"
	"os"
	"path"
	"strconv"
	"strings"
	"time"
)

var (
	root_folder *string // TODO: Find a way to be cleaner !
	uses_gzip   *bool
)

const (
	serverUA      = "Alexis/0.1"
	fs_maxbufsize = 4096 // 4096 bits = default page size on OSX
)

/* Go is the first programming language with a templating engine embeddeed
 * but with no min function. */
func min(x int64, y int64) int64 {
	if x < y {
		return x
	}
	return y
}

func main() {
	// Get current working directory to get the file from it
	cwd, err := os.Getwd()
	if err != nil {
		fmt.Printf("Error while getting current directory.")
		return
	}

	// Command line parsing
	bind := flag.String("bind", ":8080", "Bind address")
	root_folder = flag.String("root", cwd, "Root folder")
	// TODO: investigate why gzip isn't working properly
	// setting this to false
	uses_gzip = flag.Bool("gzip", false, "Enables gzip/zlib compression")

	flag.Parse()

	http.Handle("/", http.HandlerFunc(handleFile))

	fmt.Printf("Sharing %s on %s ...\n", *root_folder, *bind)
	http.ListenAndServe((*bind), nil)
}

// Manages directory listings
type DirListing struct {
	Name           string
	Children_dir   []string
	Children_files []string
	ServerUA       string
}

func copyToArray(src *list.List) []string {
	dst := make([]string, src.Len())

	i := 0
	for e := src.Front(); e != nil; e = e.Next() {
		dst[i] = e.Value.(string)
		i = i + 1
	}

	return dst
}

func handleDirectory(f *os.File, w http.ResponseWriter, req *http.Request) {
	names, _ := f.Readdir(-1)

	// First, check if there is any index in this folder.
	for _, val := range names {
		if val.Name() == "index.html" {
			serveFile(path.Join(f.Name(), "index.html"), w, req)
			return
		}
	}

	// Otherwise, generate folder content.
	children_dir_tmp := list.New()
	children_files_tmp := list.New()

	for _, val := range names {
		if val.Name()[0] == '.' {
			continue
		} // Remove hidden files from listing

		if val.IsDir() {
			children_dir_tmp.PushBack(val.Name())
		} else {
			children_files_tmp.PushBack(val.Name())
		}
	}

	// And transfer the content to the final array structure
	children_dir := copyToArray(children_dir_tmp)
	children_files := copyToArray(children_files_tmp)

	tmpl := template.Must(template.New("tpl").Parse(dirlisting_tpl))

	data := DirListing{
		Name: req.URL.Path,
		Children_dir: children_dir, 
		Children_files: children_files,
		ServerUA: serverUA,
	}
	
	err := tmpl.Execute(w, data)
	if err != nil {
		http.Error(w, "500 Internal Error: Error executing the template.", 500)
	}
}

func serveFile(filepath string, w http.ResponseWriter, req *http.Request) {
	// Opening the file handle
	f, err := os.Open(filepath)
	if err != nil {
		http.Error(w, "404 Not Found : Error while opening the file.", 404)
		return
	}

	defer f.Close()

	// Checking if the opened handle is really a file
	statinfo, err := f.Stat()
	if err != nil {
		http.Error(w, "500 Internal Error : stat() failure.", 500)
		return
	}

	if statinfo.IsDir() { // If it's a directory, open it !
		handleDirectory(f, w, req)
		return
	}

	if statinfo.Mode().Type() == fs.ModeSocket { // If it's a socket, forbid it !
		http.Error(w, "403 Forbidden : you can't access this resource.", 403)
		return
	}

	// Manages If-Modified-Since and add Last-Modified (taken from Golang code)
	if t, _ := time.Parse(http.TimeFormat, req.Header.Get("If-Modified-Since")); !t.IsZero() && t.Equal(time.Unix(0, 0)) {
		w.WriteHeader(http.StatusNotModified)
		return
	}
	w.Header().Set("Last-Modified", statinfo.ModTime().Format(http.TimeFormat))

	// Content-Type handling
	query, err := url.ParseQuery(req.URL.RawQuery)

	if err == nil && len(query["dl"]) > 0 { // The user explicitedly wanted to download the file (Dropbox style!)
		w.Header().Set("Content-Type", "application/octet-stream")
	} else {
		// Fetching file's mimetype and giving it to the browser
		if mimetype := mime.TypeByExtension(path.Ext(filepath)); mimetype != "" {
			w.Header().Set("Content-Type", mimetype)
		} else {
			w.Header().Set("Content-Type", "application/octet-stream")
		}
	}

	// Add Content-Length
	w.Header().Set("Content-Length", strconv.FormatInt(statinfo.Size(), 10))

	// Manage Content-Range (TODO: Manage end byte and multiple Content-Range)
	if req.Header.Get("Range") != "" {
		start_byte := parseRange(req.Header.Get("Range"))

		if start_byte < statinfo.Size() {
			f.Seek(start_byte, 0)
		} else {
			start_byte = 0
		}

		w.Header().Set("Content-Range",
			fmt.Sprintf("bytes %d-%d/%d", start_byte, statinfo.Size()-1, statinfo.Size()))
	}

	// Manage gzip/zlib compression
	output_writer := w.(io.Writer)

	if (*uses_gzip) == true && req.Header.Get("Accept-Encoding") != "" {
		encodings := parseCSV(req.Header.Get("Accept-Encoding"))

		for _, val := range encodings {
			if val == "gzip" {
				w.Header().Set("Accept-Encoding", "gzip")
				output_writer, _ = gzip.NewWriterLevel(w, gzip.BestSpeed)

				break
			} else if val == "deflate" {
				w.Header().Set("Accept-Encoding", "deflate")
				output_writer, _ = zlib.NewWriterLevel(w, zlib.BestSpeed)

				break
			}
		}
	}

	// Stream data out !
	buf := make([]byte, min(fs_maxbufsize, statinfo.Size()))
	n := 0
	for err == nil {
		n, err = f.Read(buf)
		output_writer.Write(buf[0:n])
	}

	// Closes current compressors
	switch output_writer.(type) {
	case *gzip.Writer:
		output_writer.(*gzip.Writer).Close()
	case io.WriteCloser:
		output_writer.(io.WriteCloser).Close()
	}

	f.Close()
}

func handleFile(w http.ResponseWriter, req *http.Request) {
	w.Header().Set("Server", serverUA)

	filepath := path.Join((*root_folder), path.Clean(req.URL.Path))
	serveFile(filepath, w, req)

	fmt.Printf("\"%s %s %s\"\n",
		req.Method,
		req.URL,
		req.Proto)
		/*,
		req.Referer,
		req.UserAgent) // TODO: Improve this crappy logging
		*/
}

func parseCSV(data string) []string {
	splitted := strings.Split(data, ",")

	data_tmp := make([]string, len(splitted))

	for i, val := range splitted {
		data_tmp[i] = strings.TrimSpace(val)
	}

	return data_tmp
}

func parseRange(data string) int64 {
	stop := (int64)(0)
	part := 0
	for i := 0; i < len(data) && part < 2; i = i + 1 {
		if part == 0 { // part = 0 <=> equal isn't met.
			if data[i] == '=' {
				part = 1
			}

			continue
		}

		if part == 1 { // part = 1 <=> we've met the equal, parse beginning
			if data[i] == ',' || data[i] == '-' {
				part = 2 // part = 2 <=> OK DUDE.
			} else {
				if 48 <= data[i] && data[i] <= 57 { // If it's a digit ...
					// ... convert the char to integer and add it!
					stop = (stop * 10) + (((int64)(data[i])) - 48)
				} else {
					part = 2 // Parsing error! No error needed : 0 = from start.
				}
			}
		}
	}

	return stop
}

var dirlisting_tpl string = `
<title>Index of {{.Name}}</title>
<style type="text/css">
a, a:active {text-decoration: none; color: blue;}
a:visited {color: #48468F;}
a:hover, a:focus {text-decoration: underline; color: red;}
body {background-color: #F5F5F5;}
h2 {margin-bottom: 12px;}
table {margin-left: 12px;}
th, td { font: 14px monospace; text-align: left;}
th { font-weight: bold; padding-right: 14px; padding-bottom: 3px;}
td {padding-right: 14px;}
td.s, th.s {text-align: right;}
div.list { background-color: white; border-top: 1px solid #646464; border-bottom: 1px solid #646464; padding-top: 10px; padding-bottom: 14px;}
div.foot { font: 14px monospace; color: #787878; padding-top: 4px;}
</style>
</head>
<body>
<h2>Index of {{.Name}}</h2>
<div class="list">
<table summary="Directory Listing" cellpadding="0" cellspacing="0">
<thead><tr><th class="n">Name</th><th class="t">Type</th><th class="dl">Options</th></tr></thead>
<tbody>
<tr><td class="n"><a href="../">Parent Directory</a>/</td><td class="t">Directory</td><td class="dl"></td></tr>
{{range .Children_dir}}
<tr><td class="n"><a href="{{.}}/">{{.}}/</a></td><td class="t">Directory</td><td class="dl"></td></tr>
{{end}}
{{range .Children_files}}
<tr><td class="n"><a href="{{.}}">{{.}}</a></td><td class="t">&nbsp;</td><td class="dl"><a href="{{.}}?dl">Download</a></td></tr>
{{end}}
</tbody>
</table>
</div>
<div class="foot">{{.ServerUA}}</div>
</body>
</html>
`