fs.go

Documentation: net/http

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // HTTP file system request handler
  
  package http
  
  import (
  	"errors"
  	"fmt"
  	"io"
  	"mime"
  	"mime/multipart"
  	"net/textproto"
  	"net/url"
  	"os"
  	"path"
  	"path/filepath"
  	"sort"
  	"strconv"
  	"strings"
  	"time"
  )
  
  // A Dir implements FileSystem using the native file system restricted to a
  // specific directory tree.
  //
  // While the FileSystem.Open method takes '/'-separated paths, a Dir's string
  // value is a filename on the native file system, not a URL, so it is separated
  // by filepath.Separator, which isn't necessarily '/'.
  //
  // Note that Dir will allow access to files and directories starting with a
  // period, which could expose sensitive directories like a .git directory or
  // sensitive files like .htpasswd. To exclude files with a leading period,
  // remove the files/directories from the server or create a custom FileSystem
  // implementation.
  //
  // An empty Dir is treated as ".".
  type Dir string
  
  // mapDirOpenError maps the provided non-nil error from opening name
  // to a possibly better non-nil error. In particular, it turns OS-specific errors
  // about opening files in non-directories into os.ErrNotExist. See Issue 18984.
  func mapDirOpenError(originalErr error, name string) error {
  	if os.IsNotExist(originalErr) || os.IsPermission(originalErr) {
  		return originalErr
  	}
  
  	parts := strings.Split(name, string(filepath.Separator))
  	for i := range parts {
  		if parts[i] == "" {
  			continue
  		}
  		fi, err := os.Stat(strings.Join(parts[:i+1], string(filepath.Separator)))
  		if err != nil {
  			return originalErr
  		}
  		if !fi.IsDir() {
  			return os.ErrNotExist
  		}
  	}
  	return originalErr
  }
  
  func (d Dir) Open(name string) (File, error) {
  	if filepath.Separator != '/' && strings.ContainsRune(name, filepath.Separator) {
  		return nil, errors.New("http: invalid character in file path")
  	}
  	dir := string(d)
  	if dir == "" {
  		dir = "."
  	}
  	fullName := filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name)))
  	f, err := os.Open(fullName)
  	if err != nil {
  		return nil, mapDirOpenError(err, fullName)
  	}
  	return f, nil
  }
  
  // A FileSystem implements access to a collection of named files.
  // The elements in a file path are separated by slash ('/', U+002F)
  // characters, regardless of host operating system convention.
  type FileSystem interface {
  	Open(name string) (File, error)
  }
  
  // A File is returned by a FileSystem's Open method and can be
  // served by the FileServer implementation.
  //
  // The methods should behave the same as those on an *os.File.
  type File interface {
  	io.Closer
  	io.Reader
  	io.Seeker
  	Readdir(count int) ([]os.FileInfo, error)
  	Stat() (os.FileInfo, error)
  }
  
  func dirList(w ResponseWriter, r *Request, f File) {
  	dirs, err := f.Readdir(-1)
  	if err != nil {
  		logf(r, "http: error reading directory: %v", err)
  		Error(w, "Error reading directory", StatusInternalServerError)
  		return
  	}
  	sort.Slice(dirs, func(i, j int) bool { return dirs[i].Name() < dirs[j].Name() })
  
  	w.Header().Set("Content-Type", "text/html; charset=utf-8")
  	fmt.Fprintf(w, "<pre>\n")
  	for _, d := range dirs {
  		name := d.Name()
  		if d.IsDir() {
  			name += "/"
  		}
  		// name may contain '?' or '#', which must be escaped to remain
  		// part of the URL path, and not indicate the start of a query
  		// string or fragment.
  		url := url.URL{Path: name}
  		fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
  	}
  	fmt.Fprintf(w, "</pre>\n")
  }
  
  // ServeContent replies to the request using the content in the
  // provided ReadSeeker. The main benefit of ServeContent over io.Copy
  // is that it handles Range requests properly, sets the MIME type, and
  // handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
  // and If-Range requests.
  //
  // If the response's Content-Type header is not set, ServeContent
  // first tries to deduce the type from name's file extension and,
  // if that fails, falls back to reading the first block of the content
  // and passing it to DetectContentType.
  // The name is otherwise unused; in particular it can be empty and is
  // never sent in the response.
  //
  // If modtime is not the zero time or Unix epoch, ServeContent
  // includes it in a Last-Modified header in the response. If the
  // request includes an If-Modified-Since header, ServeContent uses
  // modtime to decide whether the content needs to be sent at all.
  //
  // The content's Seek method must work: ServeContent uses
  // a seek to the end of the content to determine its size.
  //
  // If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
  // ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
  //
  // Note that *os.File implements the io.ReadSeeker interface.
  func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
  	sizeFunc := func() (int64, error) {
  		size, err := content.Seek(0, io.SeekEnd)
  		if err != nil {
  			return 0, errSeeker
  		}
  		_, err = content.Seek(0, io.SeekStart)
  		if err != nil {
  			return 0, errSeeker
  		}
  		return size, nil
  	}
  	serveContent(w, req, name, modtime, sizeFunc, content)
  }
  
  // errSeeker is returned by ServeContent's sizeFunc when the content
  // doesn't seek properly. The underlying Seeker's error text isn't
  // included in the sizeFunc reply so it's not sent over HTTP to end
  // users.
  var errSeeker = errors.New("seeker can't seek")
  
  // errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
  // all of the byte-range-spec values is greater than the content size.
  var errNoOverlap = errors.New("invalid range: failed to overlap")
  
  // if name is empty, filename is unknown. (used for mime type, before sniffing)
  // if modtime.IsZero(), modtime is unknown.
  // content must be seeked to the beginning of the file.
  // The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
  func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
  	setLastModified(w, modtime)
  	done, rangeReq := checkPreconditions(w, r, modtime)
  	if done {
  		return
  	}
  
  	code := StatusOK
  
  	// If Content-Type isn't set, use the file's extension to find it, but
  	// if the Content-Type is unset explicitly, do not sniff the type.
  	ctypes, haveType := w.Header()["Content-Type"]
  	var ctype string
  	if !haveType {
  		ctype = mime.TypeByExtension(filepath.Ext(name))
  		if ctype == "" {
  			// read a chunk to decide between utf-8 text and binary
  			var buf [sniffLen]byte
  			n, _ := io.ReadFull(content, buf[:])
  			ctype = DetectContentType(buf[:n])
  			_, err := content.Seek(0, io.SeekStart) // rewind to output whole file
  			if err != nil {
  				Error(w, "seeker can't seek", StatusInternalServerError)
  				return
  			}
  		}
  		w.Header().Set("Content-Type", ctype)
  	} else if len(ctypes) > 0 {
  		ctype = ctypes[0]
  	}
  
  	size, err := sizeFunc()
  	if err != nil {
  		Error(w, err.Error(), StatusInternalServerError)
  		return
  	}
  
  	// handle Content-Range header.
  	sendSize := size
  	var sendContent io.Reader = content
  	if size >= 0 {
  		ranges, err := parseRange(rangeReq, size)
  		if err != nil {
  			if err == errNoOverlap {
  				w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
  			}
  			Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
  			return
  		}
  		if sumRangesSize(ranges) > size {
  			// The total number of bytes in all the ranges
  			// is larger than the size of the file by
  			// itself, so this is probably an attack, or a
  			// dumb client. Ignore the range request.
  			ranges = nil
  		}
  		switch {
  		case len(ranges) == 1:
  			// RFC 2616, Section 14.16:
  			// "When an HTTP message includes the content of a single
  			// range (for example, a response to a request for a
  			// single range, or to a request for a set of ranges
  			// that overlap without any holes), this content is
  			// transmitted with a Content-Range header, and a
  			// Content-Length header showing the number of bytes
  			// actually transferred.
  			// ...
  			// A response to a request for a single range MUST NOT
  			// be sent using the multipart/byteranges media type."
  			ra := ranges[0]
  			if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
  				Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
  				return
  			}
  			sendSize = ra.length
  			code = StatusPartialContent
  			w.Header().Set("Content-Range", ra.contentRange(size))
  		case len(ranges) > 1:
  			sendSize = rangesMIMESize(ranges, ctype, size)
  			code = StatusPartialContent
  
  			pr, pw := io.Pipe()
  			mw := multipart.NewWriter(pw)
  			w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
  			sendContent = pr
  			defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
  			go func() {
  				for _, ra := range ranges {
  					part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
  					if err != nil {
  						pw.CloseWithError(err)
  						return
  					}
  					if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
  						pw.CloseWithError(err)
  						return
  					}
  					if _, err := io.CopyN(part, content, ra.length); err != nil {
  						pw.CloseWithError(err)
  						return
  					}
  				}
  				mw.Close()
  				pw.Close()
  			}()
  		}
  
  		w.Header().Set("Accept-Ranges", "bytes")
  		if w.Header().Get("Content-Encoding") == "" {
  			w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
  		}
  	}
  
  	w.WriteHeader(code)
  
  	if r.Method != "HEAD" {
  		io.CopyN(w, sendContent, sendSize)
  	}
  }
  
  // scanETag determines if a syntactically valid ETag is present at s. If so,
  // the ETag and remaining text after consuming ETag is returned. Otherwise,
  // it returns "", "".
  func scanETag(s string) (etag string, remain string) {
  	s = textproto.TrimString(s)
  	start := 0
  	if strings.HasPrefix(s, "W/") {
  		start = 2
  	}
  	if len(s[start:]) < 2 || s[start] != '"' {
  		return "", ""
  	}
  	// ETag is either W/"text" or "text".
  	// See RFC 7232 2.3.
  	for i := start + 1; i < len(s); i++ {
  		c := s[i]
  		switch {
  		// Character values allowed in ETags.
  		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
  		case c == '"':
  			return s[:i+1], s[i+1:]
  		default:
  			return "", ""
  		}
  	}
  	return "", ""
  }
  
  // etagStrongMatch reports whether a and b match using strong ETag comparison.
  // Assumes a and b are valid ETags.
  func etagStrongMatch(a, b string) bool {
  	return a == b && a != "" && a[0] == '"'
  }
  
  // etagWeakMatch reports whether a and b match using weak ETag comparison.
  // Assumes a and b are valid ETags.
  func etagWeakMatch(a, b string) bool {
  	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
  }
  
  // condResult is the result of an HTTP request precondition check.
  // See https://tools.ietf.org/html/rfc7232 section 3.
  type condResult int
  
  const (
  	condNone condResult = iota
  	condTrue
  	condFalse
  )
  
  func checkIfMatch(w ResponseWriter, r *Request) condResult {
  	im := r.Header.Get("If-Match")
  	if im == "" {
  		return condNone
  	}
  	for {
  		im = textproto.TrimString(im)
  		if len(im) == 0 {
  			break
  		}
  		if im[0] == ',' {
  			im = im[1:]
  			continue
  		}
  		if im[0] == '*' {
  			return condTrue
  		}
  		etag, remain := scanETag(im)
  		if etag == "" {
  			break
  		}
  		if etagStrongMatch(etag, w.Header().get("Etag")) {
  			return condTrue
  		}
  		im = remain
  	}
  
  	return condFalse
  }
  
  func checkIfUnmodifiedSince(r *Request, modtime time.Time) condResult {
  	ius := r.Header.Get("If-Unmodified-Since")
  	if ius == "" || isZeroTime(modtime) {
  		return condNone
  	}
  	if t, err := ParseTime(ius); err == nil {
  		// The Date-Modified header truncates sub-second precision, so
  		// use mtime < t+1s instead of mtime <= t to check for unmodified.
  		if modtime.Before(t.Add(1 * time.Second)) {
  			return condTrue
  		}
  		return condFalse
  	}
  	return condNone
  }
  
  func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
  	inm := r.Header.get("If-None-Match")
  	if inm == "" {
  		return condNone
  	}
  	buf := inm
  	for {
  		buf = textproto.TrimString(buf)
  		if len(buf) == 0 {
  			break
  		}
  		if buf[0] == ',' {
  			buf = buf[1:]
  		}
  		if buf[0] == '*' {
  			return condFalse
  		}
  		etag, remain := scanETag(buf)
  		if etag == "" {
  			break
  		}
  		if etagWeakMatch(etag, w.Header().get("Etag")) {
  			return condFalse
  		}
  		buf = remain
  	}
  	return condTrue
  }
  
  func checkIfModifiedSince(r *Request, modtime time.Time) condResult {
  	if r.Method != "GET" && r.Method != "HEAD" {
  		return condNone
  	}
  	ims := r.Header.Get("If-Modified-Since")
  	if ims == "" || isZeroTime(modtime) {
  		return condNone
  	}
  	t, err := ParseTime(ims)
  	if err != nil {
  		return condNone
  	}
  	// The Date-Modified header truncates sub-second precision, so
  	// use mtime < t+1s instead of mtime <= t to check for unmodified.
  	if modtime.Before(t.Add(1 * time.Second)) {
  		return condFalse
  	}
  	return condTrue
  }
  
  func checkIfRange(w ResponseWriter, r *Request, modtime time.Time) condResult {
  	if r.Method != "GET" && r.Method != "HEAD" {
  		return condNone
  	}
  	ir := r.Header.get("If-Range")
  	if ir == "" {
  		return condNone
  	}
  	etag, _ := scanETag(ir)
  	if etag != "" {
  		if etagStrongMatch(etag, w.Header().Get("Etag")) {
  			return condTrue
  		} else {
  			return condFalse
  		}
  	}
  	// The If-Range value is typically the ETag value, but it may also be
  	// the modtime date. See golang.org/issue/8367.
  	if modtime.IsZero() {
  		return condFalse
  	}
  	t, err := ParseTime(ir)
  	if err != nil {
  		return condFalse
  	}
  	if t.Unix() == modtime.Unix() {
  		return condTrue
  	}
  	return condFalse
  }
  
  var unixEpochTime = time.Unix(0, 0)
  
  // isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
  func isZeroTime(t time.Time) bool {
  	return t.IsZero() || t.Equal(unixEpochTime)
  }
  
  func setLastModified(w ResponseWriter, modtime time.Time) {
  	if !isZeroTime(modtime) {
  		w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
  	}
  }
  
  func writeNotModified(w ResponseWriter) {
  	// RFC 7232 section 4.1:
  	// a sender SHOULD NOT generate representation metadata other than the
  	// above listed fields unless said metadata exists for the purpose of
  	// guiding cache updates (e.g., Last-Modified might be useful if the
  	// response does not have an ETag field).
  	h := w.Header()
  	delete(h, "Content-Type")
  	delete(h, "Content-Length")
  	if h.Get("Etag") != "" {
  		delete(h, "Last-Modified")
  	}
  	w.WriteHeader(StatusNotModified)
  }
  
  // checkPreconditions evaluates request preconditions and reports whether a precondition
  // resulted in sending StatusNotModified or StatusPreconditionFailed.
  func checkPreconditions(w ResponseWriter, r *Request, modtime time.Time) (done bool, rangeHeader string) {
  	// This function carefully follows RFC 7232 section 6.
  	ch := checkIfMatch(w, r)
  	if ch == condNone {
  		ch = checkIfUnmodifiedSince(r, modtime)
  	}
  	if ch == condFalse {
  		w.WriteHeader(StatusPreconditionFailed)
  		return true, ""
  	}
  	switch checkIfNoneMatch(w, r) {
  	case condFalse:
  		if r.Method == "GET" || r.Method == "HEAD" {
  			writeNotModified(w)
  			return true, ""
  		} else {
  			w.WriteHeader(StatusPreconditionFailed)
  			return true, ""
  		}
  	case condNone:
  		if checkIfModifiedSince(r, modtime) == condFalse {
  			writeNotModified(w)
  			return true, ""
  		}
  	}
  
  	rangeHeader = r.Header.get("Range")
  	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
  		rangeHeader = ""
  	}
  	return false, rangeHeader
  }
  
  // name is '/'-separated, not filepath.Separator.
  func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
  	const indexPage = "/index.html"
  
  	// redirect .../index.html to .../
  	// can't use Redirect() because that would make the path absolute,
  	// which would be a problem running under StripPrefix
  	if strings.HasSuffix(r.URL.Path, indexPage) {
  		localRedirect(w, r, "./")
  		return
  	}
  
  	f, err := fs.Open(name)
  	if err != nil {
  		msg, code := toHTTPError(err)
  		Error(w, msg, code)
  		return
  	}
  	defer f.Close()
  
  	d, err := f.Stat()
  	if err != nil {
  		msg, code := toHTTPError(err)
  		Error(w, msg, code)
  		return
  	}
  
  	if redirect {
  		// redirect to canonical path: / at end of directory url
  		// r.URL.Path always begins with /
  		url := r.URL.Path
  		if d.IsDir() {
  			if url[len(url)-1] != '/' {
  				localRedirect(w, r, path.Base(url)+"/")
  				return
  			}
  		} else {
  			if url[len(url)-1] == '/' {
  				localRedirect(w, r, "../"+path.Base(url))
  				return
  			}
  		}
  	}
  
  	// redirect if the directory name doesn't end in a slash
  	if d.IsDir() {
  		url := r.URL.Path
  		if url[len(url)-1] != '/' {
  			localRedirect(w, r, path.Base(url)+"/")
  			return
  		}
  	}
  
  	// use contents of index.html for directory, if present
  	if d.IsDir() {
  		index := strings.TrimSuffix(name, "/") + indexPage
  		ff, err := fs.Open(index)
  		if err == nil {
  			defer ff.Close()
  			dd, err := ff.Stat()
  			if err == nil {
  				name = index
  				d = dd
  				f = ff
  			}
  		}
  	}
  
  	// Still a directory? (we didn't find an index.html file)
  	if d.IsDir() {
  		if checkIfModifiedSince(r, d.ModTime()) == condFalse {
  			writeNotModified(w)
  			return
  		}
  		w.Header().Set("Last-Modified", d.ModTime().UTC().Format(TimeFormat))
  		dirList(w, r, f)
  		return
  	}
  
  	// serveContent will check modification time
  	sizeFunc := func() (int64, error) { return d.Size(), nil }
  	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
  }
  
  // toHTTPError returns a non-specific HTTP error message and status code
  // for a given non-nil error value. It's important that toHTTPError does not
  // actually return err.Error(), since msg and httpStatus are returned to users,
  // and historically Go's ServeContent always returned just "404 Not Found" for
  // all errors. We don't want to start leaking information in error messages.
  func toHTTPError(err error) (msg string, httpStatus int) {
  	if os.IsNotExist(err) {
  		return "404 page not found", StatusNotFound
  	}
  	if os.IsPermission(err) {
  		return "403 Forbidden", StatusForbidden
  	}
  	// Default:
  	return "500 Internal Server Error", StatusInternalServerError
  }
  
  // localRedirect gives a Moved Permanently response.
  // It does not convert relative paths to absolute paths like Redirect does.
  func localRedirect(w ResponseWriter, r *Request, newPath string) {
  	if q := r.URL.RawQuery; q != "" {
  		newPath += "?" + q
  	}
  	w.Header().Set("Location", newPath)
  	w.WriteHeader(StatusMovedPermanently)
  }
  
  // ServeFile replies to the request with the contents of the named
  // file or directory.
  //
  // If the provided file or directory name is a relative path, it is
  // interpreted relative to the current directory and may ascend to parent
  // directories. If the provided name is constructed from user input, it
  // should be sanitized before calling ServeFile. As a precaution, ServeFile
  // will reject requests where r.URL.Path contains a ".." path element.
  //
  // As a special case, ServeFile redirects any request where r.URL.Path
  // ends in "/index.html" to the same path, without the final
  // "index.html". To avoid such redirects either modify the path or
  // use ServeContent.
  func ServeFile(w ResponseWriter, r *Request, name string) {
  	if containsDotDot(r.URL.Path) {
  		// Too many programs use r.URL.Path to construct the argument to
  		// serveFile. Reject the request under the assumption that happened
  		// here and ".." may not be wanted.
  		// Note that name might not contain "..", for example if code (still
  		// incorrectly) used filepath.Join(myDir, r.URL.Path).
  		Error(w, "invalid URL path", StatusBadRequest)
  		return
  	}
  	dir, file := filepath.Split(name)
  	serveFile(w, r, Dir(dir), file, false)
  }
  
  func containsDotDot(v string) bool {
  	if !strings.Contains(v, "..") {
  		return false
  	}
  	for _, ent := range strings.FieldsFunc(v, isSlashRune) {
  		if ent == ".." {
  			return true
  		}
  	}
  	return false
  }
  
  func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
  
  type fileHandler struct {
  	root FileSystem
  }
  
  // FileServer returns a handler that serves HTTP requests
  // with the contents of the file system rooted at root.
  //
  // To use the operating system's file system implementation,
  // use http.Dir:
  //
  //     http.Handle("/", http.FileServer(http.Dir("/tmp")))
  //
  // As a special case, the returned file server redirects any request
  // ending in "/index.html" to the same path, without the final
  // "index.html".
  func FileServer(root FileSystem) Handler {
  	return &fileHandler{root}
  }
  
  func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
  	upath := r.URL.Path
  	if !strings.HasPrefix(upath, "/") {
  		upath = "/" + upath
  		r.URL.Path = upath
  	}
  	serveFile(w, r, f.root, path.Clean(upath), true)
  }
  
  // httpRange specifies the byte range to be sent to the client.
  type httpRange struct {
  	start, length int64
  }
  
  func (r httpRange) contentRange(size int64) string {
  	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
  }
  
  func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
  	return textproto.MIMEHeader{
  		"Content-Range": {r.contentRange(size)},
  		"Content-Type":  {contentType},
  	}
  }
  
  // parseRange parses a Range header string as per RFC 2616.
  // errNoOverlap is returned if none of the ranges overlap.
  func parseRange(s string, size int64) ([]httpRange, error) {
  	if s == "" {
  		return nil, nil // header not present
  	}
  	const b = "bytes="
  	if !strings.HasPrefix(s, b) {
  		return nil, errors.New("invalid range")
  	}
  	var ranges []httpRange
  	noOverlap := false
  	for _, ra := range strings.Split(s[len(b):], ",") {
  		ra = strings.TrimSpace(ra)
  		if ra == "" {
  			continue
  		}
  		i := strings.Index(ra, "-")
  		if i < 0 {
  			return nil, errors.New("invalid range")
  		}
  		start, end := strings.TrimSpace(ra[:i]), strings.TrimSpace(ra[i+1:])
  		var r httpRange
  		if start == "" {
  			// If no start is specified, end specifies the
  			// range start relative to the end of the file.
  			i, err := strconv.ParseInt(end, 10, 64)
  			if err != nil {
  				return nil, errors.New("invalid range")
  			}
  			if i > size {
  				i = size
  			}
  			r.start = size - i
  			r.length = size - r.start
  		} else {
  			i, err := strconv.ParseInt(start, 10, 64)
  			if err != nil || i < 0 {
  				return nil, errors.New("invalid range")
  			}
  			if i >= size {
  				// If the range begins after the size of the content,
  				// then it does not overlap.
  				noOverlap = true
  				continue
  			}
  			r.start = i
  			if end == "" {
  				// If no end is specified, range extends to end of the file.
  				r.length = size - r.start
  			} else {
  				i, err := strconv.ParseInt(end, 10, 64)
  				if err != nil || r.start > i {
  					return nil, errors.New("invalid range")
  				}
  				if i >= size {
  					i = size - 1
  				}
  				r.length = i - r.start + 1
  			}
  		}
  		ranges = append(ranges, r)
  	}
  	if noOverlap && len(ranges) == 0 {
  		// The specified ranges did not overlap with the content.
  		return nil, errNoOverlap
  	}
  	return ranges, nil
  }
  
  // countingWriter counts how many bytes have been written to it.
  type countingWriter int64
  
  func (w *countingWriter) Write(p []byte) (n int, err error) {
  	*w += countingWriter(len(p))
  	return len(p), nil
  }
  
  // rangesMIMESize returns the number of bytes it takes to encode the
  // provided ranges as a multipart response.
  func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
  	var w countingWriter
  	mw := multipart.NewWriter(&w)
  	for _, ra := range ranges {
  		mw.CreatePart(ra.mimeHeader(contentType, contentSize))
  		encSize += ra.length
  	}
  	mw.Close()
  	encSize += int64(w)
  	return
  }
  
  func sumRangesSize(ranges []httpRange) (size int64) {
  	for _, ra := range ranges {
  		size += ra.length
  	}
  	return
  }
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