libgo/go/net/http/fs.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // HTTP file system request handler
   6
   7 package http
   8
   9 import (
  10         "errors"
  11         "fmt"
  12         "io"
  13         "mime"
  14         "mime/multipart"
  15         "net/textproto"
  16         "net/url"
  17         "os"
  18         "path"
  19         "path/filepath"
  20         "sort"
  21         "strconv"
  22         "strings"
  23         "time"
  24 )
  25
  26 // A Dir implements FileSystem using the native file system restricted to a
  27 // specific directory tree.
  28 //
  29 // While the FileSystem.Open method takes '/'-separated paths, a Dir's string
  30 // value is a filename on the native file system, not a URL, so it is separated
  31 // by filepath.Separator, which isn't necessarily '/'.
  32 //
  33 // Note that Dir will allow access to files and directories starting with a
  34 // period, which could expose sensitive directories like a .git directory or
  35 // sensitive files like .htpasswd. To exclude files with a leading period,
  36 // remove the files/directories from the server or create a custom FileSystem
  37 // implementation.
  38 //
  39 // An empty Dir is treated as ".".
  40 type Dir string
  41
  42 // mapDirOpenError maps the provided non-nil error from opening name
  43 // to a possibly better non-nil error. In particular, it turns OS-specific errors
  44 // about opening files in non-directories into os.ErrNotExist. See Issue 18984.
  45 func mapDirOpenError(originalErr error, name string) error {
  46         if os.IsNotExist(originalErr) || os.IsPermission(originalErr) {
  47                 return originalErr
  48         }
  49
  50         parts := strings.Split(name, string(filepath.Separator))
  51         for i := range parts {
  52                 if parts[i] == "" {
  53                         continue
  54                 }
  55                 fi, err := os.Stat(strings.Join(parts[:i+1], string(filepath.Separator)))
  56                 if err != nil {
  57                         return originalErr
  58                 }
  59                 if !fi.IsDir() {
  60                         return os.ErrNotExist
  61                 }
  62         }
  63         return originalErr
  64 }
  65
  66 // Open implements FileSystem using os.Open, opening files for reading rooted
  67 // and relative to the directory d.
  68 func (d Dir) Open(name string) (File, error) {
  69         if filepath.Separator != '/' && strings.ContainsRune(name, filepath.Separator) {
  70                 return nil, errors.New("http: invalid character in file path")
  71         }
  72         dir := string(d)
  73         if dir == "" {
  74                 dir = "."
  75         }
  76         fullName := filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name)))
  77         f, err := os.Open(fullName)
  78         if err != nil {
  79                 return nil, mapDirOpenError(err, fullName)
  80         }
  81         return f, nil
  82 }
  83
  84 // A FileSystem implements access to a collection of named files.
  85 // The elements in a file path are separated by slash ('/', U+002F)
  86 // characters, regardless of host operating system convention.
  87 type FileSystem interface {
  88         Open(name string) (File, error)
  89 }
  90
  91 // A File is returned by a FileSystem's Open method and can be
  92 // served by the FileServer implementation.
  93 //
  94 // The methods should behave the same as those on an *os.File.
  95 type File interface {
  96         io.Closer
  97         io.Reader
  98         io.Seeker
  99         Readdir(count int) ([]os.FileInfo, error)
 100         Stat() (os.FileInfo, error)
 101 }
 102
 103 func dirList(w ResponseWriter, r *Request, f File) {
 104         dirs, err := f.Readdir(-1)
 105         if err != nil {
 106                 logf(r, "http: error reading directory: %v", err)
 107                 Error(w, "Error reading directory", StatusInternalServerError)
 108                 return
 109         }
 110         sort.Slice(dirs, func(i, j int) bool { return dirs[i].Name() < dirs[j].Name() })
 111
 112         w.Header().Set("Content-Type", "text/html; charset=utf-8")
 113         fmt.Fprintf(w, "<pre>\n")
 114         for _, d := range dirs {
 115                 name := d.Name()
 116                 if d.IsDir() {
 117                         name += "/"
 118                 }
 119                 // name may contain '?' or '#', which must be escaped to remain
 120                 // part of the URL path, and not indicate the start of a query
 121                 // string or fragment.
 122                 url := url.URL{Path: name}
 123                 fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
 124         }
 125         fmt.Fprintf(w, "</pre>\n")
 126 }
 127
 128 // ServeContent replies to the request using the content in the
 129 // provided ReadSeeker. The main benefit of ServeContent over io.Copy
 130 // is that it handles Range requests properly, sets the MIME type, and
 131 // handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
 132 // and If-Range requests.
 133 //
 134 // If the response's Content-Type header is not set, ServeContent
 135 // first tries to deduce the type from name's file extension and,
 136 // if that fails, falls back to reading the first block of the content
 137 // and passing it to DetectContentType.
 138 // The name is otherwise unused; in particular it can be empty and is
 139 // never sent in the response.
 140 //
 141 // If modtime is not the zero time or Unix epoch, ServeContent
 142 // includes it in a Last-Modified header in the response. If the
 143 // request includes an If-Modified-Since header, ServeContent uses
 144 // modtime to decide whether the content needs to be sent at all.
 145 //
 146 // The content's Seek method must work: ServeContent uses
 147 // a seek to the end of the content to determine its size.
 148 //
 149 // If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
 150 // ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
 151 //
 152 // Note that *os.File implements the io.ReadSeeker interface.
 153 func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
 154         sizeFunc := func() (int64, error) {
 155                 size, err := content.Seek(0, io.SeekEnd)
 156                 if err != nil {
 157                         return 0, errSeeker
 158                 }
 159                 _, err = content.Seek(0, io.SeekStart)
 160                 if err != nil {
 161                         return 0, errSeeker
 162                 }
 163                 return size, nil
 164         }
 165         serveContent(w, req, name, modtime, sizeFunc, content)
 166 }
 167
 168 // errSeeker is returned by ServeContent's sizeFunc when the content
 169 // doesn't seek properly. The underlying Seeker's error text isn't
 170 // included in the sizeFunc reply so it's not sent over HTTP to end
 171 // users.
 172 var errSeeker = errors.New("seeker can't seek")
 173
 174 // errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
 175 // all of the byte-range-spec values is greater than the content size.
 176 var errNoOverlap = errors.New("invalid range: failed to overlap")
 177
 178 // if name is empty, filename is unknown. (used for mime type, before sniffing)
 179 // if modtime.IsZero(), modtime is unknown.
 180 // content must be seeked to the beginning of the file.
 181 // The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
 182 func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
 183         setLastModified(w, modtime)
 184         done, rangeReq := checkPreconditions(w, r, modtime)
 185         if done {
 186                 return
 187         }
 188
 189         code := StatusOK
 190
 191         // If Content-Type isn't set, use the file's extension to find it, but
 192         // if the Content-Type is unset explicitly, do not sniff the type.
 193         ctypes, haveType := w.Header()["Content-Type"]
 194         var ctype string
 195         if !haveType {
 196                 ctype = mime.TypeByExtension(filepath.Ext(name))
 197                 if ctype == "" {
 198                         // read a chunk to decide between utf-8 text and binary
 199                         var buf [sniffLen]byte
 200                         n, _ := io.ReadFull(content, buf[:])
 201                         ctype = DetectContentType(buf[:n])
 202                         _, err := content.Seek(0, io.SeekStart) // rewind to output whole file
 203                         if err != nil {
 204                                 Error(w, "seeker can't seek", StatusInternalServerError)
 205                                 return
 206                         }
 207                 }
 208                 w.Header().Set("Content-Type", ctype)
 209         } else if len(ctypes) > 0 {
 210                 ctype = ctypes[0]
 211         }
 212
 213         size, err := sizeFunc()
 214         if err != nil {
 215                 Error(w, err.Error(), StatusInternalServerError)
 216                 return
 217         }
 218
 219         // handle Content-Range header.
 220         sendSize := size
 221         var sendContent io.Reader = content
 222         if size >= 0 {
 223                 ranges, err := parseRange(rangeReq, size)
 224                 if err != nil {
 225                         if err == errNoOverlap {
 226                                 w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
 227                         }
 228                         Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
 229                         return
 230                 }
 231                 if sumRangesSize(ranges) > size {
 232                         // The total number of bytes in all the ranges
 233                         // is larger than the size of the file by
 234                         // itself, so this is probably an attack, or a
 235                         // dumb client. Ignore the range request.
 236                         ranges = nil
 237                 }
 238                 switch {
 239                 case len(ranges) == 1:
 240                         // RFC 7233, Section 4.1:
 241                         // "If a single part is being transferred, the server
 242                         // generating the 206 response MUST generate a
 243                         // Content-Range header field, describing what range
 244                         // of the selected representation is enclosed, and a
 245                         // payload consisting of the range.
 246                         // ...
 247                         // A server MUST NOT generate a multipart response to
 248                         // a request for a single range, since a client that
 249                         // does not request multiple parts might not support
 250                         // multipart responses."
 251                         ra := ranges[0]
 252                         if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
 253                                 Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
 254                                 return
 255                         }
 256                         sendSize = ra.length
 257                         code = StatusPartialContent
 258                         w.Header().Set("Content-Range", ra.contentRange(size))
 259                 case len(ranges) > 1:
 260                         sendSize = rangesMIMESize(ranges, ctype, size)
 261                         code = StatusPartialContent
 262
 263                         pr, pw := io.Pipe()
 264                         mw := multipart.NewWriter(pw)
 265                         w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
 266                         sendContent = pr
 267                         defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
 268                         go func() {
 269                                 for _, ra := range ranges {
 270                                         part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
 271                                         if err != nil {
 272                                                 pw.CloseWithError(err)
 273                                                 return
 274                                         }
 275                                         if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
 276                                                 pw.CloseWithError(err)
 277                                                 return
 278                                         }
 279                                         if _, err := io.CopyN(part, content, ra.length); err != nil {
 280                                                 pw.CloseWithError(err)
 281                                                 return
 282                                         }
 283                                 }
 284                                 mw.Close()
 285                                 pw.Close()
 286                         }()
 287                 }
 288
 289                 w.Header().Set("Accept-Ranges", "bytes")
 290                 if w.Header().Get("Content-Encoding") == "" {
 291                         w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
 292                 }
 293         }
 294
 295         w.WriteHeader(code)
 296
 297         if r.Method != "HEAD" {
 298                 io.CopyN(w, sendContent, sendSize)
 299         }
 300 }
 301
 302 // scanETag determines if a syntactically valid ETag is present at s. If so,
 303 // the ETag and remaining text after consuming ETag is returned. Otherwise,
 304 // it returns "", "".
 305 func scanETag(s string) (etag string, remain string) {
 306         s = textproto.TrimString(s)
 307         start := 0
 308         if strings.HasPrefix(s, "W/") {
 309                 start = 2
 310         }
 311         if len(s[start:]) < 2 || s[start] != '"' {
 312                 return "", ""
 313         }
 314         // ETag is either W/"text" or "text".
 315         // See RFC 7232 2.3.
 316         for i := start + 1; i < len(s); i++ {
 317                 c := s[i]
 318                 switch {
 319                 // Character values allowed in ETags.
 320                 case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
 321                 case c == '"':
 322                         return s[:i+1], s[i+1:]
 323                 default:
 324                         return "", ""
 325                 }
 326         }
 327         return "", ""
 328 }
 329
 330 // etagStrongMatch reports whether a and b match using strong ETag comparison.
 331 // Assumes a and b are valid ETags.
 332 func etagStrongMatch(a, b string) bool {
 333         return a == b && a != "" && a[0] == '"'
 334 }
 335
 336 // etagWeakMatch reports whether a and b match using weak ETag comparison.
 337 // Assumes a and b are valid ETags.
 338 func etagWeakMatch(a, b string) bool {
 339         return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
 340 }
 341
 342 // condResult is the result of an HTTP request precondition check.
 343 // See https://tools.ietf.org/html/rfc7232 section 3.
 344 type condResult int
 345
 346 const (
 347         condNone condResult = iota
 348         condTrue
 349         condFalse
 350 )
 351
 352 func checkIfMatch(w ResponseWriter, r *Request) condResult {
 353         im := r.Header.Get("If-Match")
 354         if im == "" {
 355                 return condNone
 356         }
 357         for {
 358                 im = textproto.TrimString(im)
 359                 if len(im) == 0 {
 360                         break
 361                 }
 362                 if im[0] == ',' {
 363                         im = im[1:]
 364                         continue
 365                 }
 366                 if im[0] == '*' {
 367                         return condTrue
 368                 }
 369                 etag, remain := scanETag(im)
 370                 if etag == "" {
 371                         break
 372                 }
 373                 if etagStrongMatch(etag, w.Header().get("Etag")) {
 374                         return condTrue
 375                 }
 376                 im = remain
 377         }
 378
 379         return condFalse
 380 }
 381
 382 func checkIfUnmodifiedSince(r *Request, modtime time.Time) condResult {
 383         ius := r.Header.Get("If-Unmodified-Since")
 384         if ius == "" || isZeroTime(modtime) {
 385                 return condNone
 386         }
 387         t, err := ParseTime(ius)
 388         if err != nil {
 389                 return condNone
 390         }
 391
 392         // The Last-Modified header truncates sub-second precision so
 393         // the modtime needs to be truncated too.
 394         modtime = modtime.Truncate(time.Second)
 395         if modtime.Before(t) || modtime.Equal(t) {
 396                 return condTrue
 397         }
 398         return condFalse
 399 }
 400
 401 func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
 402         inm := r.Header.get("If-None-Match")
 403         if inm == "" {
 404                 return condNone
 405         }
 406         buf := inm
 407         for {
 408                 buf = textproto.TrimString(buf)
 409                 if len(buf) == 0 {
 410                         break
 411                 }
 412                 if buf[0] == ',' {
 413                         buf = buf[1:]
 414                 }
 415                 if buf[0] == '*' {
 416                         return condFalse
 417                 }
 418                 etag, remain := scanETag(buf)
 419                 if etag == "" {
 420                         break
 421                 }
 422                 if etagWeakMatch(etag, w.Header().get("Etag")) {
 423                         return condFalse
 424                 }
 425                 buf = remain
 426         }
 427         return condTrue
 428 }
 429
 430 func checkIfModifiedSince(r *Request, modtime time.Time) condResult {
 431         if r.Method != "GET" && r.Method != "HEAD" {
 432                 return condNone
 433         }
 434         ims := r.Header.Get("If-Modified-Since")
 435         if ims == "" || isZeroTime(modtime) {
 436                 return condNone
 437         }
 438         t, err := ParseTime(ims)
 439         if err != nil {
 440                 return condNone
 441         }
 442         // The Last-Modified header truncates sub-second precision so
 443         // the modtime needs to be truncated too.
 444         modtime = modtime.Truncate(time.Second)
 445         if modtime.Before(t) || modtime.Equal(t) {
 446                 return condFalse
 447         }
 448         return condTrue
 449 }
 450
 451 func checkIfRange(w ResponseWriter, r *Request, modtime time.Time) condResult {
 452         if r.Method != "GET" && r.Method != "HEAD" {
 453                 return condNone
 454         }
 455         ir := r.Header.get("If-Range")
 456         if ir == "" {
 457                 return condNone
 458         }
 459         etag, _ := scanETag(ir)
 460         if etag != "" {
 461                 if etagStrongMatch(etag, w.Header().Get("Etag")) {
 462                         return condTrue
 463                 } else {
 464                         return condFalse
 465                 }
 466         }
 467         // The If-Range value is typically the ETag value, but it may also be
 468         // the modtime date. See golang.org/issue/8367.
 469         if modtime.IsZero() {
 470                 return condFalse
 471         }
 472         t, err := ParseTime(ir)
 473         if err != nil {
 474                 return condFalse
 475         }
 476         if t.Unix() == modtime.Unix() {
 477                 return condTrue
 478         }
 479         return condFalse
 480 }
 481
 482 var unixEpochTime = time.Unix(0, 0)
 483
 484 // isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
 485 func isZeroTime(t time.Time) bool {
 486         return t.IsZero() || t.Equal(unixEpochTime)
 487 }
 488
 489 func setLastModified(w ResponseWriter, modtime time.Time) {
 490         if !isZeroTime(modtime) {
 491                 w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
 492         }
 493 }
 494
 495 func writeNotModified(w ResponseWriter) {
 496         // RFC 7232 section 4.1:
 497         // a sender SHOULD NOT generate representation metadata other than the
 498         // above listed fields unless said metadata exists for the purpose of
 499         // guiding cache updates (e.g., Last-Modified might be useful if the
 500         // response does not have an ETag field).
 501         h := w.Header()
 502         delete(h, "Content-Type")
 503         delete(h, "Content-Length")
 504         if h.Get("Etag") != "" {
 505                 delete(h, "Last-Modified")
 506         }
 507         w.WriteHeader(StatusNotModified)
 508 }
 509
 510 // checkPreconditions evaluates request preconditions and reports whether a precondition
 511 // resulted in sending StatusNotModified or StatusPreconditionFailed.
 512 func checkPreconditions(w ResponseWriter, r *Request, modtime time.Time) (done bool, rangeHeader string) {
 513         // This function carefully follows RFC 7232 section 6.
 514         ch := checkIfMatch(w, r)
 515         if ch == condNone {
 516                 ch = checkIfUnmodifiedSince(r, modtime)
 517         }
 518         if ch == condFalse {
 519                 w.WriteHeader(StatusPreconditionFailed)
 520                 return true, ""
 521         }
 522         switch checkIfNoneMatch(w, r) {
 523         case condFalse:
 524                 if r.Method == "GET" || r.Method == "HEAD" {
 525                         writeNotModified(w)
 526                         return true, ""
 527                 } else {
 528                         w.WriteHeader(StatusPreconditionFailed)
 529                         return true, ""
 530                 }
 531         case condNone:
 532                 if checkIfModifiedSince(r, modtime) == condFalse {
 533                         writeNotModified(w)
 534                         return true, ""
 535                 }
 536         }
 537
 538         rangeHeader = r.Header.get("Range")
 539         if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
 540                 rangeHeader = ""
 541         }
 542         return false, rangeHeader
 543 }
 544
 545 // name is '/'-separated, not filepath.Separator.
 546 func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
 547         const indexPage = "/index.html"
 548
 549         // redirect .../index.html to .../
 550         // can't use Redirect() because that would make the path absolute,
 551         // which would be a problem running under StripPrefix
 552         if strings.HasSuffix(r.URL.Path, indexPage) {
 553                 localRedirect(w, r, "./")
 554                 return
 555         }
 556
 557         f, err := fs.Open(name)
 558         if err != nil {
 559                 msg, code := toHTTPError(err)
 560                 Error(w, msg, code)
 561                 return
 562         }
 563         defer f.Close()
 564
 565         d, err := f.Stat()
 566         if err != nil {
 567                 msg, code := toHTTPError(err)
 568                 Error(w, msg, code)
 569                 return
 570         }
 571
 572         if redirect {
 573                 // redirect to canonical path: / at end of directory url
 574                 // r.URL.Path always begins with /
 575                 url := r.URL.Path
 576                 if d.IsDir() {
 577                         if url[len(url)-1] != '/' {
 578                                 localRedirect(w, r, path.Base(url)+"/")
 579                                 return
 580                         }
 581                 } else {
 582                         if url[len(url)-1] == '/' {
 583                                 localRedirect(w, r, "../"+path.Base(url))
 584                                 return
 585                         }
 586                 }
 587         }
 588
 589         if d.IsDir() {
 590                 url := r.URL.Path
 591                 // redirect if the directory name doesn't end in a slash
 592                 if url == "" || url[len(url)-1] != '/' {
 593                         localRedirect(w, r, path.Base(url)+"/")
 594                         return
 595                 }
 596
 597                 // use contents of index.html for directory, if present
 598                 index := strings.TrimSuffix(name, "/") + indexPage
 599                 ff, err := fs.Open(index)
 600                 if err == nil {
 601                         defer ff.Close()
 602                         dd, err := ff.Stat()
 603                         if err == nil {
 604                                 name = index
 605                                 d = dd
 606                                 f = ff
 607                         }
 608                 }
 609         }
 610
 611         // Still a directory? (we didn't find an index.html file)
 612         if d.IsDir() {
 613                 if checkIfModifiedSince(r, d.ModTime()) == condFalse {
 614                         writeNotModified(w)
 615                         return
 616                 }
 617                 setLastModified(w, d.ModTime())
 618                 dirList(w, r, f)
 619                 return
 620         }
 621
 622         // serveContent will check modification time
 623         sizeFunc := func() (int64, error) { return d.Size(), nil }
 624         serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
 625 }
 626
 627 // toHTTPError returns a non-specific HTTP error message and status code
 628 // for a given non-nil error value. It's important that toHTTPError does not
 629 // actually return err.Error(), since msg and httpStatus are returned to users,
 630 // and historically Go's ServeContent always returned just "404 Not Found" for
 631 // all errors. We don't want to start leaking information in error messages.
 632 func toHTTPError(err error) (msg string, httpStatus int) {
 633         if os.IsNotExist(err) {
 634                 return "404 page not found", StatusNotFound
 635         }
 636         if os.IsPermission(err) {
 637                 return "403 Forbidden", StatusForbidden
 638         }
 639         // Default:
 640         return "500 Internal Server Error", StatusInternalServerError
 641 }
 642
 643 // localRedirect gives a Moved Permanently response.
 644 // It does not convert relative paths to absolute paths like Redirect does.
 645 func localRedirect(w ResponseWriter, r *Request, newPath string) {
 646         if q := r.URL.RawQuery; q != "" {
 647                 newPath += "?" + q
 648         }
 649         w.Header().Set("Location", newPath)
 650         w.WriteHeader(StatusMovedPermanently)
 651 }
 652
 653 // ServeFile replies to the request with the contents of the named
 654 // file or directory.
 655 //
 656 // If the provided file or directory name is a relative path, it is
 657 // interpreted relative to the current directory and may ascend to
 658 // parent directories. If the provided name is constructed from user
 659 // input, it should be sanitized before calling ServeFile.
 660 //
 661 // As a precaution, ServeFile will reject requests where r.URL.Path
 662 // contains a ".." path element; this protects against callers who
 663 // might unsafely use filepath.Join on r.URL.Path without sanitizing
 664 // it and then use that filepath.Join result as the name argument.
 665 //
 666 // As another special case, ServeFile redirects any request where r.URL.Path
 667 // ends in "/index.html" to the same path, without the final
 668 // "index.html". To avoid such redirects either modify the path or
 669 // use ServeContent.
 670 //
 671 // Outside of those two special cases, ServeFile does not use
 672 // r.URL.Path for selecting the file or directory to serve; only the
 673 // file or directory provided in the name argument is used.
 674 func ServeFile(w ResponseWriter, r *Request, name string) {
 675         if containsDotDot(r.URL.Path) {
 676                 // Too many programs use r.URL.Path to construct the argument to
 677                 // serveFile. Reject the request under the assumption that happened
 678                 // here and ".." may not be wanted.
 679                 // Note that name might not contain "..", for example if code (still
 680                 // incorrectly) used filepath.Join(myDir, r.URL.Path).
 681                 Error(w, "invalid URL path", StatusBadRequest)
 682                 return
 683         }
 684         dir, file := filepath.Split(name)
 685         serveFile(w, r, Dir(dir), file, false)
 686 }
 687
 688 func containsDotDot(v string) bool {
 689         if !strings.Contains(v, "..") {
 690                 return false
 691         }
 692         for _, ent := range strings.FieldsFunc(v, isSlashRune) {
 693                 if ent == ".." {
 694                         return true
 695                 }
 696         }
 697         return false
 698 }
 699
 700 func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
 701
 702 type fileHandler struct {
 703         root FileSystem
 704 }
 705
 706 // FileServer returns a handler that serves HTTP requests
 707 // with the contents of the file system rooted at root.
 708 //
 709 // To use the operating system's file system implementation,
 710 // use http.Dir:
 711 //
 712 //     http.Handle("/", http.FileServer(http.Dir("/tmp")))
 713 //
 714 // As a special case, the returned file server redirects any request
 715 // ending in "/index.html" to the same path, without the final
 716 // "index.html".
 717 func FileServer(root FileSystem) Handler {
 718         return &fileHandler{root}
 719 }
 720
 721 func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
 722         upath := r.URL.Path
 723         if !strings.HasPrefix(upath, "/") {
 724                 upath = "/" + upath
 725                 r.URL.Path = upath
 726         }
 727         serveFile(w, r, f.root, path.Clean(upath), true)
 728 }
 729
 730 // httpRange specifies the byte range to be sent to the client.
 731 type httpRange struct {
 732         start, length int64
 733 }
 734
 735 func (r httpRange) contentRange(size int64) string {
 736         return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
 737 }
 738
 739 func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
 740         return textproto.MIMEHeader{
 741                 "Content-Range": {r.contentRange(size)},
 742                 "Content-Type":  {contentType},
 743         }
 744 }
 745
 746 // parseRange parses a Range header string as per RFC 7233.
 747 // errNoOverlap is returned if none of the ranges overlap.
 748 func parseRange(s string, size int64) ([]httpRange, error) {
 749         if s == "" {
 750                 return nil, nil // header not present
 751         }
 752         const b = "bytes="
 753         if !strings.HasPrefix(s, b) {
 754                 return nil, errors.New("invalid range")
 755         }
 756         var ranges []httpRange
 757         noOverlap := false
 758         for _, ra := range strings.Split(s[len(b):], ",") {
 759                 ra = strings.TrimSpace(ra)
 760                 if ra == "" {
 761                         continue
 762                 }
 763                 i := strings.Index(ra, "-")
 764                 if i < 0 {
 765                         return nil, errors.New("invalid range")
 766                 }
 767                 start, end := strings.TrimSpace(ra[:i]), strings.TrimSpace(ra[i+1:])
 768                 var r httpRange
 769                 if start == "" {
 770                         // If no start is specified, end specifies the
 771                         // range start relative to the end of the file.
 772                         i, err := strconv.ParseInt(end, 10, 64)
 773                         if err != nil {
 774                                 return nil, errors.New("invalid range")
 775                         }
 776                         if i > size {
 777                                 i = size
 778                         }
 779                         r.start = size - i
 780                         r.length = size - r.start
 781                 } else {
 782                         i, err := strconv.ParseInt(start, 10, 64)
 783                         if err != nil || i < 0 {
 784                                 return nil, errors.New("invalid range")
 785                         }
 786                         if i >= size {
 787                                 // If the range begins after the size of the content,
 788                                 // then it does not overlap.
 789                                 noOverlap = true
 790                                 continue
 791                         }
 792                         r.start = i
 793                         if end == "" {
 794                                 // If no end is specified, range extends to end of the file.
 795                                 r.length = size - r.start
 796                         } else {
 797                                 i, err := strconv.ParseInt(end, 10, 64)
 798                                 if err != nil || r.start > i {
 799                                         return nil, errors.New("invalid range")
 800                                 }
 801                                 if i >= size {
 802                                         i = size - 1
 803                                 }
 804                                 r.length = i - r.start + 1
 805                         }
 806                 }
 807                 ranges = append(ranges, r)
 808         }
 809         if noOverlap && len(ranges) == 0 {
 810                 // The specified ranges did not overlap with the content.
 811                 return nil, errNoOverlap
 812         }
 813         return ranges, nil
 814 }
 815
 816 // countingWriter counts how many bytes have been written to it.
 817 type countingWriter int64
 818
 819 func (w *countingWriter) Write(p []byte) (n int, err error) {
 820         *w += countingWriter(len(p))
 821         return len(p), nil
 822 }
 823
 824 // rangesMIMESize returns the number of bytes it takes to encode the
 825 // provided ranges as a multipart response.
 826 func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
 827         var w countingWriter
 828         mw := multipart.NewWriter(&w)
 829         for _, ra := range ranges {
 830                 mw.CreatePart(ra.mimeHeader(contentType, contentSize))
 831                 encSize += ra.length
 832         }
 833         mw.Close()
 834         encSize += int64(w)
 835         return
 836 }
 837
 838 func sumRangesSize(ranges []httpRange) (size int64) {
 839         for _, ra := range ranges {
 840                 size += ra.length
 841         }
 842         return
 843 }