fs.go

Documentation: net/http

     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  func (d Dir) Open(name string) (File, error) {
    67  	if filepath.Separator != '/' && strings.ContainsRune(name, filepath.Separator) {
    68  		return nil, errors.New("http: invalid character in file path")
    69  	}
    70  	dir := string(d)
    71  	if dir == "" {
    72  		dir = "."
    73  	}
    74  	fullName := filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name)))
    75  	f, err := os.Open(fullName)
    76  	if err != nil {
    77  		return nil, mapDirOpenError(err, fullName)
    78  	}
    79  	return f, nil
    80  }
    81  
    82  // A FileSystem implements access to a collection of named files.
    83  // The elements in a file path are separated by slash ('/', U+002F)
    84  // characters, regardless of host operating system convention.
    85  type FileSystem interface {
    86  	Open(name string) (File, error)
    87  }
    88  
    89  // A File is returned by a FileSystem's Open method and can be
    90  // served by the FileServer implementation.
    91  //
    92  // The methods should behave the same as those on an *os.File.
    93  type File interface {
    94  	io.Closer
    95  	io.Reader
    96  	io.Seeker
    97  	Readdir(count int) ([]os.FileInfo, error)
    98  	Stat() (os.FileInfo, error)
    99  }
   100  
   101  func dirList(w ResponseWriter, r *Request, f File) {
   102  	dirs, err := f.Readdir(-1)
   103  	if err != nil {
   104  		logf(r, "http: error reading directory: %v", err)
   105  		Error(w, "Error reading directory", StatusInternalServerError)
   106  		return
   107  	}
   108  	sort.Slice(dirs, func(i, j int) bool { return dirs[i].Name() < dirs[j].Name() })
   109  
   110  	w.Header().Set("Content-Type", "text/html; charset=utf-8")
   111  	fmt.Fprintf(w, "<pre>\n")
   112  	for _, d := range dirs {
   113  		name := d.Name()
   114  		if d.IsDir() {
   115  			name += "/"
   116  		}
   117  		// name may contain '?' or '#', which must be escaped to remain
   118  		// part of the URL path, and not indicate the start of a query
   119  		// string or fragment.
   120  		url := url.URL{Path: name}
   121  		fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
   122  	}
   123  	fmt.Fprintf(w, "</pre>\n")
   124  }
   125  
   126  // ServeContent replies to the request using the content in the
   127  // provided ReadSeeker. The main benefit of ServeContent over io.Copy
   128  // is that it handles Range requests properly, sets the MIME type, and
   129  // handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
   130  // and If-Range requests.
   131  //
   132  // If the response's Content-Type header is not set, ServeContent
   133  // first tries to deduce the type from name's file extension and,
   134  // if that fails, falls back to reading the first block of the content
   135  // and passing it to DetectContentType.
   136  // The name is otherwise unused; in particular it can be empty and is
   137  // never sent in the response.
   138  //
   139  // If modtime is not the zero time or Unix epoch, ServeContent
   140  // includes it in a Last-Modified header in the response. If the
   141  // request includes an If-Modified-Since header, ServeContent uses
   142  // modtime to decide whether the content needs to be sent at all.
   143  //
   144  // The content's Seek method must work: ServeContent uses
   145  // a seek to the end of the content to determine its size.
   146  //
   147  // If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
   148  // ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
   149  //
   150  // Note that *os.File implements the io.ReadSeeker interface.
   151  func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
   152  	sizeFunc := func() (int64, error) {
   153  		size, err := content.Seek(0, io.SeekEnd)
   154  		if err != nil {
   155  			return 0, errSeeker
   156  		}
   157  		_, err = content.Seek(0, io.SeekStart)
   158  		if err != nil {
   159  			return 0, errSeeker
   160  		}
   161  		return size, nil
   162  	}
   163  	serveContent(w, req, name, modtime, sizeFunc, content)
   164  }
   165  
   166  // errSeeker is returned by ServeContent's sizeFunc when the content
   167  // doesn't seek properly. The underlying Seeker's error text isn't
   168  // included in the sizeFunc reply so it's not sent over HTTP to end
   169  // users.
   170  var errSeeker = errors.New("seeker can't seek")
   171  
   172  // errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
   173  // all of the byte-range-spec values is greater than the content size.
   174  var errNoOverlap = errors.New("invalid range: failed to overlap")
   175  
   176  // if name is empty, filename is unknown. (used for mime type, before sniffing)
   177  // if modtime.IsZero(), modtime is unknown.
   178  // content must be seeked to the beginning of the file.
   179  // The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
   180  func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
   181  	setLastModified(w, modtime)
   182  	done, rangeReq := checkPreconditions(w, r, modtime)
   183  	if done {
   184  		return
   185  	}
   186  
   187  	code := StatusOK
   188  
   189  	// If Content-Type isn't set, use the file's extension to find it, but
   190  	// if the Content-Type is unset explicitly, do not sniff the type.
   191  	ctypes, haveType := w.Header()["Content-Type"]
   192  	var ctype string
   193  	if !haveType {
   194  		ctype = mime.TypeByExtension(filepath.Ext(name))
   195  		if ctype == "" {
   196  			// read a chunk to decide between utf-8 text and binary
   197  			var buf [sniffLen]byte
   198  			n, _ := io.ReadFull(content, buf[:])
   199  			ctype = DetectContentType(buf[:n])
   200  			_, err := content.Seek(0, io.SeekStart) // rewind to output whole file
   201  			if err != nil {
   202  				Error(w, "seeker can't seek", StatusInternalServerError)
   203  				return
   204  			}
   205  		}
   206  		w.Header().Set("Content-Type", ctype)
   207  	} else if len(ctypes) > 0 {
   208  		ctype = ctypes[0]
   209  	}
   210  
   211  	size, err := sizeFunc()
   212  	if err != nil {
   213  		Error(w, err.Error(), StatusInternalServerError)
   214  		return
   215  	}
   216  
   217  	// handle Content-Range header.
   218  	sendSize := size
   219  	var sendContent io.Reader = content
   220  	if size >= 0 {
   221  		ranges, err := parseRange(rangeReq, size)
   222  		if err != nil {
   223  			if err == errNoOverlap {
   224  				w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
   225  			}
   226  			Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   227  			return
   228  		}
   229  		if sumRangesSize(ranges) > size {
   230  			// The total number of bytes in all the ranges
   231  			// is larger than the size of the file by
   232  			// itself, so this is probably an attack, or a
   233  			// dumb client. Ignore the range request.
   234  			ranges = nil
   235  		}
   236  		switch {
   237  		case len(ranges) == 1:
   238  			// RFC 2616, Section 14.16:
   239  			// "When an HTTP message includes the content of a single
   240  			// range (for example, a response to a request for a
   241  			// single range, or to a request for a set of ranges
   242  			// that overlap without any holes), this content is
   243  			// transmitted with a Content-Range header, and a
   244  			// Content-Length header showing the number of bytes
   245  			// actually transferred.
   246  			// ...
   247  			// A response to a request for a single range MUST NOT
   248  			// be sent using the multipart/byteranges media type."
   249  			ra := ranges[0]
   250  			if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   251  				Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   252  				return
   253  			}
   254  			sendSize = ra.length
   255  			code = StatusPartialContent
   256  			w.Header().Set("Content-Range", ra.contentRange(size))
   257  		case len(ranges) > 1:
   258  			sendSize = rangesMIMESize(ranges, ctype, size)
   259  			code = StatusPartialContent
   260  
   261  			pr, pw := io.Pipe()
   262  			mw := multipart.NewWriter(pw)
   263  			w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
   264  			sendContent = pr
   265  			defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
   266  			go func() {
   267  				for _, ra := range ranges {
   268  					part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
   269  					if err != nil {
   270  						pw.CloseWithError(err)
   271  						return
   272  					}
   273  					if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   274  						pw.CloseWithError(err)
   275  						return
   276  					}
   277  					if _, err := io.CopyN(part, content, ra.length); err != nil {
   278  						pw.CloseWithError(err)
   279  						return
   280  					}
   281  				}
   282  				mw.Close()
   283  				pw.Close()
   284  			}()
   285  		}
   286  
   287  		w.Header().Set("Accept-Ranges", "bytes")
   288  		if w.Header().Get("Content-Encoding") == "" {
   289  			w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
   290  		}
   291  	}
   292  
   293  	w.WriteHeader(code)
   294  
   295  	if r.Method != "HEAD" {
   296  		io.CopyN(w, sendContent, sendSize)
   297  	}
   298  }
   299  
   300  // scanETag determines if a syntactically valid ETag is present at s. If so,
   301  // the ETag and remaining text after consuming ETag is returned. Otherwise,
   302  // it returns "", "".
   303  func scanETag(s string) (etag string, remain string) {
   304  	s = textproto.TrimString(s)
   305  	start := 0
   306  	if strings.HasPrefix(s, "W/") {
   307  		start = 2
   308  	}
   309  	if len(s[start:]) < 2 || s[start] != '"' {
   310  		return "", ""
   311  	}
   312  	// ETag is either W/"text" or "text".
   313  	// See RFC 7232 2.3.
   314  	for i := start + 1; i < len(s); i++ {
   315  		c := s[i]
   316  		switch {
   317  		// Character values allowed in ETags.
   318  		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
   319  		case c == '"':
   320  			return s[:i+1], s[i+1:]
   321  		default:
   322  			return "", ""
   323  		}
   324  	}
   325  	return "", ""
   326  }
   327  
   328  // etagStrongMatch reports whether a and b match using strong ETag comparison.
   329  // Assumes a and b are valid ETags.
   330  func etagStrongMatch(a, b string) bool {
   331  	return a == b && a != "" && a[0] == '"'
   332  }
   333  
   334  // etagWeakMatch reports whether a and b match using weak ETag comparison.
   335  // Assumes a and b are valid ETags.
   336  func etagWeakMatch(a, b string) bool {
   337  	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
   338  }
   339  
   340  // condResult is the result of an HTTP request precondition check.
   341  // See https://tools.ietf.org/html/rfc7232 section 3.
   342  type condResult int
   343  
   344  const (
   345  	condNone condResult = iota
   346  	condTrue
   347  	condFalse
   348  )
   349  
   350  func checkIfMatch(w ResponseWriter, r *Request) condResult {
   351  	im := r.Header.Get("If-Match")
   352  	if im == "" {
   353  		return condNone
   354  	}
   355  	for {
   356  		im = textproto.TrimString(im)
   357  		if len(im) == 0 {
   358  			break
   359  		}
   360  		if im[0] == ',' {
   361  			im = im[1:]
   362  			continue
   363  		}
   364  		if im[0] == '*' {
   365  			return condTrue
   366  		}
   367  		etag, remain := scanETag(im)
   368  		if etag == "" {
   369  			break
   370  		}
   371  		if etagStrongMatch(etag, w.Header().get("Etag")) {
   372  			return condTrue
   373  		}
   374  		im = remain
   375  	}
   376  
   377  	return condFalse
   378  }
   379  
   380  func checkIfUnmodifiedSince(r *Request, modtime time.Time) condResult {
   381  	ius := r.Header.Get("If-Unmodified-Since")
   382  	if ius == "" || isZeroTime(modtime) {
   383  		return condNone
   384  	}
   385  	if t, err := ParseTime(ius); err == nil {
   386  		// The Date-Modified header truncates sub-second precision, so
   387  		// use mtime < t+1s instead of mtime <= t to check for unmodified.
   388  		if modtime.Before(t.Add(1 * time.Second)) {
   389  			return condTrue
   390  		}
   391  		return condFalse
   392  	}
   393  	return condNone
   394  }
   395  
   396  func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
   397  	inm := r.Header.get("If-None-Match")
   398  	if inm == "" {
   399  		return condNone
   400  	}
   401  	buf := inm
   402  	for {
   403  		buf = textproto.TrimString(buf)
   404  		if len(buf) == 0 {
   405  			break
   406  		}
   407  		if buf[0] == ',' {
   408  			buf = buf[1:]
   409  		}
   410  		if buf[0] == '*' {
   411  			return condFalse
   412  		}
   413  		etag, remain := scanETag(buf)
   414  		if etag == "" {
   415  			break
   416  		}
   417  		if etagWeakMatch(etag, w.Header().get("Etag")) {
   418  			return condFalse
   419  		}
   420  		buf = remain
   421  	}
   422  	return condTrue
   423  }
   424  
   425  func checkIfModifiedSince(r *Request, modtime time.Time) condResult {
   426  	if r.Method != "GET" && r.Method != "HEAD" {
   427  		return condNone
   428  	}
   429  	ims := r.Header.Get("If-Modified-Since")
   430  	if ims == "" || isZeroTime(modtime) {
   431  		return condNone
   432  	}
   433  	t, err := ParseTime(ims)
   434  	if err != nil {
   435  		return condNone
   436  	}
   437  	// The Date-Modified header truncates sub-second precision, so
   438  	// use mtime < t+1s instead of mtime <= t to check for unmodified.
   439  	if modtime.Before(t.Add(1 * time.Second)) {
   440  		return condFalse
   441  	}
   442  	return condTrue
   443  }
   444  
   445  func checkIfRange(w ResponseWriter, r *Request, modtime time.Time) condResult {
   446  	if r.Method != "GET" && r.Method != "HEAD" {
   447  		return condNone
   448  	}
   449  	ir := r.Header.get("If-Range")
   450  	if ir == "" {
   451  		return condNone
   452  	}
   453  	etag, _ := scanETag(ir)
   454  	if etag != "" {
   455  		if etagStrongMatch(etag, w.Header().Get("Etag")) {
   456  			return condTrue
   457  		} else {
   458  			return condFalse
   459  		}
   460  	}
   461  	// The If-Range value is typically the ETag value, but it may also be
   462  	// the modtime date. See golang.org/issue/8367.
   463  	if modtime.IsZero() {
   464  		return condFalse
   465  	}
   466  	t, err := ParseTime(ir)
   467  	if err != nil {
   468  		return condFalse
   469  	}
   470  	if t.Unix() == modtime.Unix() {
   471  		return condTrue
   472  	}
   473  	return condFalse
   474  }
   475  
   476  var unixEpochTime = time.Unix(0, 0)
   477  
   478  // isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
   479  func isZeroTime(t time.Time) bool {
   480  	return t.IsZero() || t.Equal(unixEpochTime)
   481  }
   482  
   483  func setLastModified(w ResponseWriter, modtime time.Time) {
   484  	if !isZeroTime(modtime) {
   485  		w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
   486  	}
   487  }
   488  
   489  func writeNotModified(w ResponseWriter) {
   490  	// RFC 7232 section 4.1:
   491  	// a sender SHOULD NOT generate representation metadata other than the
   492  	// above listed fields unless said metadata exists for the purpose of
   493  	// guiding cache updates (e.g., Last-Modified might be useful if the
   494  	// response does not have an ETag field).
   495  	h := w.Header()
   496  	delete(h, "Content-Type")
   497  	delete(h, "Content-Length")
   498  	if h.Get("Etag") != "" {
   499  		delete(h, "Last-Modified")
   500  	}
   501  	w.WriteHeader(StatusNotModified)
   502  }
   503  
   504  // checkPreconditions evaluates request preconditions and reports whether a precondition
   505  // resulted in sending StatusNotModified or StatusPreconditionFailed.
   506  func checkPreconditions(w ResponseWriter, r *Request, modtime time.Time) (done bool, rangeHeader string) {
   507  	// This function carefully follows RFC 7232 section 6.
   508  	ch := checkIfMatch(w, r)
   509  	if ch == condNone {
   510  		ch = checkIfUnmodifiedSince(r, modtime)
   511  	}
   512  	if ch == condFalse {
   513  		w.WriteHeader(StatusPreconditionFailed)
   514  		return true, ""
   515  	}
   516  	switch checkIfNoneMatch(w, r) {
   517  	case condFalse:
   518  		if r.Method == "GET" || r.Method == "HEAD" {
   519  			writeNotModified(w)
   520  			return true, ""
   521  		} else {
   522  			w.WriteHeader(StatusPreconditionFailed)
   523  			return true, ""
   524  		}
   525  	case condNone:
   526  		if checkIfModifiedSince(r, modtime) == condFalse {
   527  			writeNotModified(w)
   528  			return true, ""
   529  		}
   530  	}
   531  
   532  	rangeHeader = r.Header.get("Range")
   533  	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
   534  		rangeHeader = ""
   535  	}
   536  	return false, rangeHeader
   537  }
   538  
   539  // name is '/'-separated, not filepath.Separator.
   540  func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
   541  	const indexPage = "/index.html"
   542  
   543  	// redirect .../index.html to .../
   544  	// can't use Redirect() because that would make the path absolute,
   545  	// which would be a problem running under StripPrefix
   546  	if strings.HasSuffix(r.URL.Path, indexPage) {
   547  		localRedirect(w, r, "./")
   548  		return
   549  	}
   550  
   551  	f, err := fs.Open(name)
   552  	if err != nil {
   553  		msg, code := toHTTPError(err)
   554  		Error(w, msg, code)
   555  		return
   556  	}
   557  	defer f.Close()
   558  
   559  	d, err := f.Stat()
   560  	if err != nil {
   561  		msg, code := toHTTPError(err)
   562  		Error(w, msg, code)
   563  		return
   564  	}
   565  
   566  	if redirect {
   567  		// redirect to canonical path: / at end of directory url
   568  		// r.URL.Path always begins with /
   569  		url := r.URL.Path
   570  		if d.IsDir() {
   571  			if url[len(url)-1] != '/' {
   572  				localRedirect(w, r, path.Base(url)+"/")
   573  				return
   574  			}
   575  		} else {
   576  			if url[len(url)-1] == '/' {
   577  				localRedirect(w, r, "../"+path.Base(url))
   578  				return
   579  			}
   580  		}
   581  	}
   582  
   583  	// redirect if the directory name doesn't end in a slash
   584  	if d.IsDir() {
   585  		url := r.URL.Path
   586  		if url[len(url)-1] != '/' {
   587  			localRedirect(w, r, path.Base(url)+"/")
   588  			return
   589  		}
   590  	}
   591  
   592  	// use contents of index.html for directory, if present
   593  	if d.IsDir() {
   594  		index := strings.TrimSuffix(name, "/") + indexPage
   595  		ff, err := fs.Open(index)
   596  		if err == nil {
   597  			defer ff.Close()
   598  			dd, err := ff.Stat()
   599  			if err == nil {
   600  				name = index
   601  				d = dd
   602  				f = ff
   603  			}
   604  		}
   605  	}
   606  
   607  	// Still a directory? (we didn't find an index.html file)
   608  	if d.IsDir() {
   609  		if checkIfModifiedSince(r, d.ModTime()) == condFalse {
   610  			writeNotModified(w)
   611  			return
   612  		}
   613  		w.Header().Set("Last-Modified", d.ModTime().UTC().Format(TimeFormat))
   614  		dirList(w, r, f)
   615  		return
   616  	}
   617  
   618  	// serveContent will check modification time
   619  	sizeFunc := func() (int64, error) { return d.Size(), nil }
   620  	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
   621  }
   622  
   623  // toHTTPError returns a non-specific HTTP error message and status code
   624  // for a given non-nil error value. It's important that toHTTPError does not
   625  // actually return err.Error(), since msg and httpStatus are returned to users,
   626  // and historically Go's ServeContent always returned just "404 Not Found" for
   627  // all errors. We don't want to start leaking information in error messages.
   628  func toHTTPError(err error) (msg string, httpStatus int) {
   629  	if os.IsNotExist(err) {
   630  		return "404 page not found", StatusNotFound
   631  	}
   632  	if os.IsPermission(err) {
   633  		return "403 Forbidden", StatusForbidden
   634  	}
   635  	// Default:
   636  	return "500 Internal Server Error", StatusInternalServerError
   637  }
   638  
   639  // localRedirect gives a Moved Permanently response.
   640  // It does not convert relative paths to absolute paths like Redirect does.
   641  func localRedirect(w ResponseWriter, r *Request, newPath string) {
   642  	if q := r.URL.RawQuery; q != "" {
   643  		newPath += "?" + q
   644  	}
   645  	w.Header().Set("Location", newPath)
   646  	w.WriteHeader(StatusMovedPermanently)
   647  }
   648  
   649  // ServeFile replies to the request with the contents of the named
   650  // file or directory.
   651  //
   652  // If the provided file or directory name is a relative path, it is
   653  // interpreted relative to the current directory and may ascend to parent
   654  // directories. If the provided name is constructed from user input, it
   655  // should be sanitized before calling ServeFile. As a precaution, ServeFile
   656  // will reject requests where r.URL.Path contains a ".." path element.
   657  //
   658  // As a special case, ServeFile redirects any request where r.URL.Path
   659  // ends in "/index.html" to the same path, without the final
   660  // "index.html". To avoid such redirects either modify the path or
   661  // use ServeContent.
   662  func ServeFile(w ResponseWriter, r *Request, name string) {
   663  	if containsDotDot(r.URL.Path) {
   664  		// Too many programs use r.URL.Path to construct the argument to
   665  		// serveFile. Reject the request under the assumption that happened
   666  		// here and ".." may not be wanted.
   667  		// Note that name might not contain "..", for example if code (still
   668  		// incorrectly) used filepath.Join(myDir, r.URL.Path).
   669  		Error(w, "invalid URL path", StatusBadRequest)
   670  		return
   671  	}
   672  	dir, file := filepath.Split(name)
   673  	serveFile(w, r, Dir(dir), file, false)
   674  }
   675  
   676  func containsDotDot(v string) bool {
   677  	if !strings.Contains(v, "..") {
   678  		return false
   679  	}
   680  	for _, ent := range strings.FieldsFunc(v, isSlashRune) {
   681  		if ent == ".." {
   682  			return true
   683  		}
   684  	}
   685  	return false
   686  }
   687  
   688  func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
   689  
   690  type fileHandler struct {
   691  	root FileSystem
   692  }
   693  
   694  // FileServer returns a handler that serves HTTP requests
   695  // with the contents of the file system rooted at root.
   696  //
   697  // To use the operating system's file system implementation,
   698  // use http.Dir:
   699  //
   700  //     http.Handle("/", http.FileServer(http.Dir("/tmp")))
   701  //
   702  // As a special case, the returned file server redirects any request
   703  // ending in "/index.html" to the same path, without the final
   704  // "index.html".
   705  func FileServer(root FileSystem) Handler {
   706  	return &fileHandler{root}
   707  }
   708  
   709  func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
   710  	upath := r.URL.Path
   711  	if !strings.HasPrefix(upath, "/") {
   712  		upath = "/" + upath
   713  		r.URL.Path = upath
   714  	}
   715  	serveFile(w, r, f.root, path.Clean(upath), true)
   716  }
   717  
   718  // httpRange specifies the byte range to be sent to the client.
   719  type httpRange struct {
   720  	start, length int64
   721  }
   722  
   723  func (r httpRange) contentRange(size int64) string {
   724  	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
   725  }
   726  
   727  func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
   728  	return textproto.MIMEHeader{
   729  		"Content-Range": {r.contentRange(size)},
   730  		"Content-Type":  {contentType},
   731  	}
   732  }
   733  
   734  // parseRange parses a Range header string as per RFC 2616.
   735  // errNoOverlap is returned if none of the ranges overlap.
   736  func parseRange(s string, size int64) ([]httpRange, error) {
   737  	if s == "" {
   738  		return nil, nil // header not present
   739  	}
   740  	const b = "bytes="
   741  	if !strings.HasPrefix(s, b) {
   742  		return nil, errors.New("invalid range")
   743  	}
   744  	var ranges []httpRange
   745  	noOverlap := false
   746  	for _, ra := range strings.Split(s[len(b):], ",") {
   747  		ra = strings.TrimSpace(ra)
   748  		if ra == "" {
   749  			continue
   750  		}
   751  		i := strings.Index(ra, "-")
   752  		if i < 0 {
   753  			return nil, errors.New("invalid range")
   754  		}
   755  		start, end := strings.TrimSpace(ra[:i]), strings.TrimSpace(ra[i+1:])
   756  		var r httpRange
   757  		if start == "" {
   758  			// If no start is specified, end specifies the
   759  			// range start relative to the end of the file.
   760  			i, err := strconv.ParseInt(end, 10, 64)
   761  			if err != nil {
   762  				return nil, errors.New("invalid range")
   763  			}
   764  			if i > size {
   765  				i = size
   766  			}
   767  			r.start = size - i
   768  			r.length = size - r.start
   769  		} else {
   770  			i, err := strconv.ParseInt(start, 10, 64)
   771  			if err != nil || i < 0 {
   772  				return nil, errors.New("invalid range")
   773  			}
   774  			if i >= size {
   775  				// If the range begins after the size of the content,
   776  				// then it does not overlap.
   777  				noOverlap = true
   778  				continue
   779  			}
   780  			r.start = i
   781  			if end == "" {
   782  				// If no end is specified, range extends to end of the file.
   783  				r.length = size - r.start
   784  			} else {
   785  				i, err := strconv.ParseInt(end, 10, 64)
   786  				if err != nil || r.start > i {
   787  					return nil, errors.New("invalid range")
   788  				}
   789  				if i >= size {
   790  					i = size - 1
   791  				}
   792  				r.length = i - r.start + 1
   793  			}
   794  		}
   795  		ranges = append(ranges, r)
   796  	}
   797  	if noOverlap && len(ranges) == 0 {
   798  		// The specified ranges did not overlap with the content.
   799  		return nil, errNoOverlap
   800  	}
   801  	return ranges, nil
   802  }
   803  
   804  // countingWriter counts how many bytes have been written to it.
   805  type countingWriter int64
   806  
   807  func (w *countingWriter) Write(p []byte) (n int, err error) {
   808  	*w += countingWriter(len(p))
   809  	return len(p), nil
   810  }
   811  
   812  // rangesMIMESize returns the number of bytes it takes to encode the
   813  // provided ranges as a multipart response.
   814  func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
   815  	var w countingWriter
   816  	mw := multipart.NewWriter(&w)
   817  	for _, ra := range ranges {
   818  		mw.CreatePart(ra.mimeHeader(contentType, contentSize))
   819  		encSize += ra.length
   820  	}
   821  	mw.Close()
   822  	encSize += int64(w)
   823  	return
   824  }
   825  
   826  func sumRangesSize(ranges []httpRange) (size int64) {
   827  	for _, ra := range ranges {
   828  		size += ra.length
   829  	}
   830  	return
   831  }
   832
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