Source file src/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  	"internal/safefilepath"
    13  	"io"
    14  	"io/fs"
    15  	"mime"
    16  	"mime/multipart"
    17  	"net/textproto"
    18  	"net/url"
    19  	"os"
    20  	"path"
    21  	"path/filepath"
    22  	"sort"
    23  	"strconv"
    24  	"strings"
    25  	"time"
    26  )
    27  
    28  // A Dir implements FileSystem using the native file system restricted to a
    29  // specific directory tree.
    30  //
    31  // While the FileSystem.Open method takes '/'-separated paths, a Dir's string
    32  // value is a filename on the native file system, not a URL, so it is separated
    33  // by filepath.Separator, which isn't necessarily '/'.
    34  //
    35  // Note that Dir could expose sensitive files and directories. Dir will follow
    36  // symlinks pointing out of the directory tree, which can be especially dangerous
    37  // if serving from a directory in which users are able to create arbitrary symlinks.
    38  // Dir will also allow access to files and directories starting with a period,
    39  // which could expose sensitive directories like .git or sensitive files like
    40  // .htpasswd. To exclude files with a leading period, remove the files/directories
    41  // from the server or create a custom FileSystem implementation.
    42  //
    43  // An empty Dir is treated as ".".
    44  type Dir string
    45  
    46  // mapOpenError maps the provided non-nil error from opening name
    47  // to a possibly better non-nil error. In particular, it turns OS-specific errors
    48  // about opening files in non-directories into fs.ErrNotExist. See Issues 18984 and 49552.
    49  func mapOpenError(originalErr error, name string, sep rune, stat func(string) (fs.FileInfo, error)) error {
    50  	if errors.Is(originalErr, fs.ErrNotExist) || errors.Is(originalErr, fs.ErrPermission) {
    51  		return originalErr
    52  	}
    53  
    54  	parts := strings.Split(name, string(sep))
    55  	for i := range parts {
    56  		if parts[i] == "" {
    57  			continue
    58  		}
    59  		fi, err := stat(strings.Join(parts[:i+1], string(sep)))
    60  		if err != nil {
    61  			return originalErr
    62  		}
    63  		if !fi.IsDir() {
    64  			return fs.ErrNotExist
    65  		}
    66  	}
    67  	return originalErr
    68  }
    69  
    70  // Open implements FileSystem using os.Open, opening files for reading rooted
    71  // and relative to the directory d.
    72  func (d Dir) Open(name string) (File, error) {
    73  	path, err := safefilepath.FromFS(path.Clean("/" + name))
    74  	if err != nil {
    75  		return nil, errors.New("http: invalid or unsafe file path")
    76  	}
    77  	dir := string(d)
    78  	if dir == "" {
    79  		dir = "."
    80  	}
    81  	fullName := filepath.Join(dir, path)
    82  	f, err := os.Open(fullName)
    83  	if err != nil {
    84  		return nil, mapOpenError(err, fullName, filepath.Separator, os.Stat)
    85  	}
    86  	return f, nil
    87  }
    88  
    89  // A FileSystem implements access to a collection of named files.
    90  // The elements in a file path are separated by slash ('/', U+002F)
    91  // characters, regardless of host operating system convention.
    92  // See the FileServer function to convert a FileSystem to a Handler.
    93  //
    94  // This interface predates the fs.FS interface, which can be used instead:
    95  // the FS adapter function converts an fs.FS to a FileSystem.
    96  type FileSystem interface {
    97  	Open(name string) (File, error)
    98  }
    99  
   100  // A File is returned by a FileSystem's Open method and can be
   101  // served by the FileServer implementation.
   102  //
   103  // The methods should behave the same as those on an *os.File.
   104  type File interface {
   105  	io.Closer
   106  	io.Reader
   107  	io.Seeker
   108  	Readdir(count int) ([]fs.FileInfo, error)
   109  	Stat() (fs.FileInfo, error)
   110  }
   111  
   112  type anyDirs interface {
   113  	len() int
   114  	name(i int) string
   115  	isDir(i int) bool
   116  }
   117  
   118  type fileInfoDirs []fs.FileInfo
   119  
   120  func (d fileInfoDirs) len() int          { return len(d) }
   121  func (d fileInfoDirs) isDir(i int) bool  { return d[i].IsDir() }
   122  func (d fileInfoDirs) name(i int) string { return d[i].Name() }
   123  
   124  type dirEntryDirs []fs.DirEntry
   125  
   126  func (d dirEntryDirs) len() int          { return len(d) }
   127  func (d dirEntryDirs) isDir(i int) bool  { return d[i].IsDir() }
   128  func (d dirEntryDirs) name(i int) string { return d[i].Name() }
   129  
   130  func dirList(w ResponseWriter, r *Request, f File) {
   131  	// Prefer to use ReadDir instead of Readdir,
   132  	// because the former doesn't require calling
   133  	// Stat on every entry of a directory on Unix.
   134  	var dirs anyDirs
   135  	var err error
   136  	if d, ok := f.(fs.ReadDirFile); ok {
   137  		var list dirEntryDirs
   138  		list, err = d.ReadDir(-1)
   139  		dirs = list
   140  	} else {
   141  		var list fileInfoDirs
   142  		list, err = f.Readdir(-1)
   143  		dirs = list
   144  	}
   145  
   146  	if err != nil {
   147  		logf(r, "http: error reading directory: %v", err)
   148  		Error(w, "Error reading directory", StatusInternalServerError)
   149  		return
   150  	}
   151  	sort.Slice(dirs, func(i, j int) bool { return dirs.name(i) < dirs.name(j) })
   152  
   153  	w.Header().Set("Content-Type", "text/html; charset=utf-8")
   154  	fmt.Fprintf(w, "<pre>\n")
   155  	for i, n := 0, dirs.len(); i < n; i++ {
   156  		name := dirs.name(i)
   157  		if dirs.isDir(i) {
   158  			name += "/"
   159  		}
   160  		// name may contain '?' or '#', which must be escaped to remain
   161  		// part of the URL path, and not indicate the start of a query
   162  		// string or fragment.
   163  		url := url.URL{Path: name}
   164  		fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
   165  	}
   166  	fmt.Fprintf(w, "</pre>\n")
   167  }
   168  
   169  // ServeContent replies to the request using the content in the
   170  // provided ReadSeeker. The main benefit of ServeContent over io.Copy
   171  // is that it handles Range requests properly, sets the MIME type, and
   172  // handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
   173  // and If-Range requests.
   174  //
   175  // If the response's Content-Type header is not set, ServeContent
   176  // first tries to deduce the type from name's file extension and,
   177  // if that fails, falls back to reading the first block of the content
   178  // and passing it to DetectContentType.
   179  // The name is otherwise unused; in particular it can be empty and is
   180  // never sent in the response.
   181  //
   182  // If modtime is not the zero time or Unix epoch, ServeContent
   183  // includes it in a Last-Modified header in the response. If the
   184  // request includes an If-Modified-Since header, ServeContent uses
   185  // modtime to decide whether the content needs to be sent at all.
   186  //
   187  // The content's Seek method must work: ServeContent uses
   188  // a seek to the end of the content to determine its size.
   189  //
   190  // If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
   191  // ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
   192  //
   193  // Note that *os.File implements the io.ReadSeeker interface.
   194  func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
   195  	sizeFunc := func() (int64, error) {
   196  		size, err := content.Seek(0, io.SeekEnd)
   197  		if err != nil {
   198  			return 0, errSeeker
   199  		}
   200  		_, err = content.Seek(0, io.SeekStart)
   201  		if err != nil {
   202  			return 0, errSeeker
   203  		}
   204  		return size, nil
   205  	}
   206  	serveContent(w, req, name, modtime, sizeFunc, content)
   207  }
   208  
   209  // errSeeker is returned by ServeContent's sizeFunc when the content
   210  // doesn't seek properly. The underlying Seeker's error text isn't
   211  // included in the sizeFunc reply so it's not sent over HTTP to end
   212  // users.
   213  var errSeeker = errors.New("seeker can't seek")
   214  
   215  // errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
   216  // all of the byte-range-spec values is greater than the content size.
   217  var errNoOverlap = errors.New("invalid range: failed to overlap")
   218  
   219  // if name is empty, filename is unknown. (used for mime type, before sniffing)
   220  // if modtime.IsZero(), modtime is unknown.
   221  // content must be seeked to the beginning of the file.
   222  // The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
   223  func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
   224  	setLastModified(w, modtime)
   225  	done, rangeReq := checkPreconditions(w, r, modtime)
   226  	if done {
   227  		return
   228  	}
   229  
   230  	code := StatusOK
   231  
   232  	// If Content-Type isn't set, use the file's extension to find it, but
   233  	// if the Content-Type is unset explicitly, do not sniff the type.
   234  	ctypes, haveType := w.Header()["Content-Type"]
   235  	var ctype string
   236  	if !haveType {
   237  		ctype = mime.TypeByExtension(filepath.Ext(name))
   238  		if ctype == "" {
   239  			// read a chunk to decide between utf-8 text and binary
   240  			var buf [sniffLen]byte
   241  			n, _ := io.ReadFull(content, buf[:])
   242  			ctype = DetectContentType(buf[:n])
   243  			_, err := content.Seek(0, io.SeekStart) // rewind to output whole file
   244  			if err != nil {
   245  				Error(w, "seeker can't seek", StatusInternalServerError)
   246  				return
   247  			}
   248  		}
   249  		w.Header().Set("Content-Type", ctype)
   250  	} else if len(ctypes) > 0 {
   251  		ctype = ctypes[0]
   252  	}
   253  
   254  	size, err := sizeFunc()
   255  	if err != nil {
   256  		Error(w, err.Error(), StatusInternalServerError)
   257  		return
   258  	}
   259  	if size < 0 {
   260  		// Should never happen but just to be sure
   261  		Error(w, "negative content size computed", StatusInternalServerError)
   262  		return
   263  	}
   264  
   265  	// handle Content-Range header.
   266  	sendSize := size
   267  	var sendContent io.Reader = content
   268  	ranges, err := parseRange(rangeReq, size)
   269  	switch err {
   270  	case nil:
   271  	case errNoOverlap:
   272  		if size == 0 {
   273  			// Some clients add a Range header to all requests to
   274  			// limit the size of the response. If the file is empty,
   275  			// ignore the range header and respond with a 200 rather
   276  			// than a 416.
   277  			ranges = nil
   278  			break
   279  		}
   280  		w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
   281  		fallthrough
   282  	default:
   283  		Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   284  		return
   285  	}
   286  
   287  	if sumRangesSize(ranges) > size {
   288  		// The total number of bytes in all the ranges
   289  		// is larger than the size of the file by
   290  		// itself, so this is probably an attack, or a
   291  		// dumb client. Ignore the range request.
   292  		ranges = nil
   293  	}
   294  	switch {
   295  	case len(ranges) == 1:
   296  		// RFC 7233, Section 4.1:
   297  		// "If a single part is being transferred, the server
   298  		// generating the 206 response MUST generate a
   299  		// Content-Range header field, describing what range
   300  		// of the selected representation is enclosed, and a
   301  		// payload consisting of the range.
   302  		// ...
   303  		// A server MUST NOT generate a multipart response to
   304  		// a request for a single range, since a client that
   305  		// does not request multiple parts might not support
   306  		// multipart responses."
   307  		ra := ranges[0]
   308  		if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   309  			Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   310  			return
   311  		}
   312  		sendSize = ra.length
   313  		code = StatusPartialContent
   314  		w.Header().Set("Content-Range", ra.contentRange(size))
   315  	case len(ranges) > 1:
   316  		sendSize = rangesMIMESize(ranges, ctype, size)
   317  		code = StatusPartialContent
   318  
   319  		pr, pw := io.Pipe()
   320  		mw := multipart.NewWriter(pw)
   321  		w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
   322  		sendContent = pr
   323  		defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
   324  		go func() {
   325  			for _, ra := range ranges {
   326  				part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
   327  				if err != nil {
   328  					pw.CloseWithError(err)
   329  					return
   330  				}
   331  				if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   332  					pw.CloseWithError(err)
   333  					return
   334  				}
   335  				if _, err := io.CopyN(part, content, ra.length); err != nil {
   336  					pw.CloseWithError(err)
   337  					return
   338  				}
   339  			}
   340  			mw.Close()
   341  			pw.Close()
   342  		}()
   343  	}
   344  
   345  	w.Header().Set("Accept-Ranges", "bytes")
   346  	if w.Header().Get("Content-Encoding") == "" {
   347  		w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
   348  	}
   349  
   350  	w.WriteHeader(code)
   351  
   352  	if r.Method != "HEAD" {
   353  		io.CopyN(w, sendContent, sendSize)
   354  	}
   355  }
   356  
   357  // scanETag determines if a syntactically valid ETag is present at s. If so,
   358  // the ETag and remaining text after consuming ETag is returned. Otherwise,
   359  // it returns "", "".
   360  func scanETag(s string) (etag string, remain string) {
   361  	s = textproto.TrimString(s)
   362  	start := 0
   363  	if strings.HasPrefix(s, "W/") {
   364  		start = 2
   365  	}
   366  	if len(s[start:]) < 2 || s[start] != '"' {
   367  		return "", ""
   368  	}
   369  	// ETag is either W/"text" or "text".
   370  	// See RFC 7232 2.3.
   371  	for i := start + 1; i < len(s); i++ {
   372  		c := s[i]
   373  		switch {
   374  		// Character values allowed in ETags.
   375  		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
   376  		case c == '"':
   377  			return s[:i+1], s[i+1:]
   378  		default:
   379  			return "", ""
   380  		}
   381  	}
   382  	return "", ""
   383  }
   384  
   385  // etagStrongMatch reports whether a and b match using strong ETag comparison.
   386  // Assumes a and b are valid ETags.
   387  func etagStrongMatch(a, b string) bool {
   388  	return a == b && a != "" && a[0] == '"'
   389  }
   390  
   391  // etagWeakMatch reports whether a and b match using weak ETag comparison.
   392  // Assumes a and b are valid ETags.
   393  func etagWeakMatch(a, b string) bool {
   394  	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
   395  }
   396  
   397  // condResult is the result of an HTTP request precondition check.
   398  // See https://tools.ietf.org/html/rfc7232 section 3.
   399  type condResult int
   400  
   401  const (
   402  	condNone condResult = iota
   403  	condTrue
   404  	condFalse
   405  )
   406  
   407  func checkIfMatch(w ResponseWriter, r *Request) condResult {
   408  	im := r.Header.Get("If-Match")
   409  	if im == "" {
   410  		return condNone
   411  	}
   412  	for {
   413  		im = textproto.TrimString(im)
   414  		if len(im) == 0 {
   415  			break
   416  		}
   417  		if im[0] == ',' {
   418  			im = im[1:]
   419  			continue
   420  		}
   421  		if im[0] == '*' {
   422  			return condTrue
   423  		}
   424  		etag, remain := scanETag(im)
   425  		if etag == "" {
   426  			break
   427  		}
   428  		if etagStrongMatch(etag, w.Header().get("Etag")) {
   429  			return condTrue
   430  		}
   431  		im = remain
   432  	}
   433  
   434  	return condFalse
   435  }
   436  
   437  func checkIfUnmodifiedSince(r *Request, modtime time.Time) condResult {
   438  	ius := r.Header.Get("If-Unmodified-Since")
   439  	if ius == "" || isZeroTime(modtime) {
   440  		return condNone
   441  	}
   442  	t, err := ParseTime(ius)
   443  	if err != nil {
   444  		return condNone
   445  	}
   446  
   447  	// The Last-Modified header truncates sub-second precision so
   448  	// the modtime needs to be truncated too.
   449  	modtime = modtime.Truncate(time.Second)
   450  	if ret := modtime.Compare(t); ret <= 0 {
   451  		return condTrue
   452  	}
   453  	return condFalse
   454  }
   455  
   456  func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
   457  	inm := r.Header.get("If-None-Match")
   458  	if inm == "" {
   459  		return condNone
   460  	}
   461  	buf := inm
   462  	for {
   463  		buf = textproto.TrimString(buf)
   464  		if len(buf) == 0 {
   465  			break
   466  		}
   467  		if buf[0] == ',' {
   468  			buf = buf[1:]
   469  			continue
   470  		}
   471  		if buf[0] == '*' {
   472  			return condFalse
   473  		}
   474  		etag, remain := scanETag(buf)
   475  		if etag == "" {
   476  			break
   477  		}
   478  		if etagWeakMatch(etag, w.Header().get("Etag")) {
   479  			return condFalse
   480  		}
   481  		buf = remain
   482  	}
   483  	return condTrue
   484  }
   485  
   486  func checkIfModifiedSince(r *Request, modtime time.Time) condResult {
   487  	if r.Method != "GET" && r.Method != "HEAD" {
   488  		return condNone
   489  	}
   490  	ims := r.Header.Get("If-Modified-Since")
   491  	if ims == "" || isZeroTime(modtime) {
   492  		return condNone
   493  	}
   494  	t, err := ParseTime(ims)
   495  	if err != nil {
   496  		return condNone
   497  	}
   498  	// The Last-Modified header truncates sub-second precision so
   499  	// the modtime needs to be truncated too.
   500  	modtime = modtime.Truncate(time.Second)
   501  	if ret := modtime.Compare(t); ret <= 0 {
   502  		return condFalse
   503  	}
   504  	return condTrue
   505  }
   506  
   507  func checkIfRange(w ResponseWriter, r *Request, modtime time.Time) condResult {
   508  	if r.Method != "GET" && r.Method != "HEAD" {
   509  		return condNone
   510  	}
   511  	ir := r.Header.get("If-Range")
   512  	if ir == "" {
   513  		return condNone
   514  	}
   515  	etag, _ := scanETag(ir)
   516  	if etag != "" {
   517  		if etagStrongMatch(etag, w.Header().Get("Etag")) {
   518  			return condTrue
   519  		} else {
   520  			return condFalse
   521  		}
   522  	}
   523  	// The If-Range value is typically the ETag value, but it may also be
   524  	// the modtime date. See golang.org/issue/8367.
   525  	if modtime.IsZero() {
   526  		return condFalse
   527  	}
   528  	t, err := ParseTime(ir)
   529  	if err != nil {
   530  		return condFalse
   531  	}
   532  	if t.Unix() == modtime.Unix() {
   533  		return condTrue
   534  	}
   535  	return condFalse
   536  }
   537  
   538  var unixEpochTime = time.Unix(0, 0)
   539  
   540  // isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
   541  func isZeroTime(t time.Time) bool {
   542  	return t.IsZero() || t.Equal(unixEpochTime)
   543  }
   544  
   545  func setLastModified(w ResponseWriter, modtime time.Time) {
   546  	if !isZeroTime(modtime) {
   547  		w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
   548  	}
   549  }
   550  
   551  func writeNotModified(w ResponseWriter) {
   552  	// RFC 7232 section 4.1:
   553  	// a sender SHOULD NOT generate representation metadata other than the
   554  	// above listed fields unless said metadata exists for the purpose of
   555  	// guiding cache updates (e.g., Last-Modified might be useful if the
   556  	// response does not have an ETag field).
   557  	h := w.Header()
   558  	delete(h, "Content-Type")
   559  	delete(h, "Content-Length")
   560  	delete(h, "Content-Encoding")
   561  	if h.Get("Etag") != "" {
   562  		delete(h, "Last-Modified")
   563  	}
   564  	w.WriteHeader(StatusNotModified)
   565  }
   566  
   567  // checkPreconditions evaluates request preconditions and reports whether a precondition
   568  // resulted in sending StatusNotModified or StatusPreconditionFailed.
   569  func checkPreconditions(w ResponseWriter, r *Request, modtime time.Time) (done bool, rangeHeader string) {
   570  	// This function carefully follows RFC 7232 section 6.
   571  	ch := checkIfMatch(w, r)
   572  	if ch == condNone {
   573  		ch = checkIfUnmodifiedSince(r, modtime)
   574  	}
   575  	if ch == condFalse {
   576  		w.WriteHeader(StatusPreconditionFailed)
   577  		return true, ""
   578  	}
   579  	switch checkIfNoneMatch(w, r) {
   580  	case condFalse:
   581  		if r.Method == "GET" || r.Method == "HEAD" {
   582  			writeNotModified(w)
   583  			return true, ""
   584  		} else {
   585  			w.WriteHeader(StatusPreconditionFailed)
   586  			return true, ""
   587  		}
   588  	case condNone:
   589  		if checkIfModifiedSince(r, modtime) == condFalse {
   590  			writeNotModified(w)
   591  			return true, ""
   592  		}
   593  	}
   594  
   595  	rangeHeader = r.Header.get("Range")
   596  	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
   597  		rangeHeader = ""
   598  	}
   599  	return false, rangeHeader
   600  }
   601  
   602  // name is '/'-separated, not filepath.Separator.
   603  func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
   604  	const indexPage = "/index.html"
   605  
   606  	// redirect .../index.html to .../
   607  	// can't use Redirect() because that would make the path absolute,
   608  	// which would be a problem running under StripPrefix
   609  	if strings.HasSuffix(r.URL.Path, indexPage) {
   610  		localRedirect(w, r, "./")
   611  		return
   612  	}
   613  
   614  	f, err := fs.Open(name)
   615  	if err != nil {
   616  		msg, code := toHTTPError(err)
   617  		Error(w, msg, code)
   618  		return
   619  	}
   620  	defer f.Close()
   621  
   622  	d, err := f.Stat()
   623  	if err != nil {
   624  		msg, code := toHTTPError(err)
   625  		Error(w, msg, code)
   626  		return
   627  	}
   628  
   629  	if redirect {
   630  		// redirect to canonical path: / at end of directory url
   631  		// r.URL.Path always begins with /
   632  		url := r.URL.Path
   633  		if d.IsDir() {
   634  			if url[len(url)-1] != '/' {
   635  				localRedirect(w, r, path.Base(url)+"/")
   636  				return
   637  			}
   638  		} else {
   639  			if url[len(url)-1] == '/' {
   640  				localRedirect(w, r, "../"+path.Base(url))
   641  				return
   642  			}
   643  		}
   644  	}
   645  
   646  	if d.IsDir() {
   647  		url := r.URL.Path
   648  		// redirect if the directory name doesn't end in a slash
   649  		if url == "" || url[len(url)-1] != '/' {
   650  			localRedirect(w, r, path.Base(url)+"/")
   651  			return
   652  		}
   653  
   654  		// use contents of index.html for directory, if present
   655  		index := strings.TrimSuffix(name, "/") + indexPage
   656  		ff, err := fs.Open(index)
   657  		if err == nil {
   658  			defer ff.Close()
   659  			dd, err := ff.Stat()
   660  			if err == nil {
   661  				d = dd
   662  				f = ff
   663  			}
   664  		}
   665  	}
   666  
   667  	// Still a directory? (we didn't find an index.html file)
   668  	if d.IsDir() {
   669  		if checkIfModifiedSince(r, d.ModTime()) == condFalse {
   670  			writeNotModified(w)
   671  			return
   672  		}
   673  		setLastModified(w, d.ModTime())
   674  		dirList(w, r, f)
   675  		return
   676  	}
   677  
   678  	// serveContent will check modification time
   679  	sizeFunc := func() (int64, error) { return d.Size(), nil }
   680  	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
   681  }
   682  
   683  // toHTTPError returns a non-specific HTTP error message and status code
   684  // for a given non-nil error value. It's important that toHTTPError does not
   685  // actually return err.Error(), since msg and httpStatus are returned to users,
   686  // and historically Go's ServeContent always returned just "404 Not Found" for
   687  // all errors. We don't want to start leaking information in error messages.
   688  func toHTTPError(err error) (msg string, httpStatus int) {
   689  	if errors.Is(err, fs.ErrNotExist) {
   690  		return "404 page not found", StatusNotFound
   691  	}
   692  	if errors.Is(err, fs.ErrPermission) {
   693  		return "403 Forbidden", StatusForbidden
   694  	}
   695  	// Default:
   696  	return "500 Internal Server Error", StatusInternalServerError
   697  }
   698  
   699  // localRedirect gives a Moved Permanently response.
   700  // It does not convert relative paths to absolute paths like Redirect does.
   701  func localRedirect(w ResponseWriter, r *Request, newPath string) {
   702  	if q := r.URL.RawQuery; q != "" {
   703  		newPath += "?" + q
   704  	}
   705  	w.Header().Set("Location", newPath)
   706  	w.WriteHeader(StatusMovedPermanently)
   707  }
   708  
   709  // ServeFile replies to the request with the contents of the named
   710  // file or directory.
   711  //
   712  // If the provided file or directory name is a relative path, it is
   713  // interpreted relative to the current directory and may ascend to
   714  // parent directories. If the provided name is constructed from user
   715  // input, it should be sanitized before calling ServeFile.
   716  //
   717  // As a precaution, ServeFile will reject requests where r.URL.Path
   718  // contains a ".." path element; this protects against callers who
   719  // might unsafely use filepath.Join on r.URL.Path without sanitizing
   720  // it and then use that filepath.Join result as the name argument.
   721  //
   722  // As another special case, ServeFile redirects any request where r.URL.Path
   723  // ends in "/index.html" to the same path, without the final
   724  // "index.html". To avoid such redirects either modify the path or
   725  // use ServeContent.
   726  //
   727  // Outside of those two special cases, ServeFile does not use
   728  // r.URL.Path for selecting the file or directory to serve; only the
   729  // file or directory provided in the name argument is used.
   730  func ServeFile(w ResponseWriter, r *Request, name string) {
   731  	if containsDotDot(r.URL.Path) {
   732  		// Too many programs use r.URL.Path to construct the argument to
   733  		// serveFile. Reject the request under the assumption that happened
   734  		// here and ".." may not be wanted.
   735  		// Note that name might not contain "..", for example if code (still
   736  		// incorrectly) used filepath.Join(myDir, r.URL.Path).
   737  		Error(w, "invalid URL path", StatusBadRequest)
   738  		return
   739  	}
   740  	dir, file := filepath.Split(name)
   741  	serveFile(w, r, Dir(dir), file, false)
   742  }
   743  
   744  func containsDotDot(v string) bool {
   745  	if !strings.Contains(v, "..") {
   746  		return false
   747  	}
   748  	for _, ent := range strings.FieldsFunc(v, isSlashRune) {
   749  		if ent == ".." {
   750  			return true
   751  		}
   752  	}
   753  	return false
   754  }
   755  
   756  func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
   757  
   758  type fileHandler struct {
   759  	root FileSystem
   760  }
   761  
   762  type ioFS struct {
   763  	fsys fs.FS
   764  }
   765  
   766  type ioFile struct {
   767  	file fs.File
   768  }
   769  
   770  func (f ioFS) Open(name string) (File, error) {
   771  	if name == "/" {
   772  		name = "."
   773  	} else {
   774  		name = strings.TrimPrefix(name, "/")
   775  	}
   776  	file, err := f.fsys.Open(name)
   777  	if err != nil {
   778  		return nil, mapOpenError(err, name, '/', func(path string) (fs.FileInfo, error) {
   779  			return fs.Stat(f.fsys, path)
   780  		})
   781  	}
   782  	return ioFile{file}, nil
   783  }
   784  
   785  func (f ioFile) Close() error               { return f.file.Close() }
   786  func (f ioFile) Read(b []byte) (int, error) { return f.file.Read(b) }
   787  func (f ioFile) Stat() (fs.FileInfo, error) { return f.file.Stat() }
   788  
   789  var errMissingSeek = errors.New("io.File missing Seek method")
   790  var errMissingReadDir = errors.New("io.File directory missing ReadDir method")
   791  
   792  func (f ioFile) Seek(offset int64, whence int) (int64, error) {
   793  	s, ok := f.file.(io.Seeker)
   794  	if !ok {
   795  		return 0, errMissingSeek
   796  	}
   797  	return s.Seek(offset, whence)
   798  }
   799  
   800  func (f ioFile) ReadDir(count int) ([]fs.DirEntry, error) {
   801  	d, ok := f.file.(fs.ReadDirFile)
   802  	if !ok {
   803  		return nil, errMissingReadDir
   804  	}
   805  	return d.ReadDir(count)
   806  }
   807  
   808  func (f ioFile) Readdir(count int) ([]fs.FileInfo, error) {
   809  	d, ok := f.file.(fs.ReadDirFile)
   810  	if !ok {
   811  		return nil, errMissingReadDir
   812  	}
   813  	var list []fs.FileInfo
   814  	for {
   815  		dirs, err := d.ReadDir(count - len(list))
   816  		for _, dir := range dirs {
   817  			info, err := dir.Info()
   818  			if err != nil {
   819  				// Pretend it doesn't exist, like (*os.File).Readdir does.
   820  				continue
   821  			}
   822  			list = append(list, info)
   823  		}
   824  		if err != nil {
   825  			return list, err
   826  		}
   827  		if count < 0 || len(list) >= count {
   828  			break
   829  		}
   830  	}
   831  	return list, nil
   832  }
   833  
   834  // FS converts fsys to a FileSystem implementation,
   835  // for use with FileServer and NewFileTransport.
   836  // The files provided by fsys must implement io.Seeker.
   837  func FS(fsys fs.FS) FileSystem {
   838  	return ioFS{fsys}
   839  }
   840  
   841  // FileServer returns a handler that serves HTTP requests
   842  // with the contents of the file system rooted at root.
   843  //
   844  // As a special case, the returned file server redirects any request
   845  // ending in "/index.html" to the same path, without the final
   846  // "index.html".
   847  //
   848  // To use the operating system's file system implementation,
   849  // use http.Dir:
   850  //
   851  //	http.Handle("/", http.FileServer(http.Dir("/tmp")))
   852  //
   853  // To use an fs.FS implementation, use http.FS to convert it:
   854  //
   855  //	http.Handle("/", http.FileServer(http.FS(fsys)))
   856  func FileServer(root FileSystem) Handler {
   857  	return &fileHandler{root}
   858  }
   859  
   860  func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
   861  	const options = MethodOptions + ", " + MethodGet + ", " + MethodHead
   862  
   863  	switch r.Method {
   864  	case MethodGet, MethodHead:
   865  		if !strings.HasPrefix(r.URL.Path, "/") {
   866  			r.URL.Path = "/" + r.URL.Path
   867  		}
   868  		serveFile(w, r, f.root, path.Clean(r.URL.Path), true)
   869  
   870  	case MethodOptions:
   871  		w.Header().Set("Allow", options)
   872  
   873  	default:
   874  		w.Header().Set("Allow", options)
   875  		Error(w, "read-only", StatusMethodNotAllowed)
   876  	}
   877  }
   878  
   879  // httpRange specifies the byte range to be sent to the client.
   880  type httpRange struct {
   881  	start, length int64
   882  }
   883  
   884  func (r httpRange) contentRange(size int64) string {
   885  	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
   886  }
   887  
   888  func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
   889  	return textproto.MIMEHeader{
   890  		"Content-Range": {r.contentRange(size)},
   891  		"Content-Type":  {contentType},
   892  	}
   893  }
   894  
   895  // parseRange parses a Range header string as per RFC 7233.
   896  // errNoOverlap is returned if none of the ranges overlap.
   897  func parseRange(s string, size int64) ([]httpRange, error) {
   898  	if s == "" {
   899  		return nil, nil // header not present
   900  	}
   901  	const b = "bytes="
   902  	if !strings.HasPrefix(s, b) {
   903  		return nil, errors.New("invalid range")
   904  	}
   905  	var ranges []httpRange
   906  	noOverlap := false
   907  	for _, ra := range strings.Split(s[len(b):], ",") {
   908  		ra = textproto.TrimString(ra)
   909  		if ra == "" {
   910  			continue
   911  		}
   912  		start, end, ok := strings.Cut(ra, "-")
   913  		if !ok {
   914  			return nil, errors.New("invalid range")
   915  		}
   916  		start, end = textproto.TrimString(start), textproto.TrimString(end)
   917  		var r httpRange
   918  		if start == "" {
   919  			// If no start is specified, end specifies the
   920  			// range start relative to the end of the file,
   921  			// and we are dealing with <suffix-length>
   922  			// which has to be a non-negative integer as per
   923  			// RFC 7233 Section 2.1 "Byte-Ranges".
   924  			if end == "" || end[0] == '-' {
   925  				return nil, errors.New("invalid range")
   926  			}
   927  			i, err := strconv.ParseInt(end, 10, 64)
   928  			if i < 0 || err != nil {
   929  				return nil, errors.New("invalid range")
   930  			}
   931  			if i > size {
   932  				i = size
   933  			}
   934  			r.start = size - i
   935  			r.length = size - r.start
   936  		} else {
   937  			i, err := strconv.ParseInt(start, 10, 64)
   938  			if err != nil || i < 0 {
   939  				return nil, errors.New("invalid range")
   940  			}
   941  			if i >= size {
   942  				// If the range begins after the size of the content,
   943  				// then it does not overlap.
   944  				noOverlap = true
   945  				continue
   946  			}
   947  			r.start = i
   948  			if end == "" {
   949  				// If no end is specified, range extends to end of the file.
   950  				r.length = size - r.start
   951  			} else {
   952  				i, err := strconv.ParseInt(end, 10, 64)
   953  				if err != nil || r.start > i {
   954  					return nil, errors.New("invalid range")
   955  				}
   956  				if i >= size {
   957  					i = size - 1
   958  				}
   959  				r.length = i - r.start + 1
   960  			}
   961  		}
   962  		ranges = append(ranges, r)
   963  	}
   964  	if noOverlap && len(ranges) == 0 {
   965  		// The specified ranges did not overlap with the content.
   966  		return nil, errNoOverlap
   967  	}
   968  	return ranges, nil
   969  }
   970  
   971  // countingWriter counts how many bytes have been written to it.
   972  type countingWriter int64
   973  
   974  func (w *countingWriter) Write(p []byte) (n int, err error) {
   975  	*w += countingWriter(len(p))
   976  	return len(p), nil
   977  }
   978  
   979  // rangesMIMESize returns the number of bytes it takes to encode the
   980  // provided ranges as a multipart response.
   981  func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
   982  	var w countingWriter
   983  	mw := multipart.NewWriter(&w)
   984  	for _, ra := range ranges {
   985  		mw.CreatePart(ra.mimeHeader(contentType, contentSize))
   986  		encSize += ra.length
   987  	}
   988  	mw.Close()
   989  	encSize += int64(w)
   990  	return
   991  }
   992  
   993  func sumRangesSize(ranges []httpRange) (size int64) {
   994  	for _, ra := range ranges {
   995  		size += ra.length
   996  	}
   997  	return
   998  }
   999  

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