Source file src/net/http/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  	"internal/godebug"
    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 directory path 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 := path.Clean("/" + name)[1:]
    74  	if path == "" {
    75  		path = "."
    76  	}
    77  	path, err := filepath.Localize(path)
    78  	if err != nil {
    79  		return nil, errors.New("http: invalid or unsafe file path")
    80  	}
    81  	dir := string(d)
    82  	if dir == "" {
    83  		dir = "."
    84  	}
    85  	fullName := filepath.Join(dir, path)
    86  	f, err := os.Open(fullName)
    87  	if err != nil {
    88  		return nil, mapOpenError(err, fullName, filepath.Separator, os.Stat)
    89  	}
    90  	return f, nil
    91  }
    92  
    93  // A FileSystem implements access to a collection of named files.
    94  // The elements in a file path are separated by slash ('/', U+002F)
    95  // characters, regardless of host operating system convention.
    96  // See the [FileServer] function to convert a FileSystem to a [Handler].
    97  //
    98  // This interface predates the [fs.FS] interface, which can be used instead:
    99  // the [FS] adapter function converts an fs.FS to a FileSystem.
   100  type FileSystem interface {
   101  	Open(name string) (File, error)
   102  }
   103  
   104  // A File is returned by a [FileSystem]'s Open method and can be
   105  // served by the [FileServer] implementation.
   106  //
   107  // The methods should behave the same as those on an [*os.File].
   108  type File interface {
   109  	io.Closer
   110  	io.Reader
   111  	io.Seeker
   112  	Readdir(count int) ([]fs.FileInfo, error)
   113  	Stat() (fs.FileInfo, error)
   114  }
   115  
   116  type anyDirs interface {
   117  	len() int
   118  	name(i int) string
   119  	isDir(i int) bool
   120  }
   121  
   122  type fileInfoDirs []fs.FileInfo
   123  
   124  func (d fileInfoDirs) len() int          { return len(d) }
   125  func (d fileInfoDirs) isDir(i int) bool  { return d[i].IsDir() }
   126  func (d fileInfoDirs) name(i int) string { return d[i].Name() }
   127  
   128  type dirEntryDirs []fs.DirEntry
   129  
   130  func (d dirEntryDirs) len() int          { return len(d) }
   131  func (d dirEntryDirs) isDir(i int) bool  { return d[i].IsDir() }
   132  func (d dirEntryDirs) name(i int) string { return d[i].Name() }
   133  
   134  func dirList(w ResponseWriter, r *Request, f File) {
   135  	// Prefer to use ReadDir instead of Readdir,
   136  	// because the former doesn't require calling
   137  	// Stat on every entry of a directory on Unix.
   138  	var dirs anyDirs
   139  	var err error
   140  	if d, ok := f.(fs.ReadDirFile); ok {
   141  		var list dirEntryDirs
   142  		list, err = d.ReadDir(-1)
   143  		dirs = list
   144  	} else {
   145  		var list fileInfoDirs
   146  		list, err = f.Readdir(-1)
   147  		dirs = list
   148  	}
   149  
   150  	if err != nil {
   151  		logf(r, "http: error reading directory: %v", err)
   152  		Error(w, "Error reading directory", StatusInternalServerError)
   153  		return
   154  	}
   155  	sort.Slice(dirs, func(i, j int) bool { return dirs.name(i) < dirs.name(j) })
   156  
   157  	w.Header().Set("Content-Type", "text/html; charset=utf-8")
   158  	fmt.Fprintf(w, "<!doctype html>\n")
   159  	fmt.Fprintf(w, "<meta name=\"viewport\" content=\"width=device-width\">\n")
   160  	fmt.Fprintf(w, "<pre>\n")
   161  	for i, n := 0, dirs.len(); i < n; i++ {
   162  		name := dirs.name(i)
   163  		if dirs.isDir(i) {
   164  			name += "/"
   165  		}
   166  		// name may contain '?' or '#', which must be escaped to remain
   167  		// part of the URL path, and not indicate the start of a query
   168  		// string or fragment.
   169  		url := url.URL{Path: name}
   170  		fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
   171  	}
   172  	fmt.Fprintf(w, "</pre>\n")
   173  }
   174  
   175  // GODEBUG=httpservecontentkeepheaders=1 restores the pre-1.23 behavior of not deleting
   176  // Cache-Control, Content-Encoding, Etag, or Last-Modified headers on ServeContent errors.
   177  var httpservecontentkeepheaders = godebug.New("httpservecontentkeepheaders")
   178  
   179  // serveError serves an error from ServeFile, ServeFileFS, and ServeContent.
   180  // Because those can all be configured by the caller by setting headers like
   181  // Etag, Last-Modified, and Cache-Control to send on a successful response,
   182  // the error path needs to clear them, since they may not be meant for errors.
   183  func serveError(w ResponseWriter, text string, code int) {
   184  	h := w.Header()
   185  
   186  	nonDefault := false
   187  	for _, k := range []string{
   188  		"Cache-Control",
   189  		"Content-Encoding",
   190  		"Etag",
   191  		"Last-Modified",
   192  	} {
   193  		if !h.has(k) {
   194  			continue
   195  		}
   196  		if httpservecontentkeepheaders.Value() == "1" {
   197  			nonDefault = true
   198  		} else {
   199  			h.Del(k)
   200  		}
   201  	}
   202  	if nonDefault {
   203  		httpservecontentkeepheaders.IncNonDefault()
   204  	}
   205  
   206  	Error(w, text, code)
   207  }
   208  
   209  // ServeContent replies to the request using the content in the
   210  // provided ReadSeeker. The main benefit of ServeContent over [io.Copy]
   211  // is that it handles Range requests properly, sets the MIME type, and
   212  // handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
   213  // and If-Range requests.
   214  //
   215  // If the response's Content-Type header is not set, ServeContent
   216  // first tries to deduce the type from name's file extension and,
   217  // if that fails, falls back to reading the first block of the content
   218  // and passing it to [DetectContentType].
   219  // The name is otherwise unused; in particular it can be empty and is
   220  // never sent in the response.
   221  //
   222  // If modtime is not the zero time or Unix epoch, ServeContent
   223  // includes it in a Last-Modified header in the response. If the
   224  // request includes an If-Modified-Since header, ServeContent uses
   225  // modtime to decide whether the content needs to be sent at all.
   226  //
   227  // The content's Seek method must work: ServeContent uses
   228  // a seek to the end of the content to determine its size.
   229  // Note that [*os.File] implements the [io.ReadSeeker] interface.
   230  //
   231  // If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
   232  // ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
   233  //
   234  // If an error occurs when serving the request (for example, when
   235  // handling an invalid range request), ServeContent responds with an
   236  // error message. By default, ServeContent strips the Cache-Control,
   237  // Content-Encoding, ETag, and Last-Modified headers from error responses.
   238  // The GODEBUG setting httpservecontentkeepheaders=1 causes ServeContent
   239  // to preserve these headers.
   240  func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
   241  	sizeFunc := func() (int64, error) {
   242  		size, err := content.Seek(0, io.SeekEnd)
   243  		if err != nil {
   244  			return 0, errSeeker
   245  		}
   246  		_, err = content.Seek(0, io.SeekStart)
   247  		if err != nil {
   248  			return 0, errSeeker
   249  		}
   250  		return size, nil
   251  	}
   252  	serveContent(w, req, name, modtime, sizeFunc, content)
   253  }
   254  
   255  // errSeeker is returned by ServeContent's sizeFunc when the content
   256  // doesn't seek properly. The underlying Seeker's error text isn't
   257  // included in the sizeFunc reply so it's not sent over HTTP to end
   258  // users.
   259  var errSeeker = errors.New("seeker can't seek")
   260  
   261  // errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
   262  // all of the byte-range-spec values is greater than the content size.
   263  var errNoOverlap = errors.New("invalid range: failed to overlap")
   264  
   265  // if name is empty, filename is unknown. (used for mime type, before sniffing)
   266  // if modtime.IsZero(), modtime is unknown.
   267  // content must be seeked to the beginning of the file.
   268  // The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
   269  func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
   270  	setLastModified(w, modtime)
   271  	done, rangeReq := checkPreconditions(w, r, modtime)
   272  	if done {
   273  		return
   274  	}
   275  
   276  	code := StatusOK
   277  
   278  	// If Content-Type isn't set, use the file's extension to find it, but
   279  	// if the Content-Type is unset explicitly, do not sniff the type.
   280  	ctypes, haveType := w.Header()["Content-Type"]
   281  	var ctype string
   282  	if !haveType {
   283  		ctype = mime.TypeByExtension(filepath.Ext(name))
   284  		if ctype == "" {
   285  			// read a chunk to decide between utf-8 text and binary
   286  			var buf [sniffLen]byte
   287  			n, _ := io.ReadFull(content, buf[:])
   288  			ctype = DetectContentType(buf[:n])
   289  			_, err := content.Seek(0, io.SeekStart) // rewind to output whole file
   290  			if err != nil {
   291  				serveError(w, "seeker can't seek", StatusInternalServerError)
   292  				return
   293  			}
   294  		}
   295  		w.Header().Set("Content-Type", ctype)
   296  	} else if len(ctypes) > 0 {
   297  		ctype = ctypes[0]
   298  	}
   299  
   300  	size, err := sizeFunc()
   301  	if err != nil {
   302  		serveError(w, err.Error(), StatusInternalServerError)
   303  		return
   304  	}
   305  	if size < 0 {
   306  		// Should never happen but just to be sure
   307  		serveError(w, "negative content size computed", StatusInternalServerError)
   308  		return
   309  	}
   310  
   311  	// handle Content-Range header.
   312  	sendSize := size
   313  	var sendContent io.Reader = content
   314  	ranges, err := parseRange(rangeReq, size)
   315  	switch err {
   316  	case nil:
   317  	case errNoOverlap:
   318  		if size == 0 {
   319  			// Some clients add a Range header to all requests to
   320  			// limit the size of the response. If the file is empty,
   321  			// ignore the range header and respond with a 200 rather
   322  			// than a 416.
   323  			ranges = nil
   324  			break
   325  		}
   326  		w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
   327  		fallthrough
   328  	default:
   329  		serveError(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   330  		return
   331  	}
   332  
   333  	if sumRangesSize(ranges) > size {
   334  		// The total number of bytes in all the ranges
   335  		// is larger than the size of the file by
   336  		// itself, so this is probably an attack, or a
   337  		// dumb client. Ignore the range request.
   338  		ranges = nil
   339  	}
   340  	switch {
   341  	case len(ranges) == 1:
   342  		// RFC 7233, Section 4.1:
   343  		// "If a single part is being transferred, the server
   344  		// generating the 206 response MUST generate a
   345  		// Content-Range header field, describing what range
   346  		// of the selected representation is enclosed, and a
   347  		// payload consisting of the range.
   348  		// ...
   349  		// A server MUST NOT generate a multipart response to
   350  		// a request for a single range, since a client that
   351  		// does not request multiple parts might not support
   352  		// multipart responses."
   353  		ra := ranges[0]
   354  		if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   355  			serveError(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   356  			return
   357  		}
   358  		sendSize = ra.length
   359  		code = StatusPartialContent
   360  		w.Header().Set("Content-Range", ra.contentRange(size))
   361  	case len(ranges) > 1:
   362  		sendSize = rangesMIMESize(ranges, ctype, size)
   363  		code = StatusPartialContent
   364  
   365  		pr, pw := io.Pipe()
   366  		mw := multipart.NewWriter(pw)
   367  		w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
   368  		sendContent = pr
   369  		defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
   370  		go func() {
   371  			for _, ra := range ranges {
   372  				part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
   373  				if err != nil {
   374  					pw.CloseWithError(err)
   375  					return
   376  				}
   377  				if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   378  					pw.CloseWithError(err)
   379  					return
   380  				}
   381  				if _, err := io.CopyN(part, content, ra.length); err != nil {
   382  					pw.CloseWithError(err)
   383  					return
   384  				}
   385  			}
   386  			mw.Close()
   387  			pw.Close()
   388  		}()
   389  	}
   390  
   391  	w.Header().Set("Accept-Ranges", "bytes")
   392  
   393  	// We should be able to unconditionally set the Content-Length here.
   394  	//
   395  	// However, there is a pattern observed in the wild that this breaks:
   396  	// The user wraps the ResponseWriter in one which gzips data written to it,
   397  	// and sets "Content-Encoding: gzip".
   398  	//
   399  	// The user shouldn't be doing this; the serveContent path here depends
   400  	// on serving seekable data with a known length. If you want to compress
   401  	// on the fly, then you shouldn't be using ServeFile/ServeContent, or
   402  	// you should compress the entire file up-front and provide a seekable
   403  	// view of the compressed data.
   404  	//
   405  	// However, since we've observed this pattern in the wild, and since
   406  	// setting Content-Length here breaks code that mostly-works today,
   407  	// skip setting Content-Length if the user set Content-Encoding.
   408  	//
   409  	// If this is a range request, always set Content-Length.
   410  	// If the user isn't changing the bytes sent in the ResponseWrite,
   411  	// the Content-Length will be correct.
   412  	// If the user is changing the bytes sent, then the range request wasn't
   413  	// going to work properly anyway and we aren't worse off.
   414  	//
   415  	// A possible future improvement on this might be to look at the type
   416  	// of the ResponseWriter, and always set Content-Length if it's one
   417  	// that we recognize.
   418  	if len(ranges) > 0 || w.Header().Get("Content-Encoding") == "" {
   419  		w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
   420  	}
   421  	w.WriteHeader(code)
   422  
   423  	if r.Method != "HEAD" {
   424  		io.CopyN(w, sendContent, sendSize)
   425  	}
   426  }
   427  
   428  // scanETag determines if a syntactically valid ETag is present at s. If so,
   429  // the ETag and remaining text after consuming ETag is returned. Otherwise,
   430  // it returns "", "".
   431  func scanETag(s string) (etag string, remain string) {
   432  	s = textproto.TrimString(s)
   433  	start := 0
   434  	if strings.HasPrefix(s, "W/") {
   435  		start = 2
   436  	}
   437  	if len(s[start:]) < 2 || s[start] != '"' {
   438  		return "", ""
   439  	}
   440  	// ETag is either W/"text" or "text".
   441  	// See RFC 7232 2.3.
   442  	for i := start + 1; i < len(s); i++ {
   443  		c := s[i]
   444  		switch {
   445  		// Character values allowed in ETags.
   446  		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
   447  		case c == '"':
   448  			return s[:i+1], s[i+1:]
   449  		default:
   450  			return "", ""
   451  		}
   452  	}
   453  	return "", ""
   454  }
   455  
   456  // etagStrongMatch reports whether a and b match using strong ETag comparison.
   457  // Assumes a and b are valid ETags.
   458  func etagStrongMatch(a, b string) bool {
   459  	return a == b && a != "" && a[0] == '"'
   460  }
   461  
   462  // etagWeakMatch reports whether a and b match using weak ETag comparison.
   463  // Assumes a and b are valid ETags.
   464  func etagWeakMatch(a, b string) bool {
   465  	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
   466  }
   467  
   468  // condResult is the result of an HTTP request precondition check.
   469  // See https://tools.ietf.org/html/rfc7232 section 3.
   470  type condResult int
   471  
   472  const (
   473  	condNone condResult = iota
   474  	condTrue
   475  	condFalse
   476  )
   477  
   478  func checkIfMatch(w ResponseWriter, r *Request) condResult {
   479  	im := r.Header.Get("If-Match")
   480  	if im == "" {
   481  		return condNone
   482  	}
   483  	for {
   484  		im = textproto.TrimString(im)
   485  		if len(im) == 0 {
   486  			break
   487  		}
   488  		if im[0] == ',' {
   489  			im = im[1:]
   490  			continue
   491  		}
   492  		if im[0] == '*' {
   493  			return condTrue
   494  		}
   495  		etag, remain := scanETag(im)
   496  		if etag == "" {
   497  			break
   498  		}
   499  		if etagStrongMatch(etag, w.Header().get("Etag")) {
   500  			return condTrue
   501  		}
   502  		im = remain
   503  	}
   504  
   505  	return condFalse
   506  }
   507  
   508  func checkIfUnmodifiedSince(r *Request, modtime time.Time) condResult {
   509  	ius := r.Header.Get("If-Unmodified-Since")
   510  	if ius == "" || isZeroTime(modtime) {
   511  		return condNone
   512  	}
   513  	t, err := ParseTime(ius)
   514  	if err != nil {
   515  		return condNone
   516  	}
   517  
   518  	// The Last-Modified header truncates sub-second precision so
   519  	// the modtime needs to be truncated too.
   520  	modtime = modtime.Truncate(time.Second)
   521  	if ret := modtime.Compare(t); ret <= 0 {
   522  		return condTrue
   523  	}
   524  	return condFalse
   525  }
   526  
   527  func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
   528  	inm := r.Header.get("If-None-Match")
   529  	if inm == "" {
   530  		return condNone
   531  	}
   532  	buf := inm
   533  	for {
   534  		buf = textproto.TrimString(buf)
   535  		if len(buf) == 0 {
   536  			break
   537  		}
   538  		if buf[0] == ',' {
   539  			buf = buf[1:]
   540  			continue
   541  		}
   542  		if buf[0] == '*' {
   543  			return condFalse
   544  		}
   545  		etag, remain := scanETag(buf)
   546  		if etag == "" {
   547  			break
   548  		}
   549  		if etagWeakMatch(etag, w.Header().get("Etag")) {
   550  			return condFalse
   551  		}
   552  		buf = remain
   553  	}
   554  	return condTrue
   555  }
   556  
   557  func checkIfModifiedSince(r *Request, modtime time.Time) condResult {
   558  	if r.Method != "GET" && r.Method != "HEAD" {
   559  		return condNone
   560  	}
   561  	ims := r.Header.Get("If-Modified-Since")
   562  	if ims == "" || isZeroTime(modtime) {
   563  		return condNone
   564  	}
   565  	t, err := ParseTime(ims)
   566  	if err != nil {
   567  		return condNone
   568  	}
   569  	// The Last-Modified header truncates sub-second precision so
   570  	// the modtime needs to be truncated too.
   571  	modtime = modtime.Truncate(time.Second)
   572  	if ret := modtime.Compare(t); ret <= 0 {
   573  		return condFalse
   574  	}
   575  	return condTrue
   576  }
   577  
   578  func checkIfRange(w ResponseWriter, r *Request, modtime time.Time) condResult {
   579  	if r.Method != "GET" && r.Method != "HEAD" {
   580  		return condNone
   581  	}
   582  	ir := r.Header.get("If-Range")
   583  	if ir == "" {
   584  		return condNone
   585  	}
   586  	etag, _ := scanETag(ir)
   587  	if etag != "" {
   588  		if etagStrongMatch(etag, w.Header().Get("Etag")) {
   589  			return condTrue
   590  		} else {
   591  			return condFalse
   592  		}
   593  	}
   594  	// The If-Range value is typically the ETag value, but it may also be
   595  	// the modtime date. See golang.org/issue/8367.
   596  	if modtime.IsZero() {
   597  		return condFalse
   598  	}
   599  	t, err := ParseTime(ir)
   600  	if err != nil {
   601  		return condFalse
   602  	}
   603  	if t.Unix() == modtime.Unix() {
   604  		return condTrue
   605  	}
   606  	return condFalse
   607  }
   608  
   609  var unixEpochTime = time.Unix(0, 0)
   610  
   611  // isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
   612  func isZeroTime(t time.Time) bool {
   613  	return t.IsZero() || t.Equal(unixEpochTime)
   614  }
   615  
   616  func setLastModified(w ResponseWriter, modtime time.Time) {
   617  	if !isZeroTime(modtime) {
   618  		w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
   619  	}
   620  }
   621  
   622  func writeNotModified(w ResponseWriter) {
   623  	// RFC 7232 section 4.1:
   624  	// a sender SHOULD NOT generate representation metadata other than the
   625  	// above listed fields unless said metadata exists for the purpose of
   626  	// guiding cache updates (e.g., Last-Modified might be useful if the
   627  	// response does not have an ETag field).
   628  	h := w.Header()
   629  	delete(h, "Content-Type")
   630  	delete(h, "Content-Length")
   631  	delete(h, "Content-Encoding")
   632  	if h.Get("Etag") != "" {
   633  		delete(h, "Last-Modified")
   634  	}
   635  	w.WriteHeader(StatusNotModified)
   636  }
   637  
   638  // checkPreconditions evaluates request preconditions and reports whether a precondition
   639  // resulted in sending StatusNotModified or StatusPreconditionFailed.
   640  func checkPreconditions(w ResponseWriter, r *Request, modtime time.Time) (done bool, rangeHeader string) {
   641  	// This function carefully follows RFC 7232 section 6.
   642  	ch := checkIfMatch(w, r)
   643  	if ch == condNone {
   644  		ch = checkIfUnmodifiedSince(r, modtime)
   645  	}
   646  	if ch == condFalse {
   647  		w.WriteHeader(StatusPreconditionFailed)
   648  		return true, ""
   649  	}
   650  	switch checkIfNoneMatch(w, r) {
   651  	case condFalse:
   652  		if r.Method == "GET" || r.Method == "HEAD" {
   653  			writeNotModified(w)
   654  			return true, ""
   655  		} else {
   656  			w.WriteHeader(StatusPreconditionFailed)
   657  			return true, ""
   658  		}
   659  	case condNone:
   660  		if checkIfModifiedSince(r, modtime) == condFalse {
   661  			writeNotModified(w)
   662  			return true, ""
   663  		}
   664  	}
   665  
   666  	rangeHeader = r.Header.get("Range")
   667  	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
   668  		rangeHeader = ""
   669  	}
   670  	return false, rangeHeader
   671  }
   672  
   673  // name is '/'-separated, not filepath.Separator.
   674  func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
   675  	const indexPage = "/index.html"
   676  
   677  	// redirect .../index.html to .../
   678  	// can't use Redirect() because that would make the path absolute,
   679  	// which would be a problem running under StripPrefix
   680  	if strings.HasSuffix(r.URL.Path, indexPage) {
   681  		localRedirect(w, r, "./")
   682  		return
   683  	}
   684  
   685  	f, err := fs.Open(name)
   686  	if err != nil {
   687  		msg, code := toHTTPError(err)
   688  		serveError(w, msg, code)
   689  		return
   690  	}
   691  	defer f.Close()
   692  
   693  	d, err := f.Stat()
   694  	if err != nil {
   695  		msg, code := toHTTPError(err)
   696  		serveError(w, msg, code)
   697  		return
   698  	}
   699  
   700  	if redirect {
   701  		// redirect to canonical path: / at end of directory url
   702  		// r.URL.Path always begins with /
   703  		url := r.URL.Path
   704  		if d.IsDir() {
   705  			if url[len(url)-1] != '/' {
   706  				localRedirect(w, r, path.Base(url)+"/")
   707  				return
   708  			}
   709  		} else if url[len(url)-1] == '/' {
   710  			base := path.Base(url)
   711  			if base == "/" || base == "." {
   712  				// The FileSystem maps a path like "/" or "/./" to a file instead of a directory.
   713  				msg := "http: attempting to traverse a non-directory"
   714  				serveError(w, msg, StatusInternalServerError)
   715  				return
   716  			}
   717  			localRedirect(w, r, "../"+base)
   718  			return
   719  		}
   720  	}
   721  
   722  	if d.IsDir() {
   723  		url := r.URL.Path
   724  		// redirect if the directory name doesn't end in a slash
   725  		if url == "" || url[len(url)-1] != '/' {
   726  			localRedirect(w, r, path.Base(url)+"/")
   727  			return
   728  		}
   729  
   730  		// use contents of index.html for directory, if present
   731  		index := strings.TrimSuffix(name, "/") + indexPage
   732  		ff, err := fs.Open(index)
   733  		if err == nil {
   734  			defer ff.Close()
   735  			dd, err := ff.Stat()
   736  			if err == nil {
   737  				d = dd
   738  				f = ff
   739  			}
   740  		}
   741  	}
   742  
   743  	// Still a directory? (we didn't find an index.html file)
   744  	if d.IsDir() {
   745  		if checkIfModifiedSince(r, d.ModTime()) == condFalse {
   746  			writeNotModified(w)
   747  			return
   748  		}
   749  		setLastModified(w, d.ModTime())
   750  		dirList(w, r, f)
   751  		return
   752  	}
   753  
   754  	// serveContent will check modification time
   755  	sizeFunc := func() (int64, error) { return d.Size(), nil }
   756  	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
   757  }
   758  
   759  // toHTTPError returns a non-specific HTTP error message and status code
   760  // for a given non-nil error value. It's important that toHTTPError does not
   761  // actually return err.Error(), since msg and httpStatus are returned to users,
   762  // and historically Go's ServeContent always returned just "404 Not Found" for
   763  // all errors. We don't want to start leaking information in error messages.
   764  func toHTTPError(err error) (msg string, httpStatus int) {
   765  	if errors.Is(err, fs.ErrNotExist) {
   766  		return "404 page not found", StatusNotFound
   767  	}
   768  	if errors.Is(err, fs.ErrPermission) {
   769  		return "403 Forbidden", StatusForbidden
   770  	}
   771  	// Default:
   772  	return "500 Internal Server Error", StatusInternalServerError
   773  }
   774  
   775  // localRedirect gives a Moved Permanently response.
   776  // It does not convert relative paths to absolute paths like Redirect does.
   777  func localRedirect(w ResponseWriter, r *Request, newPath string) {
   778  	if q := r.URL.RawQuery; q != "" {
   779  		newPath += "?" + q
   780  	}
   781  	w.Header().Set("Location", newPath)
   782  	w.WriteHeader(StatusMovedPermanently)
   783  }
   784  
   785  // ServeFile replies to the request with the contents of the named
   786  // file or directory.
   787  //
   788  // If the provided file or directory name is a relative path, it is
   789  // interpreted relative to the current directory and may ascend to
   790  // parent directories. If the provided name is constructed from user
   791  // input, it should be sanitized before calling [ServeFile].
   792  //
   793  // As a precaution, ServeFile will reject requests where r.URL.Path
   794  // contains a ".." path element; this protects against callers who
   795  // might unsafely use [filepath.Join] on r.URL.Path without sanitizing
   796  // it and then use that filepath.Join result as the name argument.
   797  //
   798  // As another special case, ServeFile redirects any request where r.URL.Path
   799  // ends in "/index.html" to the same path, without the final
   800  // "index.html". To avoid such redirects either modify the path or
   801  // use [ServeContent].
   802  //
   803  // Outside of those two special cases, ServeFile does not use
   804  // r.URL.Path for selecting the file or directory to serve; only the
   805  // file or directory provided in the name argument is used.
   806  func ServeFile(w ResponseWriter, r *Request, name string) {
   807  	if containsDotDot(r.URL.Path) {
   808  		// Too many programs use r.URL.Path to construct the argument to
   809  		// serveFile. Reject the request under the assumption that happened
   810  		// here and ".." may not be wanted.
   811  		// Note that name might not contain "..", for example if code (still
   812  		// incorrectly) used filepath.Join(myDir, r.URL.Path).
   813  		serveError(w, "invalid URL path", StatusBadRequest)
   814  		return
   815  	}
   816  	dir, file := filepath.Split(name)
   817  	serveFile(w, r, Dir(dir), file, false)
   818  }
   819  
   820  // ServeFileFS replies to the request with the contents
   821  // of the named file or directory from the file system fsys.
   822  // The files provided by fsys must implement [io.Seeker].
   823  //
   824  // If the provided name is constructed from user input, it should be
   825  // sanitized before calling [ServeFileFS].
   826  //
   827  // As a precaution, ServeFileFS will reject requests where r.URL.Path
   828  // contains a ".." path element; this protects against callers who
   829  // might unsafely use [filepath.Join] on r.URL.Path without sanitizing
   830  // it and then use that filepath.Join result as the name argument.
   831  //
   832  // As another special case, ServeFileFS redirects any request where r.URL.Path
   833  // ends in "/index.html" to the same path, without the final
   834  // "index.html". To avoid such redirects either modify the path or
   835  // use [ServeContent].
   836  //
   837  // Outside of those two special cases, ServeFileFS does not use
   838  // r.URL.Path for selecting the file or directory to serve; only the
   839  // file or directory provided in the name argument is used.
   840  func ServeFileFS(w ResponseWriter, r *Request, fsys fs.FS, name string) {
   841  	if containsDotDot(r.URL.Path) {
   842  		// Too many programs use r.URL.Path to construct the argument to
   843  		// serveFile. Reject the request under the assumption that happened
   844  		// here and ".." may not be wanted.
   845  		// Note that name might not contain "..", for example if code (still
   846  		// incorrectly) used filepath.Join(myDir, r.URL.Path).
   847  		serveError(w, "invalid URL path", StatusBadRequest)
   848  		return
   849  	}
   850  	serveFile(w, r, FS(fsys), name, false)
   851  }
   852  
   853  func containsDotDot(v string) bool {
   854  	if !strings.Contains(v, "..") {
   855  		return false
   856  	}
   857  	for _, ent := range strings.FieldsFunc(v, isSlashRune) {
   858  		if ent == ".." {
   859  			return true
   860  		}
   861  	}
   862  	return false
   863  }
   864  
   865  func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
   866  
   867  type fileHandler struct {
   868  	root FileSystem
   869  }
   870  
   871  type ioFS struct {
   872  	fsys fs.FS
   873  }
   874  
   875  type ioFile struct {
   876  	file fs.File
   877  }
   878  
   879  func (f ioFS) Open(name string) (File, error) {
   880  	if name == "/" {
   881  		name = "."
   882  	} else {
   883  		name = strings.TrimPrefix(name, "/")
   884  	}
   885  	file, err := f.fsys.Open(name)
   886  	if err != nil {
   887  		return nil, mapOpenError(err, name, '/', func(path string) (fs.FileInfo, error) {
   888  			return fs.Stat(f.fsys, path)
   889  		})
   890  	}
   891  	return ioFile{file}, nil
   892  }
   893  
   894  func (f ioFile) Close() error               { return f.file.Close() }
   895  func (f ioFile) Read(b []byte) (int, error) { return f.file.Read(b) }
   896  func (f ioFile) Stat() (fs.FileInfo, error) { return f.file.Stat() }
   897  
   898  var errMissingSeek = errors.New("io.File missing Seek method")
   899  var errMissingReadDir = errors.New("io.File directory missing ReadDir method")
   900  
   901  func (f ioFile) Seek(offset int64, whence int) (int64, error) {
   902  	s, ok := f.file.(io.Seeker)
   903  	if !ok {
   904  		return 0, errMissingSeek
   905  	}
   906  	return s.Seek(offset, whence)
   907  }
   908  
   909  func (f ioFile) ReadDir(count int) ([]fs.DirEntry, error) {
   910  	d, ok := f.file.(fs.ReadDirFile)
   911  	if !ok {
   912  		return nil, errMissingReadDir
   913  	}
   914  	return d.ReadDir(count)
   915  }
   916  
   917  func (f ioFile) Readdir(count int) ([]fs.FileInfo, error) {
   918  	d, ok := f.file.(fs.ReadDirFile)
   919  	if !ok {
   920  		return nil, errMissingReadDir
   921  	}
   922  	var list []fs.FileInfo
   923  	for {
   924  		dirs, err := d.ReadDir(count - len(list))
   925  		for _, dir := range dirs {
   926  			info, err := dir.Info()
   927  			if err != nil {
   928  				// Pretend it doesn't exist, like (*os.File).Readdir does.
   929  				continue
   930  			}
   931  			list = append(list, info)
   932  		}
   933  		if err != nil {
   934  			return list, err
   935  		}
   936  		if count < 0 || len(list) >= count {
   937  			break
   938  		}
   939  	}
   940  	return list, nil
   941  }
   942  
   943  // FS converts fsys to a [FileSystem] implementation,
   944  // for use with [FileServer] and [NewFileTransport].
   945  // The files provided by fsys must implement [io.Seeker].
   946  func FS(fsys fs.FS) FileSystem {
   947  	return ioFS{fsys}
   948  }
   949  
   950  // FileServer returns a handler that serves HTTP requests
   951  // with the contents of the file system rooted at root.
   952  //
   953  // As a special case, the returned file server redirects any request
   954  // ending in "/index.html" to the same path, without the final
   955  // "index.html".
   956  //
   957  // To use the operating system's file system implementation,
   958  // use [http.Dir]:
   959  //
   960  //	http.Handle("/", http.FileServer(http.Dir("/tmp")))
   961  //
   962  // To use an [fs.FS] implementation, use [http.FileServerFS] instead.
   963  func FileServer(root FileSystem) Handler {
   964  	return &fileHandler{root}
   965  }
   966  
   967  // FileServerFS returns a handler that serves HTTP requests
   968  // with the contents of the file system fsys.
   969  // The files provided by fsys must implement [io.Seeker].
   970  //
   971  // As a special case, the returned file server redirects any request
   972  // ending in "/index.html" to the same path, without the final
   973  // "index.html".
   974  //
   975  //	http.Handle("/", http.FileServerFS(fsys))
   976  func FileServerFS(root fs.FS) Handler {
   977  	return FileServer(FS(root))
   978  }
   979  
   980  func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
   981  	upath := r.URL.Path
   982  	if !strings.HasPrefix(upath, "/") {
   983  		upath = "/" + upath
   984  		r.URL.Path = upath
   985  	}
   986  	serveFile(w, r, f.root, path.Clean(upath), true)
   987  }
   988  
   989  // httpRange specifies the byte range to be sent to the client.
   990  type httpRange struct {
   991  	start, length int64
   992  }
   993  
   994  func (r httpRange) contentRange(size int64) string {
   995  	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
   996  }
   997  
   998  func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
   999  	return textproto.MIMEHeader{
  1000  		"Content-Range": {r.contentRange(size)},
  1001  		"Content-Type":  {contentType},
  1002  	}
  1003  }
  1004  
  1005  // parseRange parses a Range header string as per RFC 7233.
  1006  // errNoOverlap is returned if none of the ranges overlap.
  1007  func parseRange(s string, size int64) ([]httpRange, error) {
  1008  	if s == "" {
  1009  		return nil, nil // header not present
  1010  	}
  1011  	const b = "bytes="
  1012  	if !strings.HasPrefix(s, b) {
  1013  		return nil, errors.New("invalid range")
  1014  	}
  1015  	var ranges []httpRange
  1016  	noOverlap := false
  1017  	for _, ra := range strings.Split(s[len(b):], ",") {
  1018  		ra = textproto.TrimString(ra)
  1019  		if ra == "" {
  1020  			continue
  1021  		}
  1022  		start, end, ok := strings.Cut(ra, "-")
  1023  		if !ok {
  1024  			return nil, errors.New("invalid range")
  1025  		}
  1026  		start, end = textproto.TrimString(start), textproto.TrimString(end)
  1027  		var r httpRange
  1028  		if start == "" {
  1029  			// If no start is specified, end specifies the
  1030  			// range start relative to the end of the file,
  1031  			// and we are dealing with <suffix-length>
  1032  			// which has to be a non-negative integer as per
  1033  			// RFC 7233 Section 2.1 "Byte-Ranges".
  1034  			if end == "" || end[0] == '-' {
  1035  				return nil, errors.New("invalid range")
  1036  			}
  1037  			i, err := strconv.ParseInt(end, 10, 64)
  1038  			if i < 0 || err != nil {
  1039  				return nil, errors.New("invalid range")
  1040  			}
  1041  			if i > size {
  1042  				i = size
  1043  			}
  1044  			r.start = size - i
  1045  			r.length = size - r.start
  1046  		} else {
  1047  			i, err := strconv.ParseInt(start, 10, 64)
  1048  			if err != nil || i < 0 {
  1049  				return nil, errors.New("invalid range")
  1050  			}
  1051  			if i >= size {
  1052  				// If the range begins after the size of the content,
  1053  				// then it does not overlap.
  1054  				noOverlap = true
  1055  				continue
  1056  			}
  1057  			r.start = i
  1058  			if end == "" {
  1059  				// If no end is specified, range extends to end of the file.
  1060  				r.length = size - r.start
  1061  			} else {
  1062  				i, err := strconv.ParseInt(end, 10, 64)
  1063  				if err != nil || r.start > i {
  1064  					return nil, errors.New("invalid range")
  1065  				}
  1066  				if i >= size {
  1067  					i = size - 1
  1068  				}
  1069  				r.length = i - r.start + 1
  1070  			}
  1071  		}
  1072  		ranges = append(ranges, r)
  1073  	}
  1074  	if noOverlap && len(ranges) == 0 {
  1075  		// The specified ranges did not overlap with the content.
  1076  		return nil, errNoOverlap
  1077  	}
  1078  	return ranges, nil
  1079  }
  1080  
  1081  // countingWriter counts how many bytes have been written to it.
  1082  type countingWriter int64
  1083  
  1084  func (w *countingWriter) Write(p []byte) (n int, err error) {
  1085  	*w += countingWriter(len(p))
  1086  	return len(p), nil
  1087  }
  1088  
  1089  // rangesMIMESize returns the number of bytes it takes to encode the
  1090  // provided ranges as a multipart response.
  1091  func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
  1092  	var w countingWriter
  1093  	mw := multipart.NewWriter(&w)
  1094  	for _, ra := range ranges {
  1095  		mw.CreatePart(ra.mimeHeader(contentType, contentSize))
  1096  		encSize += ra.length
  1097  	}
  1098  	mw.Close()
  1099  	encSize += int64(w)
  1100  	return
  1101  }
  1102  
  1103  func sumRangesSize(ranges []httpRange) (size int64) {
  1104  	for _, ra := range ranges {
  1105  		size += ra.length
  1106  	}
  1107  	return
  1108  }
  1109  

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