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/godebug"
    13  	"io"
    14  	"io/fs"
    15  	"mime"
    16  	"mime/multipart"
    17  	"net/http/internal"
    18  	"net/textproto"
    19  	"net/url"
    20  	"os"
    21  	"path"
    22  	"path/filepath"
    23  	"sort"
    24  	"strconv"
    25  	"strings"
    26  	"time"
    27  )
    28  
    29  // A Dir implements [FileSystem] using the native file system restricted to a
    30  // specific directory tree.
    31  //
    32  // While the [FileSystem.Open] method takes '/'-separated paths, a Dir's string
    33  // value is a directory path on the native file system, not a URL, so it is separated
    34  // by [filepath.Separator], which isn't necessarily '/'.
    35  //
    36  // Note that Dir could expose sensitive files and directories. Dir will follow
    37  // symlinks pointing out of the directory tree, which can be especially dangerous
    38  // if serving from a directory in which users are able to create arbitrary symlinks.
    39  // Dir will also allow access to files and directories starting with a period,
    40  // which could expose sensitive directories like .git or sensitive files like
    41  // .htpasswd. To exclude files with a leading period, remove the files/directories
    42  // from the server or create a custom FileSystem implementation.
    43  //
    44  // An empty Dir is treated as ".".
    45  type Dir string
    46  
    47  // mapOpenError maps the provided non-nil error from opening name
    48  // to a possibly better non-nil error. In particular, it turns OS-specific errors
    49  // about opening files in non-directories into fs.ErrNotExist. See Issues 18984 and 49552.
    50  func mapOpenError(originalErr error, name string, sep rune, stat func(string) (fs.FileInfo, error)) error {
    51  	if errors.Is(originalErr, fs.ErrNotExist) || errors.Is(originalErr, fs.ErrPermission) {
    52  		return originalErr
    53  	}
    54  
    55  	parts := strings.Split(name, string(sep))
    56  	for i := range parts {
    57  		if parts[i] == "" {
    58  			continue
    59  		}
    60  		fi, err := stat(strings.Join(parts[:i+1], string(sep)))
    61  		if err != nil {
    62  			return originalErr
    63  		}
    64  		if !fi.IsDir() {
    65  			return fs.ErrNotExist
    66  		}
    67  	}
    68  	return originalErr
    69  }
    70  
    71  // errInvalidUnsafePath is returned by Dir.Open when the call to
    72  // filepath.Localize fails. filepath.Localize returns an error if the path
    73  // cannot be represented by the operating system.
    74  var errInvalidUnsafePath = errors.New("http: invalid or unsafe file path")
    75  
    76  // Open implements [FileSystem] using [os.Open], opening files for reading rooted
    77  // and relative to the directory d.
    78  func (d Dir) Open(name string) (File, error) {
    79  	path := path.Clean("/" + name)[1:]
    80  	if path == "" {
    81  		path = "."
    82  	}
    83  	path, err := filepath.Localize(path)
    84  	if err != nil {
    85  		return nil, errInvalidUnsafePath
    86  	}
    87  	dir := string(d)
    88  	if dir == "" {
    89  		dir = "."
    90  	}
    91  	fullName := filepath.Join(dir, path)
    92  	f, err := os.Open(fullName)
    93  	if err != nil {
    94  		return nil, mapOpenError(err, fullName, filepath.Separator, os.Stat)
    95  	}
    96  	return f, nil
    97  }
    98  
    99  // A FileSystem implements access to a collection of named files.
   100  // The elements in a file path are separated by slash ('/', U+002F)
   101  // characters, regardless of host operating system convention.
   102  // See the [FileServer] function to convert a FileSystem to a [Handler].
   103  //
   104  // This interface predates the [fs.FS] interface, which can be used instead:
   105  // the [FS] adapter function converts an fs.FS to a FileSystem.
   106  type FileSystem interface {
   107  	Open(name string) (File, error)
   108  }
   109  
   110  // A File is returned by a [FileSystem]'s Open method and can be
   111  // served by the [FileServer] implementation.
   112  //
   113  // The methods should behave the same as those on an [*os.File].
   114  type File interface {
   115  	io.Closer
   116  	io.Reader
   117  	io.Seeker
   118  	Readdir(count int) ([]fs.FileInfo, error)
   119  	Stat() (fs.FileInfo, error)
   120  }
   121  
   122  type anyDirs interface {
   123  	len() int
   124  	name(i int) string
   125  	isDir(i int) bool
   126  }
   127  
   128  type fileInfoDirs []fs.FileInfo
   129  
   130  func (d fileInfoDirs) len() int          { return len(d) }
   131  func (d fileInfoDirs) isDir(i int) bool  { return d[i].IsDir() }
   132  func (d fileInfoDirs) name(i int) string { return d[i].Name() }
   133  
   134  type dirEntryDirs []fs.DirEntry
   135  
   136  func (d dirEntryDirs) len() int          { return len(d) }
   137  func (d dirEntryDirs) isDir(i int) bool  { return d[i].IsDir() }
   138  func (d dirEntryDirs) name(i int) string { return d[i].Name() }
   139  
   140  func dirList(w ResponseWriter, r *Request, f File) {
   141  	// Prefer to use ReadDir instead of Readdir,
   142  	// because the former doesn't require calling
   143  	// Stat on every entry of a directory on Unix.
   144  	var dirs anyDirs
   145  	var err error
   146  	if d, ok := f.(fs.ReadDirFile); ok {
   147  		var list dirEntryDirs
   148  		list, err = d.ReadDir(-1)
   149  		dirs = list
   150  	} else {
   151  		var list fileInfoDirs
   152  		list, err = f.Readdir(-1)
   153  		dirs = list
   154  	}
   155  
   156  	if err != nil {
   157  		logf(r, "http: error reading directory: %v", err)
   158  		Error(w, "Error reading directory", StatusInternalServerError)
   159  		return
   160  	}
   161  	sort.Slice(dirs, func(i, j int) bool { return dirs.name(i) < dirs.name(j) })
   162  
   163  	w.Header().Set("Content-Type", "text/html; charset=utf-8")
   164  	fmt.Fprintf(w, "<!doctype html>\n")
   165  	fmt.Fprintf(w, "<meta name=\"viewport\" content=\"width=device-width\">\n")
   166  	fmt.Fprintf(w, "<pre>\n")
   167  	for i, n := 0, dirs.len(); i < n; i++ {
   168  		name := dirs.name(i)
   169  		if dirs.isDir(i) {
   170  			name += "/"
   171  		}
   172  		// name may contain '?' or '#', which must be escaped to remain
   173  		// part of the URL path, and not indicate the start of a query
   174  		// string or fragment.
   175  		url := url.URL{Path: name}
   176  		fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
   177  	}
   178  	fmt.Fprintf(w, "</pre>\n")
   179  }
   180  
   181  // GODEBUG=httpservecontentkeepheaders=1 restores the pre-1.23 behavior of not deleting
   182  // Cache-Control, Content-Encoding, Etag, or Last-Modified headers on ServeContent errors.
   183  var httpservecontentkeepheaders = godebug.New("httpservecontentkeepheaders")
   184  
   185  // serveError serves an error from ServeFile, ServeFileFS, and ServeContent.
   186  // Because those can all be configured by the caller by setting headers like
   187  // Etag, Last-Modified, and Cache-Control to send on a successful response,
   188  // the error path needs to clear them, since they may not be meant for errors.
   189  func serveError(w ResponseWriter, text string, code int) {
   190  	h := w.Header()
   191  
   192  	nonDefault := false
   193  	for _, k := range []string{
   194  		"Cache-Control",
   195  		"Content-Encoding",
   196  		"Etag",
   197  		"Last-Modified",
   198  	} {
   199  		if !h.has(k) {
   200  			continue
   201  		}
   202  		if httpservecontentkeepheaders.Value() == "1" {
   203  			nonDefault = true
   204  		} else {
   205  			h.Del(k)
   206  		}
   207  	}
   208  	if nonDefault {
   209  		httpservecontentkeepheaders.IncNonDefault()
   210  	}
   211  
   212  	Error(w, text, code)
   213  }
   214  
   215  // ServeContent replies to the request using the content in the
   216  // provided ReadSeeker. The main benefit of ServeContent over [io.Copy]
   217  // is that it handles Range requests properly, sets the MIME type, and
   218  // handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
   219  // and If-Range requests.
   220  //
   221  // If the response's Content-Type header is not set, ServeContent
   222  // first tries to deduce the type from name's file extension and,
   223  // if that fails, falls back to reading the first block of the content
   224  // and passing it to [DetectContentType].
   225  // The name is otherwise unused; in particular it can be empty and is
   226  // never sent in the response.
   227  //
   228  // If modtime is not the zero time or Unix epoch, ServeContent
   229  // includes it in a Last-Modified header in the response. If the
   230  // request includes an If-Modified-Since header, ServeContent uses
   231  // modtime to decide whether the content needs to be sent at all.
   232  //
   233  // The content's Seek method must work: ServeContent uses
   234  // a seek to the end of the content to determine its size.
   235  // Note that [*os.File] implements the [io.ReadSeeker] interface.
   236  //
   237  // If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
   238  // ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
   239  //
   240  // If an error occurs when serving the request (for example, when
   241  // handling an invalid range request), ServeContent responds with an
   242  // error message. By default, ServeContent strips the Cache-Control,
   243  // Content-Encoding, ETag, and Last-Modified headers from error responses.
   244  // The GODEBUG setting httpservecontentkeepheaders=1 causes ServeContent
   245  // to preserve these headers.
   246  func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
   247  	sizeFunc := func() (int64, error) {
   248  		size, err := content.Seek(0, io.SeekEnd)
   249  		if err != nil {
   250  			return 0, errSeeker
   251  		}
   252  		_, err = content.Seek(0, io.SeekStart)
   253  		if err != nil {
   254  			return 0, errSeeker
   255  		}
   256  		return size, nil
   257  	}
   258  	serveContent(w, req, name, modtime, sizeFunc, content)
   259  }
   260  
   261  // errSeeker is returned by ServeContent's sizeFunc when the content
   262  // doesn't seek properly. The underlying Seeker's error text isn't
   263  // included in the sizeFunc reply so it's not sent over HTTP to end
   264  // users.
   265  var errSeeker = errors.New("seeker can't seek")
   266  
   267  // errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
   268  // all of the byte-range-spec values is greater than the content size.
   269  var errNoOverlap = errors.New("invalid range: failed to overlap")
   270  
   271  // if name is empty, filename is unknown. (used for mime type, before sniffing)
   272  // if modtime.IsZero(), modtime is unknown.
   273  // content must be seeked to the beginning of the file.
   274  // The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
   275  func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
   276  	setLastModified(w, modtime)
   277  	done, rangeReq := checkPreconditions(w, r, modtime)
   278  	if done {
   279  		return
   280  	}
   281  
   282  	code := StatusOK
   283  
   284  	// If Content-Type isn't set, use the file's extension to find it, but
   285  	// if the Content-Type is unset explicitly, do not sniff the type.
   286  	ctypes, haveType := w.Header()["Content-Type"]
   287  	var ctype string
   288  	if !haveType {
   289  		ctype = mime.TypeByExtension(filepath.Ext(name))
   290  		if ctype == "" {
   291  			// read a chunk to decide between utf-8 text and binary
   292  			var buf [internal.SniffLen]byte
   293  			n, _ := io.ReadFull(content, buf[:])
   294  			ctype = DetectContentType(buf[:n])
   295  			_, err := content.Seek(0, io.SeekStart) // rewind to output whole file
   296  			if err != nil {
   297  				serveError(w, "seeker can't seek", StatusInternalServerError)
   298  				return
   299  			}
   300  		}
   301  		w.Header().Set("Content-Type", ctype)
   302  	} else if len(ctypes) > 0 {
   303  		ctype = ctypes[0]
   304  	}
   305  
   306  	size, err := sizeFunc()
   307  	if err != nil {
   308  		serveError(w, err.Error(), StatusInternalServerError)
   309  		return
   310  	}
   311  	if size < 0 {
   312  		// Should never happen but just to be sure
   313  		serveError(w, "negative content size computed", StatusInternalServerError)
   314  		return
   315  	}
   316  
   317  	// handle Content-Range header.
   318  	sendSize := size
   319  	var sendContent io.Reader = content
   320  	ranges, err := parseRange(rangeReq, size)
   321  	switch err {
   322  	case nil:
   323  	case errNoOverlap:
   324  		if size == 0 {
   325  			// Some clients add a Range header to all requests to
   326  			// limit the size of the response. If the file is empty,
   327  			// ignore the range header and respond with a 200 rather
   328  			// than a 416.
   329  			ranges = nil
   330  			break
   331  		}
   332  		w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
   333  		fallthrough
   334  	default:
   335  		serveError(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   336  		return
   337  	}
   338  
   339  	if sumRangesSize(ranges) > size {
   340  		// The total number of bytes in all the ranges
   341  		// is larger than the size of the file by
   342  		// itself, so this is probably an attack, or a
   343  		// dumb client. Ignore the range request.
   344  		ranges = nil
   345  	}
   346  	switch {
   347  	case len(ranges) == 1:
   348  		// RFC 7233, Section 4.1:
   349  		// "If a single part is being transferred, the server
   350  		// generating the 206 response MUST generate a
   351  		// Content-Range header field, describing what range
   352  		// of the selected representation is enclosed, and a
   353  		// payload consisting of the range.
   354  		// ...
   355  		// A server MUST NOT generate a multipart response to
   356  		// a request for a single range, since a client that
   357  		// does not request multiple parts might not support
   358  		// multipart responses."
   359  		ra := ranges[0]
   360  		if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   361  			serveError(w, err.Error(), StatusRequestedRangeNotSatisfiable)
   362  			return
   363  		}
   364  		sendSize = ra.length
   365  		code = StatusPartialContent
   366  		w.Header().Set("Content-Range", ra.contentRange(size))
   367  	case len(ranges) > 1:
   368  		sendSize = rangesMIMESize(ranges, ctype, size)
   369  		code = StatusPartialContent
   370  
   371  		pr, pw := io.Pipe()
   372  		mw := multipart.NewWriter(pw)
   373  		w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
   374  		sendContent = pr
   375  		defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
   376  		go func() {
   377  			for _, ra := range ranges {
   378  				part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
   379  				if err != nil {
   380  					pw.CloseWithError(err)
   381  					return
   382  				}
   383  				if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
   384  					pw.CloseWithError(err)
   385  					return
   386  				}
   387  				if _, err := io.CopyN(part, content, ra.length); err != nil {
   388  					pw.CloseWithError(err)
   389  					return
   390  				}
   391  			}
   392  			mw.Close()
   393  			pw.Close()
   394  		}()
   395  	}
   396  
   397  	w.Header().Set("Accept-Ranges", "bytes")
   398  
   399  	// We should be able to unconditionally set the Content-Length here.
   400  	//
   401  	// However, there is a pattern observed in the wild that this breaks:
   402  	// The user wraps the ResponseWriter in one which gzips data written to it,
   403  	// and sets "Content-Encoding: gzip".
   404  	//
   405  	// The user shouldn't be doing this; the serveContent path here depends
   406  	// on serving seekable data with a known length. If you want to compress
   407  	// on the fly, then you shouldn't be using ServeFile/ServeContent, or
   408  	// you should compress the entire file up-front and provide a seekable
   409  	// view of the compressed data.
   410  	//
   411  	// However, since we've observed this pattern in the wild, and since
   412  	// setting Content-Length here breaks code that mostly-works today,
   413  	// skip setting Content-Length if the user set Content-Encoding.
   414  	//
   415  	// If this is a range request, always set Content-Length.
   416  	// If the user isn't changing the bytes sent in the ResponseWrite,
   417  	// the Content-Length will be correct.
   418  	// If the user is changing the bytes sent, then the range request wasn't
   419  	// going to work properly anyway and we aren't worse off.
   420  	//
   421  	// A possible future improvement on this might be to look at the type
   422  	// of the ResponseWriter, and always set Content-Length if it's one
   423  	// that we recognize.
   424  	if len(ranges) > 0 || w.Header().Get("Content-Encoding") == "" {
   425  		w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
   426  	}
   427  	w.WriteHeader(code)
   428  
   429  	if r.Method != "HEAD" {
   430  		io.CopyN(w, sendContent, sendSize)
   431  	}
   432  }
   433  
   434  // scanETag determines if a syntactically valid ETag is present at s. If so,
   435  // the ETag and remaining text after consuming ETag is returned. Otherwise,
   436  // it returns "", "".
   437  func scanETag(s string) (etag string, remain string) {
   438  	s = textproto.TrimString(s)
   439  	start := 0
   440  	if strings.HasPrefix(s, "W/") {
   441  		start = 2
   442  	}
   443  	if len(s[start:]) < 2 || s[start] != '"' {
   444  		return "", ""
   445  	}
   446  	// ETag is either W/"text" or "text".
   447  	// See RFC 7232 2.3.
   448  	for i := start + 1; i < len(s); i++ {
   449  		c := s[i]
   450  		switch {
   451  		// Character values allowed in ETags.
   452  		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
   453  		case c == '"':
   454  			return s[:i+1], s[i+1:]
   455  		default:
   456  			return "", ""
   457  		}
   458  	}
   459  	return "", ""
   460  }
   461  
   462  // etagStrongMatch reports whether a and b match using strong ETag comparison.
   463  // Assumes a and b are valid ETags.
   464  func etagStrongMatch(a, b string) bool {
   465  	return a == b && a != "" && a[0] == '"'
   466  }
   467  
   468  // etagWeakMatch reports whether a and b match using weak ETag comparison.
   469  // Assumes a and b are valid ETags.
   470  func etagWeakMatch(a, b string) bool {
   471  	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
   472  }
   473  
   474  // condResult is the result of an HTTP request precondition check.
   475  // See https://tools.ietf.org/html/rfc7232 section 3.
   476  type condResult int
   477  
   478  const (
   479  	condNone condResult = iota
   480  	condTrue
   481  	condFalse
   482  )
   483  
   484  func checkIfMatch(w ResponseWriter, r *Request) condResult {
   485  	im := r.Header.Get("If-Match")
   486  	if im == "" {
   487  		return condNone
   488  	}
   489  	for {
   490  		im = textproto.TrimString(im)
   491  		if len(im) == 0 {
   492  			break
   493  		}
   494  		if im[0] == ',' {
   495  			im = im[1:]
   496  			continue
   497  		}
   498  		if im[0] == '*' {
   499  			return condTrue
   500  		}
   501  		etag, remain := scanETag(im)
   502  		if etag == "" {
   503  			break
   504  		}
   505  		if etagStrongMatch(etag, w.Header().get("Etag")) {
   506  			return condTrue
   507  		}
   508  		im = remain
   509  	}
   510  
   511  	return condFalse
   512  }
   513  
   514  func checkIfUnmodifiedSince(r *Request, modtime time.Time) condResult {
   515  	ius := r.Header.Get("If-Unmodified-Since")
   516  	if ius == "" || isZeroTime(modtime) {
   517  		return condNone
   518  	}
   519  	t, err := ParseTime(ius)
   520  	if err != nil {
   521  		return condNone
   522  	}
   523  
   524  	// The Last-Modified header truncates sub-second precision so
   525  	// the modtime needs to be truncated too.
   526  	modtime = modtime.Truncate(time.Second)
   527  	if ret := modtime.Compare(t); ret <= 0 {
   528  		return condTrue
   529  	}
   530  	return condFalse
   531  }
   532  
   533  func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
   534  	inm := r.Header.get("If-None-Match")
   535  	if inm == "" {
   536  		return condNone
   537  	}
   538  	buf := inm
   539  	for {
   540  		buf = textproto.TrimString(buf)
   541  		if len(buf) == 0 {
   542  			break
   543  		}
   544  		if buf[0] == ',' {
   545  			buf = buf[1:]
   546  			continue
   547  		}
   548  		if buf[0] == '*' {
   549  			return condFalse
   550  		}
   551  		etag, remain := scanETag(buf)
   552  		if etag == "" {
   553  			break
   554  		}
   555  		if etagWeakMatch(etag, w.Header().get("Etag")) {
   556  			return condFalse
   557  		}
   558  		buf = remain
   559  	}
   560  	return condTrue
   561  }
   562  
   563  func checkIfModifiedSince(r *Request, modtime time.Time) condResult {
   564  	if r.Method != "GET" && r.Method != "HEAD" {
   565  		return condNone
   566  	}
   567  	ims := r.Header.Get("If-Modified-Since")
   568  	if ims == "" || isZeroTime(modtime) {
   569  		return condNone
   570  	}
   571  	t, err := ParseTime(ims)
   572  	if err != nil {
   573  		return condNone
   574  	}
   575  	// The Last-Modified header truncates sub-second precision so
   576  	// the modtime needs to be truncated too.
   577  	modtime = modtime.Truncate(time.Second)
   578  	if ret := modtime.Compare(t); ret <= 0 {
   579  		return condFalse
   580  	}
   581  	return condTrue
   582  }
   583  
   584  func checkIfRange(w ResponseWriter, r *Request, modtime time.Time) condResult {
   585  	if r.Method != "GET" && r.Method != "HEAD" {
   586  		return condNone
   587  	}
   588  	ir := r.Header.get("If-Range")
   589  	if ir == "" {
   590  		return condNone
   591  	}
   592  	etag, _ := scanETag(ir)
   593  	if etag != "" {
   594  		if etagStrongMatch(etag, w.Header().Get("Etag")) {
   595  			return condTrue
   596  		} else {
   597  			return condFalse
   598  		}
   599  	}
   600  	// The If-Range value is typically the ETag value, but it may also be
   601  	// the modtime date. See golang.org/issue/8367.
   602  	if modtime.IsZero() {
   603  		return condFalse
   604  	}
   605  	t, err := ParseTime(ir)
   606  	if err != nil {
   607  		return condFalse
   608  	}
   609  	if t.Unix() == modtime.Unix() {
   610  		return condTrue
   611  	}
   612  	return condFalse
   613  }
   614  
   615  var unixEpochTime = time.Unix(0, 0)
   616  
   617  // isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
   618  func isZeroTime(t time.Time) bool {
   619  	return t.IsZero() || t.Equal(unixEpochTime)
   620  }
   621  
   622  func setLastModified(w ResponseWriter, modtime time.Time) {
   623  	if !isZeroTime(modtime) {
   624  		w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
   625  	}
   626  }
   627  
   628  func writeNotModified(w ResponseWriter) {
   629  	// RFC 7232 section 4.1:
   630  	// a sender SHOULD NOT generate representation metadata other than the
   631  	// above listed fields unless said metadata exists for the purpose of
   632  	// guiding cache updates (e.g., Last-Modified might be useful if the
   633  	// response does not have an ETag field).
   634  	h := w.Header()
   635  	delete(h, "Content-Type")
   636  	delete(h, "Content-Length")
   637  	delete(h, "Content-Encoding")
   638  	if h.Get("Etag") != "" {
   639  		delete(h, "Last-Modified")
   640  	}
   641  	w.WriteHeader(StatusNotModified)
   642  }
   643  
   644  // checkPreconditions evaluates request preconditions and reports whether a precondition
   645  // resulted in sending StatusNotModified or StatusPreconditionFailed.
   646  func checkPreconditions(w ResponseWriter, r *Request, modtime time.Time) (done bool, rangeHeader string) {
   647  	// This function carefully follows RFC 7232 section 6.
   648  	ch := checkIfMatch(w, r)
   649  	if ch == condNone {
   650  		ch = checkIfUnmodifiedSince(r, modtime)
   651  	}
   652  	if ch == condFalse {
   653  		w.WriteHeader(StatusPreconditionFailed)
   654  		return true, ""
   655  	}
   656  	switch checkIfNoneMatch(w, r) {
   657  	case condFalse:
   658  		if r.Method == "GET" || r.Method == "HEAD" {
   659  			writeNotModified(w)
   660  			return true, ""
   661  		} else {
   662  			w.WriteHeader(StatusPreconditionFailed)
   663  			return true, ""
   664  		}
   665  	case condNone:
   666  		if checkIfModifiedSince(r, modtime) == condFalse {
   667  			writeNotModified(w)
   668  			return true, ""
   669  		}
   670  	}
   671  
   672  	rangeHeader = r.Header.get("Range")
   673  	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
   674  		rangeHeader = ""
   675  	}
   676  	return false, rangeHeader
   677  }
   678  
   679  // name is '/'-separated, not filepath.Separator.
   680  func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
   681  	const indexPage = "/index.html"
   682  
   683  	// redirect .../index.html to .../
   684  	// can't use Redirect() because that would make the path absolute,
   685  	// which would be a problem running under StripPrefix
   686  	if strings.HasSuffix(r.URL.Path, indexPage) {
   687  		localRedirect(w, r, "./")
   688  		return
   689  	}
   690  
   691  	f, err := fs.Open(name)
   692  	if err != nil {
   693  		msg, code := toHTTPError(err)
   694  		serveError(w, msg, code)
   695  		return
   696  	}
   697  	defer f.Close()
   698  
   699  	d, err := f.Stat()
   700  	if err != nil {
   701  		msg, code := toHTTPError(err)
   702  		serveError(w, msg, code)
   703  		return
   704  	}
   705  
   706  	if redirect {
   707  		// redirect to canonical path: / at end of directory url
   708  		// r.URL.Path always begins with /
   709  		url := r.URL.Path
   710  		if d.IsDir() {
   711  			if url[len(url)-1] != '/' {
   712  				localRedirect(w, r, path.Base(url)+"/")
   713  				return
   714  			}
   715  		} else if url[len(url)-1] == '/' {
   716  			base := path.Base(url)
   717  			if base == "/" || base == "." {
   718  				// The FileSystem maps a path like "/" or "/./" to a file instead of a directory.
   719  				msg := "http: attempting to traverse a non-directory"
   720  				serveError(w, msg, StatusInternalServerError)
   721  				return
   722  			}
   723  			localRedirect(w, r, "../"+base)
   724  			return
   725  		}
   726  	}
   727  
   728  	if d.IsDir() {
   729  		url := r.URL.Path
   730  		// redirect if the directory name doesn't end in a slash
   731  		if url == "" || url[len(url)-1] != '/' {
   732  			localRedirect(w, r, path.Base(url)+"/")
   733  			return
   734  		}
   735  
   736  		// use contents of index.html for directory, if present
   737  		index := strings.TrimSuffix(name, "/") + indexPage
   738  		ff, err := fs.Open(index)
   739  		if err == nil {
   740  			defer ff.Close()
   741  			dd, err := ff.Stat()
   742  			if err == nil {
   743  				d = dd
   744  				f = ff
   745  			}
   746  		}
   747  	}
   748  
   749  	// Still a directory? (we didn't find an index.html file)
   750  	if d.IsDir() {
   751  		if checkIfModifiedSince(r, d.ModTime()) == condFalse {
   752  			writeNotModified(w)
   753  			return
   754  		}
   755  		setLastModified(w, d.ModTime())
   756  		dirList(w, r, f)
   757  		return
   758  	}
   759  
   760  	// serveContent will check modification time
   761  	sizeFunc := func() (int64, error) { return d.Size(), nil }
   762  	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
   763  }
   764  
   765  // toHTTPError returns a non-specific HTTP error message and status code
   766  // for a given non-nil error value. It's important that toHTTPError does not
   767  // actually return err.Error(), since msg and httpStatus are returned to users,
   768  // and historically Go's ServeContent always returned just "404 Not Found" for
   769  // all errors. We don't want to start leaking information in error messages.
   770  func toHTTPError(err error) (msg string, httpStatus int) {
   771  	if errors.Is(err, fs.ErrNotExist) {
   772  		return "404 page not found", StatusNotFound
   773  	}
   774  	if errors.Is(err, fs.ErrPermission) {
   775  		return "403 Forbidden", StatusForbidden
   776  	}
   777  	if errors.Is(err, errInvalidUnsafePath) {
   778  		return "404 page not found", StatusNotFound
   779  	}
   780  	// Default:
   781  	return "500 Internal Server Error", StatusInternalServerError
   782  }
   783  
   784  // localRedirect gives a Moved Permanently response.
   785  // It does not convert relative paths to absolute paths like Redirect does.
   786  func localRedirect(w ResponseWriter, r *Request, newPath string) {
   787  	// There is no reliable way for us to redirect correctly when the path has
   788  	// escaped slashes, since StripPrefix might be in use. Just return 404.
   789  	if p := r.URL.EscapedPath(); strings.Contains(p, "%2f") || strings.Contains(p, "%2F") {
   790  		NotFound(w, r)
   791  		return
   792  	}
   793  	if q := r.URL.RawQuery; q != "" {
   794  		newPath += "?" + q
   795  	}
   796  	w.Header().Set("Location", newPath)
   797  	w.WriteHeader(StatusMovedPermanently)
   798  }
   799  
   800  // ServeFile replies to the request with the contents of the named
   801  // file or directory.
   802  //
   803  // If the provided file or directory name is a relative path, it is
   804  // interpreted relative to the current directory and may ascend to
   805  // parent directories. If the provided name is constructed from user
   806  // input, it should be sanitized before calling [ServeFile].
   807  //
   808  // As a precaution, ServeFile will reject requests where r.URL.Path
   809  // contains a ".." path element; this protects against callers who
   810  // might unsafely use [filepath.Join] on r.URL.Path without sanitizing
   811  // it and then use that filepath.Join result as the name argument.
   812  //
   813  // As another special case, ServeFile redirects any request where r.URL.Path
   814  // ends in "/index.html" to the same path, without the final
   815  // "index.html". To avoid such redirects either modify the path or
   816  // use [ServeContent].
   817  //
   818  // Outside of those two special cases, ServeFile does not use
   819  // r.URL.Path for selecting the file or directory to serve; only the
   820  // file or directory provided in the name argument is used.
   821  func ServeFile(w ResponseWriter, r *Request, name string) {
   822  	if containsDotDot(r.URL.Path) {
   823  		// Too many programs use r.URL.Path to construct the argument to
   824  		// serveFile. Reject the request under the assumption that happened
   825  		// here and ".." may not be wanted.
   826  		// Note that name might not contain "..", for example if code (still
   827  		// incorrectly) used filepath.Join(myDir, r.URL.Path).
   828  		serveError(w, "invalid URL path", StatusBadRequest)
   829  		return
   830  	}
   831  	dir, file := filepath.Split(name)
   832  	serveFile(w, r, Dir(dir), file, false)
   833  }
   834  
   835  // ServeFileFS replies to the request with the contents
   836  // of the named file or directory from the file system fsys.
   837  // The files provided by fsys must implement [io.Seeker].
   838  //
   839  // If the provided name is constructed from user input, it should be
   840  // sanitized before calling [ServeFileFS].
   841  //
   842  // As a precaution, ServeFileFS will reject requests where r.URL.Path
   843  // contains a ".." path element; this protects against callers who
   844  // might unsafely use [filepath.Join] on r.URL.Path without sanitizing
   845  // it and then use that filepath.Join result as the name argument.
   846  //
   847  // As another special case, ServeFileFS redirects any request where r.URL.Path
   848  // ends in "/index.html" to the same path, without the final
   849  // "index.html". To avoid such redirects either modify the path or
   850  // use [ServeContent].
   851  //
   852  // Outside of those two special cases, ServeFileFS does not use
   853  // r.URL.Path for selecting the file or directory to serve; only the
   854  // file or directory provided in the name argument is used.
   855  func ServeFileFS(w ResponseWriter, r *Request, fsys fs.FS, name string) {
   856  	if containsDotDot(r.URL.Path) {
   857  		// Too many programs use r.URL.Path to construct the argument to
   858  		// serveFile. Reject the request under the assumption that happened
   859  		// here and ".." may not be wanted.
   860  		// Note that name might not contain "..", for example if code (still
   861  		// incorrectly) used filepath.Join(myDir, r.URL.Path).
   862  		serveError(w, "invalid URL path", StatusBadRequest)
   863  		return
   864  	}
   865  	serveFile(w, r, FS(fsys), name, false)
   866  }
   867  
   868  func containsDotDot(v string) bool {
   869  	if !strings.Contains(v, "..") {
   870  		return false
   871  	}
   872  	for ent := range strings.FieldsFuncSeq(v, isSlashRune) {
   873  		if ent == ".." {
   874  			return true
   875  		}
   876  	}
   877  	return false
   878  }
   879  
   880  func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
   881  
   882  type fileHandler struct {
   883  	root FileSystem
   884  }
   885  
   886  type ioFS struct {
   887  	fsys fs.FS
   888  }
   889  
   890  type ioFile struct {
   891  	file fs.File
   892  }
   893  
   894  func (f ioFS) Open(name string) (File, error) {
   895  	if name == "/" {
   896  		name = "."
   897  	} else {
   898  		name = strings.TrimPrefix(name, "/")
   899  	}
   900  	file, err := f.fsys.Open(name)
   901  	if err != nil {
   902  		return nil, mapOpenError(err, name, '/', func(path string) (fs.FileInfo, error) {
   903  			return fs.Stat(f.fsys, path)
   904  		})
   905  	}
   906  	return ioFile{file}, nil
   907  }
   908  
   909  func (f ioFile) Close() error               { return f.file.Close() }
   910  func (f ioFile) Read(b []byte) (int, error) { return f.file.Read(b) }
   911  func (f ioFile) Stat() (fs.FileInfo, error) { return f.file.Stat() }
   912  
   913  var errMissingSeek = errors.New("io.File missing Seek method")
   914  var errMissingReadDir = errors.New("io.File directory missing ReadDir method")
   915  
   916  func (f ioFile) Seek(offset int64, whence int) (int64, error) {
   917  	s, ok := f.file.(io.Seeker)
   918  	if !ok {
   919  		return 0, errMissingSeek
   920  	}
   921  	return s.Seek(offset, whence)
   922  }
   923  
   924  func (f ioFile) ReadDir(count int) ([]fs.DirEntry, error) {
   925  	d, ok := f.file.(fs.ReadDirFile)
   926  	if !ok {
   927  		return nil, errMissingReadDir
   928  	}
   929  	return d.ReadDir(count)
   930  }
   931  
   932  func (f ioFile) Readdir(count int) ([]fs.FileInfo, error) {
   933  	d, ok := f.file.(fs.ReadDirFile)
   934  	if !ok {
   935  		return nil, errMissingReadDir
   936  	}
   937  	var list []fs.FileInfo
   938  	for {
   939  		dirs, err := d.ReadDir(count - len(list))
   940  		for _, dir := range dirs {
   941  			info, err := dir.Info()
   942  			if err != nil {
   943  				// Pretend it doesn't exist, like (*os.File).Readdir does.
   944  				continue
   945  			}
   946  			list = append(list, info)
   947  		}
   948  		if err != nil {
   949  			return list, err
   950  		}
   951  		if count < 0 || len(list) >= count {
   952  			break
   953  		}
   954  	}
   955  	return list, nil
   956  }
   957  
   958  // FS converts fsys to a [FileSystem] implementation,
   959  // for use with [FileServer] and [NewFileTransport].
   960  // The files provided by fsys must implement [io.Seeker].
   961  func FS(fsys fs.FS) FileSystem {
   962  	return ioFS{fsys}
   963  }
   964  
   965  // FileServer returns a handler that serves HTTP requests
   966  // with the contents of the file system rooted at root.
   967  //
   968  // As a special case, the returned file server redirects any request
   969  // ending in "/index.html" to the same path, without the final
   970  // "index.html".
   971  //
   972  // To use the operating system's file system implementation,
   973  // use [http.Dir]:
   974  //
   975  //	http.Handle("/", http.FileServer(http.Dir("/tmp")))
   976  //
   977  // To use an [fs.FS] implementation, use [http.FileServerFS] instead.
   978  func FileServer(root FileSystem) Handler {
   979  	return &fileHandler{root}
   980  }
   981  
   982  // FileServerFS returns a handler that serves HTTP requests
   983  // with the contents of the file system fsys.
   984  // The files provided by fsys must implement [io.Seeker].
   985  //
   986  // As a special case, the returned file server redirects any request
   987  // ending in "/index.html" to the same path, without the final
   988  // "index.html".
   989  //
   990  //	http.Handle("/", http.FileServerFS(fsys))
   991  func FileServerFS(root fs.FS) Handler {
   992  	return FileServer(FS(root))
   993  }
   994  
   995  func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
   996  	upath := r.URL.Path
   997  	if !strings.HasPrefix(upath, "/") {
   998  		upath = "/" + upath
   999  		r.URL.Path = upath
  1000  	}
  1001  	serveFile(w, r, f.root, path.Clean(upath), true)
  1002  }
  1003  
  1004  // httpRange specifies the byte range to be sent to the client.
  1005  type httpRange struct {
  1006  	start, length int64
  1007  }
  1008  
  1009  func (r httpRange) contentRange(size int64) string {
  1010  	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
  1011  }
  1012  
  1013  func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
  1014  	return textproto.MIMEHeader{
  1015  		"Content-Range": {r.contentRange(size)},
  1016  		"Content-Type":  {contentType},
  1017  	}
  1018  }
  1019  
  1020  // parseRange parses a Range header string as per RFC 7233.
  1021  // errNoOverlap is returned if none of the ranges overlap.
  1022  func parseRange(s string, size int64) ([]httpRange, error) {
  1023  	if s == "" {
  1024  		return nil, nil // header not present
  1025  	}
  1026  	const b = "bytes="
  1027  	if !strings.HasPrefix(s, b) {
  1028  		return nil, errors.New("invalid range")
  1029  	}
  1030  	var ranges []httpRange
  1031  	noOverlap := false
  1032  	for ra := range strings.SplitSeq(s[len(b):], ",") {
  1033  		ra = textproto.TrimString(ra)
  1034  		if ra == "" {
  1035  			continue
  1036  		}
  1037  		start, end, ok := strings.Cut(ra, "-")
  1038  		if !ok {
  1039  			return nil, errors.New("invalid range")
  1040  		}
  1041  		start, end = textproto.TrimString(start), textproto.TrimString(end)
  1042  		var r httpRange
  1043  		if start == "" {
  1044  			// If no start is specified, end specifies the
  1045  			// range start relative to the end of the file,
  1046  			// and we are dealing with <suffix-length>
  1047  			// which has to be a non-negative integer as per
  1048  			// RFC 7233 Section 2.1 "Byte-Ranges".
  1049  			if end == "" || end[0] == '-' {
  1050  				return nil, errors.New("invalid range")
  1051  			}
  1052  			i, err := strconv.ParseInt(end, 10, 64)
  1053  			if i < 0 || err != nil {
  1054  				return nil, errors.New("invalid range")
  1055  			}
  1056  			if i > size {
  1057  				i = size
  1058  			}
  1059  			r.start = size - i
  1060  			r.length = size - r.start
  1061  		} else {
  1062  			i, err := strconv.ParseInt(start, 10, 64)
  1063  			if err != nil || i < 0 {
  1064  				return nil, errors.New("invalid range")
  1065  			}
  1066  			if i >= size {
  1067  				// If the range begins after the size of the content,
  1068  				// then it does not overlap.
  1069  				noOverlap = true
  1070  				continue
  1071  			}
  1072  			r.start = i
  1073  			if end == "" {
  1074  				// If no end is specified, range extends to end of the file.
  1075  				r.length = size - r.start
  1076  			} else {
  1077  				i, err := strconv.ParseInt(end, 10, 64)
  1078  				if err != nil || r.start > i {
  1079  					return nil, errors.New("invalid range")
  1080  				}
  1081  				if i >= size {
  1082  					i = size - 1
  1083  				}
  1084  				r.length = i - r.start + 1
  1085  			}
  1086  		}
  1087  		ranges = append(ranges, r)
  1088  	}
  1089  	if noOverlap && len(ranges) == 0 {
  1090  		// The specified ranges did not overlap with the content.
  1091  		return nil, errNoOverlap
  1092  	}
  1093  	return ranges, nil
  1094  }
  1095  
  1096  // countingWriter counts how many bytes have been written to it.
  1097  type countingWriter int64
  1098  
  1099  func (w *countingWriter) Write(p []byte) (n int, err error) {
  1100  	*w += countingWriter(len(p))
  1101  	return len(p), nil
  1102  }
  1103  
  1104  // rangesMIMESize returns the number of bytes it takes to encode the
  1105  // provided ranges as a multipart response.
  1106  func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
  1107  	var w countingWriter
  1108  	mw := multipart.NewWriter(&w)
  1109  	for _, ra := range ranges {
  1110  		mw.CreatePart(ra.mimeHeader(contentType, contentSize))
  1111  		encSize += ra.length
  1112  	}
  1113  	mw.Close()
  1114  	encSize += int64(w)
  1115  	return
  1116  }
  1117  
  1118  func sumRangesSize(ranges []httpRange) (size int64) {
  1119  	for _, ra := range ranges {
  1120  		size += ra.length
  1121  	}
  1122  	return
  1123  }
  1124  

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