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

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