src/net/http/fs.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // HTTP file system request handler

 package http

 import (
 	"errors"
 	"fmt"
 	"io"
 	"mime"
 	"mime/multipart"
 	"net/textproto"
 	"net/url"
 	"os"
 	"path"
 	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
 	"time"
 )

 // A Dir implements FileSystem using the native file system restricted to a
 // specific directory tree.
 //
 // While the FileSystem.Open method takes '/'-separated paths, a Dir's string
 // value is a filename on the native file system, not a URL, so it is separated
 // by filepath.Separator, which isn't necessarily '/'.
 //
 // An empty Dir is treated as ".".
 type Dir string

 func (d Dir) Open(name string) (File, error) {
 	if filepath.Separator != '/' && strings.IndexRune(name, filepath.Separator) >= 0 ||
 		strings.Contains(name, "\x00") {
 		return nil, errors.New("http: invalid character in file path")
 	}
 	dir := string(d)
 	if dir == "" {
 		dir = "."
 	}
 	f, err := os.Open(filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name))))
 	if err != nil {
 		return nil, err
 	}
 	return f, nil
 }

 // A FileSystem implements access to a collection of named files.
 // The elements in a file path are separated by slash ('/', U+002F)
 // characters, regardless of host operating system convention.
 type FileSystem interface {
 	Open(name string) (File, error)
 }

 // A File is returned by a FileSystem's Open method and can be
 // served by the FileServer implementation.
 //
 // The methods should behave the same as those on an *os.File.
 type File interface {
 	io.Closer
 	io.Reader
 	io.Seeker
 	Readdir(count int) ([]os.FileInfo, error)
 	Stat() (os.FileInfo, error)
 }

 func dirList(w ResponseWriter, f File) {
 	dirs, err := f.Readdir(-1)
 	if err != nil {
 		// TODO: log err.Error() to the Server.ErrorLog, once it's possible
 		// for a handler to get at its Server via the ResponseWriter. See
 		// Issue 12438.
 		Error(w, "Error reading directory", StatusInternalServerError)
 		return
 	}
 	sort.Sort(byName(dirs))

 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	fmt.Fprintf(w, "<pre>\n")
 	for _, d := range dirs {
 		name := d.Name()
 		if d.IsDir() {
 			name += "/"
 		}
 		// name may contain '?' or '#', which must be escaped to remain
 		// part of the URL path, and not indicate the start of a query
 		// string or fragment.
 		url := url.URL{Path: name}
 		fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
 	}
 	fmt.Fprintf(w, "</pre>\n")
 }

 // ServeContent replies to the request using the content in the
 // provided ReadSeeker.  The main benefit of ServeContent over io.Copy
 // is that it handles Range requests properly, sets the MIME type, and
 // handles If-Modified-Since requests.
 //
 // If the response's Content-Type header is not set, ServeContent
 // first tries to deduce the type from name's file extension and,
 // if that fails, falls back to reading the first block of the content
 // and passing it to DetectContentType.
 // The name is otherwise unused; in particular it can be empty and is
 // never sent in the response.
 //
 // If modtime is not the zero time or Unix epoch, ServeContent
 // includes it in a Last-Modified header in the response.  If the
 // request includes an If-Modified-Since header, ServeContent uses
 // modtime to decide whether the content needs to be sent at all.
 //
 // The content's Seek method must work: ServeContent uses
 // a seek to the end of the content to determine its size.
 //
 // If the caller has set w's ETag header, ServeContent uses it to
 // handle requests using If-Range and If-None-Match.
 //
 // Note that *os.File implements the io.ReadSeeker interface.
 func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
 	sizeFunc := func() (int64, error) {
 		size, err := content.Seek(0, os.SEEK_END)
 		if err != nil {
 			return 0, errSeeker
 		}
 		_, err = content.Seek(0, os.SEEK_SET)
 		if err != nil {
 			return 0, errSeeker
 		}
 		return size, nil
 	}
 	serveContent(w, req, name, modtime, sizeFunc, content)
 }

 // errSeeker is returned by ServeContent's sizeFunc when the content
 // doesn't seek properly. The underlying Seeker's error text isn't
 // included in the sizeFunc reply so it's not sent over HTTP to end
 // users.
 var errSeeker = errors.New("seeker can't seek")

 // if name is empty, filename is unknown. (used for mime type, before sniffing)
 // if modtime.IsZero(), modtime is unknown.
 // content must be seeked to the beginning of the file.
 // The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
 func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
 	if checkLastModified(w, r, modtime) {
 		return
 	}
 	rangeReq, done := checkETag(w, r, modtime)
 	if done {
 		return
 	}

 	code := StatusOK

 	// If Content-Type isn't set, use the file's extension to find it, but
 	// if the Content-Type is unset explicitly, do not sniff the type.
 	ctypes, haveType := w.Header()["Content-Type"]
 	var ctype string
 	if !haveType {
 		ctype = mime.TypeByExtension(filepath.Ext(name))
 		if ctype == "" {
 			// read a chunk to decide between utf-8 text and binary
 			var buf [sniffLen]byte
 			n, _ := io.ReadFull(content, buf[:])
 			ctype = DetectContentType(buf[:n])
 			_, err := content.Seek(0, os.SEEK_SET) // rewind to output whole file
 			if err != nil {
 				Error(w, "seeker can't seek", StatusInternalServerError)
 				return
 			}
 		}
 		w.Header().Set("Content-Type", ctype)
 	} else if len(ctypes) > 0 {
 		ctype = ctypes[0]
 	}

 	size, err := sizeFunc()
 	if err != nil {
 		Error(w, err.Error(), StatusInternalServerError)
 		return
 	}

 	// handle Content-Range header.
 	sendSize := size
 	var sendContent io.Reader = content
 	if size >= 0 {
 		ranges, err := parseRange(rangeReq, size)
 		if err != nil {
 			Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
 			return
 		}
 		if sumRangesSize(ranges) > size {
 			// The total number of bytes in all the ranges
 			// is larger than the size of the file by
 			// itself, so this is probably an attack, or a
 			// dumb client.  Ignore the range request.
 			ranges = nil
 		}
 		switch {
 		case len(ranges) == 1:
 			// RFC 2616, Section 14.16:
 			// "When an HTTP message includes the content of a single
 			// range (for example, a response to a request for a
 			// single range, or to a request for a set of ranges
 			// that overlap without any holes), this content is
 			// transmitted with a Content-Range header, and a
 			// Content-Length header showing the number of bytes
 			// actually transferred.
 			// ...
 			// A response to a request for a single range MUST NOT
 			// be sent using the multipart/byteranges media type."
 			ra := ranges[0]
 			if _, err := content.Seek(ra.start, os.SEEK_SET); err != nil {
 				Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
 				return
 			}
 			sendSize = ra.length
 			code = StatusPartialContent
 			w.Header().Set("Content-Range", ra.contentRange(size))
 		case len(ranges) > 1:
 			sendSize = rangesMIMESize(ranges, ctype, size)
 			code = StatusPartialContent

 			pr, pw := io.Pipe()
 			mw := multipart.NewWriter(pw)
 			w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
 			sendContent = pr
 			defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
 			go func() {
 				for _, ra := range ranges {
 					part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
 					if err != nil {
 						pw.CloseWithError(err)
 						return
 					}
 					if _, err := content.Seek(ra.start, os.SEEK_SET); err != nil {
 						pw.CloseWithError(err)
 						return
 					}
 					if _, err := io.CopyN(part, content, ra.length); err != nil {
 						pw.CloseWithError(err)
 						return
 					}
 				}
 				mw.Close()
 				pw.Close()
 			}()
 		}

 		w.Header().Set("Accept-Ranges", "bytes")
 		if w.Header().Get("Content-Encoding") == "" {
 			w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
 		}
 	}

 	w.WriteHeader(code)

 	if r.Method != "HEAD" {
 		io.CopyN(w, sendContent, sendSize)
 	}
 }

 var unixEpochTime = time.Unix(0, 0)

 // modtime is the modification time of the resource to be served, or IsZero().
 // return value is whether this request is now complete.
 func checkLastModified(w ResponseWriter, r *Request, modtime time.Time) bool {
 	if modtime.IsZero() || modtime.Equal(unixEpochTime) {
 		// If the file doesn't have a modtime (IsZero), or the modtime
 		// is obviously garbage (Unix time == 0), then ignore modtimes
 		// and don't process the If-Modified-Since header.
 		return false
 	}

 	// The Date-Modified header truncates sub-second precision, so
 	// use mtime < t+1s instead of mtime <= t to check for unmodified.
 	if t, err := time.Parse(TimeFormat, r.Header.Get("If-Modified-Since")); err == nil && modtime.Before(t.Add(1*time.Second)) {
 		h := w.Header()
 		delete(h, "Content-Type")
 		delete(h, "Content-Length")
 		w.WriteHeader(StatusNotModified)
 		return true
 	}
 	w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
 	return false
 }

 // checkETag implements If-None-Match and If-Range checks.
 //
 // The ETag or modtime must have been previously set in the
 // ResponseWriter's headers.  The modtime is only compared at second
 // granularity and may be the zero value to mean unknown.
 //
 // The return value is the effective request "Range" header to use and
 // whether this request is now considered done.
 func checkETag(w ResponseWriter, r *Request, modtime time.Time) (rangeReq string, done bool) {
 	etag := w.Header().get("Etag")
 	rangeReq = r.Header.get("Range")

 	// Invalidate the range request if the entity doesn't match the one
 	// the client was expecting.
 	// "If-Range: version" means "ignore the Range: header unless version matches the
 	// current file."
 	// We only support ETag versions.
 	// The caller must have set the ETag on the response already.
 	if ir := r.Header.get("If-Range"); ir != "" && ir != etag {
 		// The If-Range value is typically the ETag value, but it may also be
 		// the modtime date. See golang.org/issue/8367.
 		timeMatches := false
 		if !modtime.IsZero() {
 			if t, err := ParseTime(ir); err == nil && t.Unix() == modtime.Unix() {
 				timeMatches = true
 			}
 		}
 		if !timeMatches {
 			rangeReq = ""
 		}
 	}

 	if inm := r.Header.get("If-None-Match"); inm != "" {
 		// Must know ETag.
 		if etag == "" {
 			return rangeReq, false
 		}

 		// TODO(bradfitz): non-GET/HEAD requests require more work:
 		// sending a different status code on matches, and
 		// also can't use weak cache validators (those with a "W/
 		// prefix).  But most users of ServeContent will be using
 		// it on GET or HEAD, so only support those for now.
 		if r.Method != "GET" && r.Method != "HEAD" {
 			return rangeReq, false
 		}

 		// TODO(bradfitz): deal with comma-separated or multiple-valued
 		// list of If-None-match values.  For now just handle the common
 		// case of a single item.
 		if inm == etag || inm == "*" {
 			h := w.Header()
 			delete(h, "Content-Type")
 			delete(h, "Content-Length")
 			w.WriteHeader(StatusNotModified)
 			return "", true
 		}
 	}
 	return rangeReq, false
 }

 // name is '/'-separated, not filepath.Separator.
 func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
 	const indexPage = "/index.html"

 	// redirect .../index.html to .../
 	// can't use Redirect() because that would make the path absolute,
 	// which would be a problem running under StripPrefix
 	if strings.HasSuffix(r.URL.Path, indexPage) {
 		localRedirect(w, r, "./")
 		return
 	}

 	f, err := fs.Open(name)
 	if err != nil {
 		msg, code := toHTTPError(err)
 		Error(w, msg, code)
 		return
 	}
 	defer f.Close()

 	d, err := f.Stat()
 	if err != nil {
 		msg, code := toHTTPError(err)
 		Error(w, msg, code)
 		return
 	}

 	if redirect {
 		// redirect to canonical path: / at end of directory url
 		// r.URL.Path always begins with /
 		url := r.URL.Path
 		if d.IsDir() {
 			if url[len(url)-1] != '/' {
 				localRedirect(w, r, path.Base(url)+"/")
 				return
 			}
 		} else {
 			if url[len(url)-1] == '/' {
 				localRedirect(w, r, "../"+path.Base(url))
 				return
 			}
 		}
 	}

 	// use contents of index.html for directory, if present
 	if d.IsDir() {
 		index := strings.TrimSuffix(name, "/") + indexPage
 		ff, err := fs.Open(index)
 		if err == nil {
 			defer ff.Close()
 			dd, err := ff.Stat()
 			if err == nil {
 				name = index
 				d = dd
 				f = ff
 			}
 		}
 	}

 	// Still a directory? (we didn't find an index.html file)
 	if d.IsDir() {
 		if checkLastModified(w, r, d.ModTime()) {
 			return
 		}
 		dirList(w, f)
 		return
 	}

 	// serveContent will check modification time
 	sizeFunc := func() (int64, error) { return d.Size(), nil }
 	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
 }

 // toHTTPError returns a non-specific HTTP error message and status code
 // for a given non-nil error value. It's important that toHTTPError does not
 // actually return err.Error(), since msg and httpStatus are returned to users,
 // and historically Go's ServeContent always returned just "404 Not Found" for
 // all errors. We don't want to start leaking information in error messages.
 func toHTTPError(err error) (msg string, httpStatus int) {
 	if os.IsNotExist(err) {
 		return "404 page not found", StatusNotFound
 	}
 	if os.IsPermission(err) {
 		return "403 Forbidden", StatusForbidden
 	}
 	// Default:
 	return "500 Internal Server Error", StatusInternalServerError
 }

 // localRedirect gives a Moved Permanently response.
 // It does not convert relative paths to absolute paths like Redirect does.
 func localRedirect(w ResponseWriter, r *Request, newPath string) {
 	if q := r.URL.RawQuery; q != "" {
 		newPath += "?" + q
 	}
 	w.Header().Set("Location", newPath)
 	w.WriteHeader(StatusMovedPermanently)
 }

 // ServeFile replies to the request with the contents of the named
 // file or directory.
 //
 // As a special case, ServeFile redirects any request where r.URL.Path
 // ends in "/index.html" to the same path, without the final
 // "index.html". To avoid such redirects either modify the path or
 // use ServeContent.
 func ServeFile(w ResponseWriter, r *Request, name string) {
 	dir, file := filepath.Split(name)
 	serveFile(w, r, Dir(dir), file, false)
 }

 type fileHandler struct {
 	root FileSystem
 }

 // FileServer returns a handler that serves HTTP requests
 // with the contents of the file system rooted at root.
 //
 // To use the operating system's file system implementation,
 // use http.Dir:
 //
 //     http.Handle("/", http.FileServer(http.Dir("/tmp")))
 //
 // As a special case, the returned file server redirects any request
 // ending in "/index.html" to the same path, without the final
 // "index.html".
 func FileServer(root FileSystem) Handler {
 	return &fileHandler{root}
 }

 func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
 	upath := r.URL.Path
 	if !strings.HasPrefix(upath, "/") {
 		upath = "/" + upath
 		r.URL.Path = upath
 	}
 	serveFile(w, r, f.root, path.Clean(upath), true)
 }

 // httpRange specifies the byte range to be sent to the client.
 type httpRange struct {
 	start, length int64
 }

 func (r httpRange) contentRange(size int64) string {
 	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
 }

 func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
 	return textproto.MIMEHeader{
 		"Content-Range": {r.contentRange(size)},
 		"Content-Type":  {contentType},
 	}
 }

 // parseRange parses a Range header string as per RFC 2616.
 func parseRange(s string, size int64) ([]httpRange, error) {
 	if s == "" {
 		return nil, nil // header not present
 	}
 	const b = "bytes="
 	if !strings.HasPrefix(s, b) {
 		return nil, errors.New("invalid range")
 	}
 	var ranges []httpRange
 	for _, ra := range strings.Split(s[len(b):], ",") {
 		ra = strings.TrimSpace(ra)
 		if ra == "" {
 			continue
 		}
 		i := strings.Index(ra, "-")
 		if i < 0 {
 			return nil, errors.New("invalid range")
 		}
 		start, end := strings.TrimSpace(ra[:i]), strings.TrimSpace(ra[i+1:])
 		var r httpRange
 		if start == "" {
 			// If no start is specified, end specifies the
 			// range start relative to the end of the file.
 			i, err := strconv.ParseInt(end, 10, 64)
 			if err != nil {
 				return nil, errors.New("invalid range")
 			}
 			if i > size {
 				i = size
 			}
 			r.start = size - i
 			r.length = size - r.start
 		} else {
 			i, err := strconv.ParseInt(start, 10, 64)
 			if err != nil || i >= size || i < 0 {
 				return nil, errors.New("invalid range")
 			}
 			r.start = i
 			if end == "" {
 				// If no end is specified, range extends to end of the file.
 				r.length = size - r.start
 			} else {
 				i, err := strconv.ParseInt(end, 10, 64)
 				if err != nil || r.start > i {
 					return nil, errors.New("invalid range")
 				}
 				if i >= size {
 					i = size - 1
 				}
 				r.length = i - r.start + 1
 			}
 		}
 		ranges = append(ranges, r)
 	}
 	return ranges, nil
 }

 // countingWriter counts how many bytes have been written to it.
 type countingWriter int64

 func (w *countingWriter) Write(p []byte) (n int, err error) {
 	*w += countingWriter(len(p))
 	return len(p), nil
 }

 // rangesMIMESize returns the number of bytes it takes to encode the
 // provided ranges as a multipart response.
 func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
 	var w countingWriter
 	mw := multipart.NewWriter(&w)
 	for _, ra := range ranges {
 		mw.CreatePart(ra.mimeHeader(contentType, contentSize))
 		encSize += ra.length
 	}
 	mw.Close()
 	encSize += int64(w)
 	return
 }

 func sumRangesSize(ranges []httpRange) (size int64) {
 	for _, ra := range ranges {
 		size += ra.length
 	}
 	return
 }

 type byName []os.FileInfo

 func (s byName) Len() int           { return len(s) }
 func (s byName) Less(i, j int) bool { return s[i].Name() < s[j].Name() }
 func (s byName) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// HTTP file system request handler

	package http

	import (
	"errors"
	"fmt"
	"io"
	"mime"
	"mime/multipart"
	"net/textproto"
	"net/url"
	"os"
	"path"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"
	)

	// A Dir implements FileSystem using the native file system restricted to a
	// specific directory tree.
	//
	// While the FileSystem.Open method takes '/'-separated paths, a Dir's string
	// value is a filename on the native file system, not a URL, so it is separated
	// by filepath.Separator, which isn't necessarily '/'.
	//
	// An empty Dir is treated as ".".
	type Dir string

	func (d Dir) Open(name string) (File, error) {
	if filepath.Separator != '/' && strings.IndexRune(name, filepath.Separator) >= 0 \|\|
	strings.Contains(name, "\x00") {
	return nil, errors.New("http: invalid character in file path")
	}
	dir := string(d)
	if dir == "" {
	dir = "."
	}
	f, err := os.Open(filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name))))
	if err != nil {
	return nil, err
	}
	return f, nil
	}

	// A FileSystem implements access to a collection of named files.
	// The elements in a file path are separated by slash ('/', U+002F)
	// characters, regardless of host operating system convention.
	type FileSystem interface {
	Open(name string) (File, error)
	}

	// A File is returned by a FileSystem's Open method and can be
	// served by the FileServer implementation.
	//
	// The methods should behave the same as those on an *os.File.
	type File interface {
	io.Closer
	io.Reader
	io.Seeker
	Readdir(count int) ([]os.FileInfo, error)
	Stat() (os.FileInfo, error)
	}

	func dirList(w ResponseWriter, f File) {
	dirs, err := f.Readdir(-1)
	if err != nil {
	// TODO: log err.Error() to the Server.ErrorLog, once it's possible
	// for a handler to get at its Server via the ResponseWriter. See
	// Issue 12438.
	Error(w, "Error reading directory", StatusInternalServerError)
	return
	}
	sort.Sort(byName(dirs))

	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	fmt.Fprintf(w, "<pre>\n")
	for _, d := range dirs {
	name := d.Name()
	if d.IsDir() {
	name += "/"
	}
	// name may contain '?' or '#', which must be escaped to remain
	// part of the URL path, and not indicate the start of a query
	// string or fragment.
	url := url.URL{Path: name}
	fmt.Fprintf(w, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
	}
	fmt.Fprintf(w, "</pre>\n")
	}

	// ServeContent replies to the request using the content in the
	// provided ReadSeeker. The main benefit of ServeContent over io.Copy
	// is that it handles Range requests properly, sets the MIME type, and
	// handles If-Modified-Since requests.
	//
	// If the response's Content-Type header is not set, ServeContent
	// first tries to deduce the type from name's file extension and,
	// if that fails, falls back to reading the first block of the content
	// and passing it to DetectContentType.
	// The name is otherwise unused; in particular it can be empty and is
	// never sent in the response.
	//
	// If modtime is not the zero time or Unix epoch, ServeContent
	// includes it in a Last-Modified header in the response. If the
	// request includes an If-Modified-Since header, ServeContent uses
	// modtime to decide whether the content needs to be sent at all.
	//
	// The content's Seek method must work: ServeContent uses
	// a seek to the end of the content to determine its size.
	//
	// If the caller has set w's ETag header, ServeContent uses it to
	// handle requests using If-Range and If-None-Match.
	//
	// Note that *os.File implements the io.ReadSeeker interface.
	func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
	sizeFunc := func() (int64, error) {
	size, err := content.Seek(0, os.SEEK_END)
	if err != nil {
	return 0, errSeeker
	}
	_, err = content.Seek(0, os.SEEK_SET)
	if err != nil {
	return 0, errSeeker
	}
	return size, nil
	}
	serveContent(w, req, name, modtime, sizeFunc, content)
	}

	// errSeeker is returned by ServeContent's sizeFunc when the content
	// doesn't seek properly. The underlying Seeker's error text isn't
	// included in the sizeFunc reply so it's not sent over HTTP to end
	// users.
	var errSeeker = errors.New("seeker can't seek")

	// if name is empty, filename is unknown. (used for mime type, before sniffing)
	// if modtime.IsZero(), modtime is unknown.
	// content must be seeked to the beginning of the file.
	// The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
	func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
	if checkLastModified(w, r, modtime) {
	return
	}
	rangeReq, done := checkETag(w, r, modtime)
	if done {
	return
	}

	code := StatusOK

	// If Content-Type isn't set, use the file's extension to find it, but
	// if the Content-Type is unset explicitly, do not sniff the type.
	ctypes, haveType := w.Header()["Content-Type"]
	var ctype string
	if !haveType {
	ctype = mime.TypeByExtension(filepath.Ext(name))
	if ctype == "" {
	// read a chunk to decide between utf-8 text and binary
	var buf [sniffLen]byte
	n, _ := io.ReadFull(content, buf[:])
	ctype = DetectContentType(buf[:n])
	_, err := content.Seek(0, os.SEEK_SET) // rewind to output whole file
	if err != nil {
	Error(w, "seeker can't seek", StatusInternalServerError)
	return
	}
	}
	w.Header().Set("Content-Type", ctype)
	} else if len(ctypes) > 0 {
	ctype = ctypes[0]
	}

	size, err := sizeFunc()
	if err != nil {
	Error(w, err.Error(), StatusInternalServerError)
	return
	}

	// handle Content-Range header.
	sendSize := size
	var sendContent io.Reader = content
	if size >= 0 {
	ranges, err := parseRange(rangeReq, size)
	if err != nil {
	Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
	return
	}
	if sumRangesSize(ranges) > size {
	// The total number of bytes in all the ranges
	// is larger than the size of the file by
	// itself, so this is probably an attack, or a
	// dumb client. Ignore the range request.
	ranges = nil
	}
	switch {
	case len(ranges) == 1:
	// RFC 2616, Section 14.16:
	// "When an HTTP message includes the content of a single
	// range (for example, a response to a request for a
	// single range, or to a request for a set of ranges
	// that overlap without any holes), this content is
	// transmitted with a Content-Range header, and a
	// Content-Length header showing the number of bytes
	// actually transferred.
	// ...
	// A response to a request for a single range MUST NOT
	// be sent using the multipart/byteranges media type."
	ra := ranges[0]
	if _, err := content.Seek(ra.start, os.SEEK_SET); err != nil {
	Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
	return
	}
	sendSize = ra.length
	code = StatusPartialContent
	w.Header().Set("Content-Range", ra.contentRange(size))
	case len(ranges) > 1:
	sendSize = rangesMIMESize(ranges, ctype, size)
	code = StatusPartialContent

	pr, pw := io.Pipe()
	mw := multipart.NewWriter(pw)
	w.Header().Set("Content-Type", "multipart/byteranges; boundary="+mw.Boundary())
	sendContent = pr
	defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
	go func() {
	for _, ra := range ranges {
	part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
	if err != nil {
	pw.CloseWithError(err)
	return
	}
	if _, err := content.Seek(ra.start, os.SEEK_SET); err != nil {
	pw.CloseWithError(err)
	return
	}
	if _, err := io.CopyN(part, content, ra.length); err != nil {
	pw.CloseWithError(err)
	return
	}
	}
	mw.Close()
	pw.Close()
	}()
	}

	w.Header().Set("Accept-Ranges", "bytes")
	if w.Header().Get("Content-Encoding") == "" {
	w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
	}
	}

	w.WriteHeader(code)

	if r.Method != "HEAD" {
	io.CopyN(w, sendContent, sendSize)
	}
	}

	var unixEpochTime = time.Unix(0, 0)

	// modtime is the modification time of the resource to be served, or IsZero().
	// return value is whether this request is now complete.
	func checkLastModified(w ResponseWriter, r *Request, modtime time.Time) bool {
	if modtime.IsZero() \|\| modtime.Equal(unixEpochTime) {
	// If the file doesn't have a modtime (IsZero), or the modtime
	// is obviously garbage (Unix time == 0), then ignore modtimes
	// and don't process the If-Modified-Since header.
	return false
	}

	// The Date-Modified header truncates sub-second precision, so
	// use mtime < t+1s instead of mtime <= t to check for unmodified.
	if t, err := time.Parse(TimeFormat, r.Header.Get("If-Modified-Since")); err == nil && modtime.Before(t.Add(1*time.Second)) {
	h := w.Header()
	delete(h, "Content-Type")
	delete(h, "Content-Length")
	w.WriteHeader(StatusNotModified)
	return true
	}
	w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
	return false
	}

	// checkETag implements If-None-Match and If-Range checks.
	//
	// The ETag or modtime must have been previously set in the
	// ResponseWriter's headers. The modtime is only compared at second
	// granularity and may be the zero value to mean unknown.
	//
	// The return value is the effective request "Range" header to use and
	// whether this request is now considered done.
	func checkETag(w ResponseWriter, r *Request, modtime time.Time) (rangeReq string, done bool) {
	etag := w.Header().get("Etag")
	rangeReq = r.Header.get("Range")

	// Invalidate the range request if the entity doesn't match the one
	// the client was expecting.
	// "If-Range: version" means "ignore the Range: header unless version matches the
	// current file."
	// We only support ETag versions.
	// The caller must have set the ETag on the response already.
	if ir := r.Header.get("If-Range"); ir != "" && ir != etag {
	// The If-Range value is typically the ETag value, but it may also be
	// the modtime date. See golang.org/issue/8367.
	timeMatches := false
	if !modtime.IsZero() {
	if t, err := ParseTime(ir); err == nil && t.Unix() == modtime.Unix() {
	timeMatches = true
	}
	}
	if !timeMatches {
	rangeReq = ""
	}
	}

	if inm := r.Header.get("If-None-Match"); inm != "" {
	// Must know ETag.
	if etag == "" {
	return rangeReq, false
	}

	// TODO(bradfitz): non-GET/HEAD requests require more work:
	// sending a different status code on matches, and
	// also can't use weak cache validators (those with a "W/
	// prefix). But most users of ServeContent will be using
	// it on GET or HEAD, so only support those for now.
	if r.Method != "GET" && r.Method != "HEAD" {
	return rangeReq, false
	}

	// TODO(bradfitz): deal with comma-separated or multiple-valued
	// list of If-None-match values. For now just handle the common
	// case of a single item.
	if inm == etag \|\| inm == "*" {
	h := w.Header()
	delete(h, "Content-Type")
	delete(h, "Content-Length")
	w.WriteHeader(StatusNotModified)
	return "", true
	}
	}
	return rangeReq, false
	}

	// name is '/'-separated, not filepath.Separator.
	func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirect bool) {
	const indexPage = "/index.html"

	// redirect .../index.html to .../
	// can't use Redirect() because that would make the path absolute,
	// which would be a problem running under StripPrefix
	if strings.HasSuffix(r.URL.Path, indexPage) {
	localRedirect(w, r, "./")
	return
	}

	f, err := fs.Open(name)
	if err != nil {
	msg, code := toHTTPError(err)
	Error(w, msg, code)
	return
	}
	defer f.Close()

	d, err := f.Stat()
	if err != nil {
	msg, code := toHTTPError(err)
	Error(w, msg, code)
	return
	}

	if redirect {
	// redirect to canonical path: / at end of directory url
	// r.URL.Path always begins with /
	url := r.URL.Path
	if d.IsDir() {
	if url[len(url)-1] != '/' {
	localRedirect(w, r, path.Base(url)+"/")
	return
	}
	} else {
	if url[len(url)-1] == '/' {
	localRedirect(w, r, "../"+path.Base(url))
	return
	}
	}
	}

	// use contents of index.html for directory, if present
	if d.IsDir() {
	index := strings.TrimSuffix(name, "/") + indexPage
	ff, err := fs.Open(index)
	if err == nil {
	defer ff.Close()
	dd, err := ff.Stat()
	if err == nil {
	name = index
	d = dd
	f = ff
	}
	}
	}

	// Still a directory? (we didn't find an index.html file)
	if d.IsDir() {
	if checkLastModified(w, r, d.ModTime()) {
	return
	}
	dirList(w, f)
	return
	}

	// serveContent will check modification time
	sizeFunc := func() (int64, error) { return d.Size(), nil }
	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
	}

	// toHTTPError returns a non-specific HTTP error message and status code
	// for a given non-nil error value. It's important that toHTTPError does not
	// actually return err.Error(), since msg and httpStatus are returned to users,
	// and historically Go's ServeContent always returned just "404 Not Found" for
	// all errors. We don't want to start leaking information in error messages.
	func toHTTPError(err error) (msg string, httpStatus int) {
	if os.IsNotExist(err) {
	return "404 page not found", StatusNotFound
	}
	if os.IsPermission(err) {
	return "403 Forbidden", StatusForbidden
	}
	// Default:
	return "500 Internal Server Error", StatusInternalServerError
	}

	// localRedirect gives a Moved Permanently response.
	// It does not convert relative paths to absolute paths like Redirect does.
	func localRedirect(w ResponseWriter, r *Request, newPath string) {
	if q := r.URL.RawQuery; q != "" {
	newPath += "?" + q
	}
	w.Header().Set("Location", newPath)
	w.WriteHeader(StatusMovedPermanently)
	}

	// ServeFile replies to the request with the contents of the named
	// file or directory.
	//
	// As a special case, ServeFile redirects any request where r.URL.Path
	// ends in "/index.html" to the same path, without the final
	// "index.html". To avoid such redirects either modify the path or
	// use ServeContent.
	func ServeFile(w ResponseWriter, r *Request, name string) {
	dir, file := filepath.Split(name)
	serveFile(w, r, Dir(dir), file, false)
	}

	type fileHandler struct {
	root FileSystem
	}

	// FileServer returns a handler that serves HTTP requests
	// with the contents of the file system rooted at root.
	//
	// To use the operating system's file system implementation,
	// use http.Dir:
	//
	// http.Handle("/", http.FileServer(http.Dir("/tmp")))
	//
	// As a special case, the returned file server redirects any request
	// ending in "/index.html" to the same path, without the final
	// "index.html".
	func FileServer(root FileSystem) Handler {
	return &fileHandler{root}
	}

	func (f fileHandler) ServeHTTP(w ResponseWriter, r Request) {
	upath := r.URL.Path
	if !strings.HasPrefix(upath, "/") {
	upath = "/" + upath
	r.URL.Path = upath
	}
	serveFile(w, r, f.root, path.Clean(upath), true)
	}

	// httpRange specifies the byte range to be sent to the client.
	type httpRange struct {
	start, length int64
	}

	func (r httpRange) contentRange(size int64) string {
	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
	}

	func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
	return textproto.MIMEHeader{
	"Content-Range": {r.contentRange(size)},
	"Content-Type": {contentType},
	}
	}

	// parseRange parses a Range header string as per RFC 2616.
	func parseRange(s string, size int64) ([]httpRange, error) {
	if s == "" {
	return nil, nil // header not present
	}
	const b = "bytes="
	if !strings.HasPrefix(s, b) {
	return nil, errors.New("invalid range")
	}
	var ranges []httpRange
	for _, ra := range strings.Split(s[len(b):], ",") {
	ra = strings.TrimSpace(ra)
	if ra == "" {
	continue
	}
	i := strings.Index(ra, "-")
	if i < 0 {
	return nil, errors.New("invalid range")
	}
	start, end := strings.TrimSpace(ra[:i]), strings.TrimSpace(ra[i+1:])
	var r httpRange
	if start == "" {
	// If no start is specified, end specifies the
	// range start relative to the end of the file.
	i, err := strconv.ParseInt(end, 10, 64)
	if err != nil {
	return nil, errors.New("invalid range")
	}
	if i > size {
	i = size
	}
	r.start = size - i
	r.length = size - r.start
	} else {
	i, err := strconv.ParseInt(start, 10, 64)
	if err != nil \|\| i >= size \|\| i < 0 {
	return nil, errors.New("invalid range")
	}
	r.start = i
	if end == "" {
	// If no end is specified, range extends to end of the file.
	r.length = size - r.start
	} else {
	i, err := strconv.ParseInt(end, 10, 64)
	if err != nil \|\| r.start > i {
	return nil, errors.New("invalid range")
	}
	if i >= size {
	i = size - 1
	}
	r.length = i - r.start + 1
	}
	}
	ranges = append(ranges, r)
	}
	return ranges, nil
	}

	// countingWriter counts how many bytes have been written to it.
	type countingWriter int64

	func (w *countingWriter) Write(p []byte) (n int, err error) {
	*w += countingWriter(len(p))
	return len(p), nil
	}

	// rangesMIMESize returns the number of bytes it takes to encode the
	// provided ranges as a multipart response.
	func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
	var w countingWriter
	mw := multipart.NewWriter(&w)
	for _, ra := range ranges {
	mw.CreatePart(ra.mimeHeader(contentType, contentSize))
	encSize += ra.length
	}
	mw.Close()
	encSize += int64(w)
	return
	}

	func sumRangesSize(ranges []httpRange) (size int64) {
	for _, ra := range ranges {
	size += ra.length
	}
	return
	}

	type byName []os.FileInfo

	func (s byName) Len() int { return len(s) }
	func (s byName) Less(i, j int) bool { return s[i].Name() < s[j].Name() }
	func (s byName) Swap(i, j int) { s[i], s[j] = s[j], s[i] }