Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 970 insertions, 0 deletions

diff --git a/src/net/http/fs.go b/src/net/http/fs.go
new file mode 100644
index 0000000..a28ae85
--- /dev/null
+++ b/src/net/http/fs.go
@@ -0,0 +1,970 @@
+// 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"
+	"io/fs"
+	"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 '/'.
+//
+// Note that Dir could expose sensitive files and directories. Dir will follow
+// symlinks pointing out of the directory tree, which can be especially dangerous
+// if serving from a directory in which users are able to create arbitrary symlinks.
+// Dir will also allow access to files and directories starting with a period,
+// which could expose sensitive directories like .git or sensitive files like
+// .htpasswd. To exclude files with a leading period, remove the files/directories
+// from the server or create a custom FileSystem implementation.
+//
+// An empty Dir is treated as ".".
+type Dir string
+
+// mapDirOpenError maps the provided non-nil error from opening name
+// to a possibly better non-nil error. In particular, it turns OS-specific errors
+// about opening files in non-directories into fs.ErrNotExist. See Issue 18984.
+func mapDirOpenError(originalErr error, name string) error {
+	if os.IsNotExist(originalErr) || os.IsPermission(originalErr) {
+		return originalErr
+	}
+
+	parts := strings.Split(name, string(filepath.Separator))
+	for i := range parts {
+		if parts[i] == "" {
+			continue
+		}
+		fi, err := os.Stat(strings.Join(parts[:i+1], string(filepath.Separator)))
+		if err != nil {
+			return originalErr
+		}
+		if !fi.IsDir() {
+			return fs.ErrNotExist
+		}
+	}
+	return originalErr
+}
+
+// Open implements FileSystem using os.Open, opening files for reading rooted
+// and relative to the directory d.
+func (d Dir) Open(name string) (File, error) {
+	if filepath.Separator != '/' && strings.ContainsRune(name, filepath.Separator) {
+		return nil, errors.New("http: invalid character in file path")
+	}
+	dir := string(d)
+	if dir == "" {
+		dir = "."
+	}
+	fullName := filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name)))
+	f, err := os.Open(fullName)
+	if err != nil {
+		return nil, mapDirOpenError(err, fullName)
+	}
+	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.
+// See the FileServer function to convert a FileSystem to a Handler.
+//
+// This interface predates the fs.FS interface, which can be used instead:
+// the FS adapter function converts an fs.FS to a FileSystem.
+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) ([]fs.FileInfo, error)
+	Stat() (fs.FileInfo, error)
+}
+
+type anyDirs interface {
+	len() int
+	name(i int) string
+	isDir(i int) bool
+}
+
+type fileInfoDirs []fs.FileInfo
+
+func (d fileInfoDirs) len() int          { return len(d) }
+func (d fileInfoDirs) isDir(i int) bool  { return d[i].IsDir() }
+func (d fileInfoDirs) name(i int) string { return d[i].Name() }
+
+type dirEntryDirs []fs.DirEntry
+
+func (d dirEntryDirs) len() int          { return len(d) }
+func (d dirEntryDirs) isDir(i int) bool  { return d[i].IsDir() }
+func (d dirEntryDirs) name(i int) string { return d[i].Name() }
+
+func dirList(w ResponseWriter, r *Request, f File) {
+	// Prefer to use ReadDir instead of Readdir,
+	// because the former doesn't require calling
+	// Stat on every entry of a directory on Unix.
+	var dirs anyDirs
+	var err error
+	if d, ok := f.(fs.ReadDirFile); ok {
+		var list dirEntryDirs
+		list, err = d.ReadDir(-1)
+		dirs = list
+	} else {
+		var list fileInfoDirs
+		list, err = f.Readdir(-1)
+		dirs = list
+	}
+
+	if err != nil {
+		logf(r, "http: error reading directory: %v", err)
+		Error(w, "Error reading directory", StatusInternalServerError)
+		return
+	}
+	sort.Slice(dirs, func(i, j int) bool { return dirs.name(i) < dirs.name(j) })
+
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	fmt.Fprintf(w, "<pre>\n")
+	for i, n := 0, dirs.len(); i < n; i++ {
+		name := dirs.name(i)
+		if dirs.isDir(i) {
+			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-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
+// and If-Range 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 formatted per RFC 7232, section 2.3,
+// ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
+//
+// 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, io.SeekEnd)
+		if err != nil {
+			return 0, errSeeker
+		}
+		_, err = content.Seek(0, io.SeekStart)
+		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")
+
+// errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
+// all of the byte-range-spec values is greater than the content size.
+var errNoOverlap = errors.New("invalid range: failed to overlap")
+
+// 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) {
+	setLastModified(w, modtime)
+	done, rangeReq := checkPreconditions(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, io.SeekStart) // 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 {
+			if err == errNoOverlap {
+				w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
+			}
+			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 7233, Section 4.1:
+			// "If a single part is being transferred, the server
+			// generating the 206 response MUST generate a
+			// Content-Range header field, describing what range
+			// of the selected representation is enclosed, and a
+			// payload consisting of the range.
+			// ...
+			// A server MUST NOT generate a multipart response to
+			// a request for a single range, since a client that
+			// does not request multiple parts might not support
+			// multipart responses."
+			ra := ranges[0]
+			if _, err := content.Seek(ra.start, io.SeekStart); 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, io.SeekStart); 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)
+	}
+}
+
+// scanETag determines if a syntactically valid ETag is present at s. If so,
+// the ETag and remaining text after consuming ETag is returned. Otherwise,
+// it returns "", "".
+func scanETag(s string) (etag string, remain string) {
+	s = textproto.TrimString(s)
+	start := 0
+	if strings.HasPrefix(s, "W/") {
+		start = 2
+	}
+	if len(s[start:]) < 2 || s[start] != '"' {
+		return "", ""
+	}
+	// ETag is either W/"text" or "text".
+	// See RFC 7232 2.3.
+	for i := start + 1; i < len(s); i++ {
+		c := s[i]
+		switch {
+		// Character values allowed in ETags.
+		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
+		case c == '"':
+			return s[:i+1], s[i+1:]
+		default:
+			return "", ""
+		}
+	}
+	return "", ""
+}
+
+// etagStrongMatch reports whether a and b match using strong ETag comparison.
+// Assumes a and b are valid ETags.
+func etagStrongMatch(a, b string) bool {
+	return a == b && a != "" && a[0] == '"'
+}
+
+// etagWeakMatch reports whether a and b match using weak ETag comparison.
+// Assumes a and b are valid ETags.
+func etagWeakMatch(a, b string) bool {
+	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
+}
+
+// condResult is the result of an HTTP request precondition check.
+// See https://tools.ietf.org/html/rfc7232 section 3.
+type condResult int
+
+const (
+	condNone condResult = iota
+	condTrue
+	condFalse
+)
+
+func checkIfMatch(w ResponseWriter, r *Request) condResult {
+	im := r.Header.Get("If-Match")
+	if im == "" {
+		return condNone
+	}
+	for {
+		im = textproto.TrimString(im)
+		if len(im) == 0 {
+			break
+		}
+		if im[0] == ',' {
+			im = im[1:]
+			continue
+		}
+		if im[0] == '*' {
+			return condTrue
+		}
+		etag, remain := scanETag(im)
+		if etag == "" {
+			break
+		}
+		if etagStrongMatch(etag, w.Header().get("Etag")) {
+			return condTrue
+		}
+		im = remain
+	}
+
+	return condFalse
+}
+
+func checkIfUnmodifiedSince(r *Request, modtime time.Time) condResult {
+	ius := r.Header.Get("If-Unmodified-Since")
+	if ius == "" || isZeroTime(modtime) {
+		return condNone
+	}
+	t, err := ParseTime(ius)
+	if err != nil {
+		return condNone
+	}
+
+	// The Last-Modified header truncates sub-second precision so
+	// the modtime needs to be truncated too.
+	modtime = modtime.Truncate(time.Second)
+	if modtime.Before(t) || modtime.Equal(t) {
+		return condTrue
+	}
+	return condFalse
+}
+
+func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
+	inm := r.Header.get("If-None-Match")
+	if inm == "" {
+		return condNone
+	}
+	buf := inm
+	for {
+		buf = textproto.TrimString(buf)
+		if len(buf) == 0 {
+			break
+		}
+		if buf[0] == ',' {
+			buf = buf[1:]
+			continue
+		}
+		if buf[0] == '*' {
+			return condFalse
+		}
+		etag, remain := scanETag(buf)
+		if etag == "" {
+			break
+		}
+		if etagWeakMatch(etag, w.Header().get("Etag")) {
+			return condFalse
+		}
+		buf = remain
+	}
+	return condTrue
+}
+
+func checkIfModifiedSince(r *Request, modtime time.Time) condResult {
+	if r.Method != "GET" && r.Method != "HEAD" {
+		return condNone
+	}
+	ims := r.Header.Get("If-Modified-Since")
+	if ims == "" || isZeroTime(modtime) {
+		return condNone
+	}
+	t, err := ParseTime(ims)
+	if err != nil {
+		return condNone
+	}
+	// The Last-Modified header truncates sub-second precision so
+	// the modtime needs to be truncated too.
+	modtime = modtime.Truncate(time.Second)
+	if modtime.Before(t) || modtime.Equal(t) {
+		return condFalse
+	}
+	return condTrue
+}
+
+func checkIfRange(w ResponseWriter, r *Request, modtime time.Time) condResult {
+	if r.Method != "GET" && r.Method != "HEAD" {
+		return condNone
+	}
+	ir := r.Header.get("If-Range")
+	if ir == "" {
+		return condNone
+	}
+	etag, _ := scanETag(ir)
+	if etag != "" {
+		if etagStrongMatch(etag, w.Header().Get("Etag")) {
+			return condTrue
+		} else {
+			return condFalse
+		}
+	}
+	// The If-Range value is typically the ETag value, but it may also be
+	// the modtime date. See golang.org/issue/8367.
+	if modtime.IsZero() {
+		return condFalse
+	}
+	t, err := ParseTime(ir)
+	if err != nil {
+		return condFalse
+	}
+	if t.Unix() == modtime.Unix() {
+		return condTrue
+	}
+	return condFalse
+}
+
+var unixEpochTime = time.Unix(0, 0)
+
+// isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
+func isZeroTime(t time.Time) bool {
+	return t.IsZero() || t.Equal(unixEpochTime)
+}
+
+func setLastModified(w ResponseWriter, modtime time.Time) {
+	if !isZeroTime(modtime) {
+		w.Header().Set("Last-Modified", modtime.UTC().Format(TimeFormat))
+	}
+}
+
+func writeNotModified(w ResponseWriter) {
+	// RFC 7232 section 4.1:
+	// a sender SHOULD NOT generate representation metadata other than the
+	// above listed fields unless said metadata exists for the purpose of
+	// guiding cache updates (e.g., Last-Modified might be useful if the
+	// response does not have an ETag field).
+	h := w.Header()
+	delete(h, "Content-Type")
+	delete(h, "Content-Length")
+	if h.Get("Etag") != "" {
+		delete(h, "Last-Modified")
+	}
+	w.WriteHeader(StatusNotModified)
+}
+
+// checkPreconditions evaluates request preconditions and reports whether a precondition
+// resulted in sending StatusNotModified or StatusPreconditionFailed.
+func checkPreconditions(w ResponseWriter, r *Request, modtime time.Time) (done bool, rangeHeader string) {
+	// This function carefully follows RFC 7232 section 6.
+	ch := checkIfMatch(w, r)
+	if ch == condNone {
+		ch = checkIfUnmodifiedSince(r, modtime)
+	}
+	if ch == condFalse {
+		w.WriteHeader(StatusPreconditionFailed)
+		return true, ""
+	}
+	switch checkIfNoneMatch(w, r) {
+	case condFalse:
+		if r.Method == "GET" || r.Method == "HEAD" {
+			writeNotModified(w)
+			return true, ""
+		} else {
+			w.WriteHeader(StatusPreconditionFailed)
+			return true, ""
+		}
+	case condNone:
+		if checkIfModifiedSince(r, modtime) == condFalse {
+			writeNotModified(w)
+			return true, ""
+		}
+	}
+
+	rangeHeader = r.Header.get("Range")
+	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
+		rangeHeader = ""
+	}
+	return false, rangeHeader
+}
+
+// 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
+			}
+		}
+	}
+
+	if d.IsDir() {
+		url := r.URL.Path
+		// redirect if the directory name doesn't end in a slash
+		if url == "" || url[len(url)-1] != '/' {
+			localRedirect(w, r, path.Base(url)+"/")
+			return
+		}
+
+		// use contents of index.html for directory, if present
+		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 checkIfModifiedSince(r, d.ModTime()) == condFalse {
+			writeNotModified(w)
+			return
+		}
+		setLastModified(w, d.ModTime())
+		dirList(w, r, 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.
+//
+// If the provided file or directory name is a relative path, it is
+// interpreted relative to the current directory and may ascend to
+// parent directories. If the provided name is constructed from user
+// input, it should be sanitized before calling ServeFile.
+//
+// As a precaution, ServeFile will reject requests where r.URL.Path
+// contains a ".." path element; this protects against callers who
+// might unsafely use filepath.Join on r.URL.Path without sanitizing
+// it and then use that filepath.Join result as the name argument.
+//
+// As another 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.
+//
+// Outside of those two special cases, ServeFile does not use
+// r.URL.Path for selecting the file or directory to serve; only the
+// file or directory provided in the name argument is used.
+func ServeFile(w ResponseWriter, r *Request, name string) {
+	if containsDotDot(r.URL.Path) {
+		// Too many programs use r.URL.Path to construct the argument to
+		// serveFile. Reject the request under the assumption that happened
+		// here and ".." may not be wanted.
+		// Note that name might not contain "..", for example if code (still
+		// incorrectly) used filepath.Join(myDir, r.URL.Path).
+		Error(w, "invalid URL path", StatusBadRequest)
+		return
+	}
+	dir, file := filepath.Split(name)
+	serveFile(w, r, Dir(dir), file, false)
+}
+
+func containsDotDot(v string) bool {
+	if !strings.Contains(v, "..") {
+		return false
+	}
+	for _, ent := range strings.FieldsFunc(v, isSlashRune) {
+		if ent == ".." {
+			return true
+		}
+	}
+	return false
+}
+
+func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
+
+type fileHandler struct {
+	root FileSystem
+}
+
+type ioFS struct {
+	fsys fs.FS
+}
+
+type ioFile struct {
+	file fs.File
+}
+
+func (f ioFS) Open(name string) (File, error) {
+	if name == "/" {
+		name = "."
+	} else {
+		name = strings.TrimPrefix(name, "/")
+	}
+	file, err := f.fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return ioFile{file}, nil
+}
+
+func (f ioFile) Close() error               { return f.file.Close() }
+func (f ioFile) Read(b []byte) (int, error) { return f.file.Read(b) }
+func (f ioFile) Stat() (fs.FileInfo, error) { return f.file.Stat() }
+
+var errMissingSeek = errors.New("io.File missing Seek method")
+var errMissingReadDir = errors.New("io.File directory missing ReadDir method")
+
+func (f ioFile) Seek(offset int64, whence int) (int64, error) {
+	s, ok := f.file.(io.Seeker)
+	if !ok {
+		return 0, errMissingSeek
+	}
+	return s.Seek(offset, whence)
+}
+
+func (f ioFile) ReadDir(count int) ([]fs.DirEntry, error) {
+	d, ok := f.file.(fs.ReadDirFile)
+	if !ok {
+		return nil, errMissingReadDir
+	}
+	return d.ReadDir(count)
+}
+
+func (f ioFile) Readdir(count int) ([]fs.FileInfo, error) {
+	d, ok := f.file.(fs.ReadDirFile)
+	if !ok {
+		return nil, errMissingReadDir
+	}
+	var list []fs.FileInfo
+	for {
+		dirs, err := d.ReadDir(count - len(list))
+		for _, dir := range dirs {
+			info, err := dir.Info()
+			if err != nil {
+				// Pretend it doesn't exist, like (*os.File).Readdir does.
+				continue
+			}
+			list = append(list, info)
+		}
+		if err != nil {
+			return list, err
+		}
+		if count < 0 || len(list) >= count {
+			break
+		}
+	}
+	return list, nil
+}
+
+// FS converts fsys to a FileSystem implementation,
+// for use with FileServer and NewFileTransport.
+func FS(fsys fs.FS) FileSystem {
+	return ioFS{fsys}
+}
+
+// FileServer returns a handler that serves HTTP requests
+// with the contents of the file system rooted at root.
+//
+// As a special case, the returned file server redirects any request
+// ending in "/index.html" to the same path, without the final
+// "index.html".
+//
+// To use the operating system's file system implementation,
+// use http.Dir:
+//
+//     http.Handle("/", http.FileServer(http.Dir("/tmp")))
+//
+// To use an fs.FS implementation, use http.FS to convert it:
+//
+//	http.Handle("/", http.FileServer(http.FS(fsys)))
+//
+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 7233.
+// errNoOverlap is returned if none of the ranges overlap.
+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
+	noOverlap := false
+	for _, ra := range strings.Split(s[len(b):], ",") {
+		ra = textproto.TrimString(ra)
+		if ra == "" {
+			continue
+		}
+		i := strings.Index(ra, "-")
+		if i < 0 {
+			return nil, errors.New("invalid range")
+		}
+		start, end := textproto.TrimString(ra[:i]), textproto.TrimString(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,
+			// and we are dealing with <suffix-length>
+			// which has to be a non-negative integer as per
+			// RFC 7233 Section 2.1 "Byte-Ranges".
+			if end == "" || end[0] == '-' {
+				return nil, errors.New("invalid range")
+			}
+			i, err := strconv.ParseInt(end, 10, 64)
+			if i < 0 || 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 < 0 {
+				return nil, errors.New("invalid range")
+			}
+			if i >= size {
+				// If the range begins after the size of the content,
+				// then it does not overlap.
+				noOverlap = true
+				continue
+			}
+			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)
+	}
+	if noOverlap && len(ranges) == 0 {
+		// The specified ranges did not overlap with the content.
+		return nil, errNoOverlap
+	}
+	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
+}