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package styx
import (
// The walk RPC is one of the more complex requests in the 9P protocol
// (though that's not saying much). There are a number of different
// responses that a server must make:
// - If the file exists: Rwalk with nwname qids
// - If no elements in the path exist: Rerror
// - If at least 1 element in the path exists: Rwalk with n(<nwname) qids
// In addition, walks are relative to another file, so a server must track
// that as well. The styx package attempts to hide this complexity from the
// (API) user.
// For any Twalk requests with nwname > 1, the styx package will synthesize
// nwname individual Twalk requests, such that each request is the next
// element in the path.
// The relative path in the Twalk request is combined with the path of fid
// to produce an absolute path to the requested file. All paths are Cleaned
// before being passed to user code.
// Thus given the following protocol message (fids replaced with their paths):
// Twalk /usr/share 6 ../include/linux/../../bin
// The user's program will instead see and respond to, *IN THIS ORDER*
// Twalk /usr
// Twalk /usr/include
// Twalk /usr/include/linux
// Twalk /usr/include
// Twalk /usr
// Twalk /usr/bin
// The order that the program sees the path in is important, as it allows
// certain synthetic file systems to create resources "on-demand", as the
// client asks for them.
type walkElem struct {
index int
qid styxproto.Qid // nil if not present
err error
type walker struct {
qids, found []styxproto.Qid
filled []int32
count int
complete chan struct{}
collect chan walkElem
newfid uint32
path string
// for cancellation
ctx context.Context
session *Session
tag uint16
func newWalker(s *Session, ctx context.Context, msg styxproto.Twalk, base string, elem ...string) *walker {
qids := make([]styxproto.Qid, len(elem))
found := qids[:0]
newpath := path.Join(base, strings.Join(elem, "/"))
w := &walker{
qids: qids,
found: found,
filled: make([]int32, len(elem)),
complete: make(chan struct{}),
collect: make(chan walkElem),
session: s,
newfid: msg.Newfid(),
path: newpath,
tag: msg.Tag(),
ctx: ctx,
return w
// runs in its own goroutine
func (w *walker) run() {
var err error
for {
select {
case <-w.ctx.Done():
break Loop
case el, ok := <-w.collect:
if !ok {
break Loop
if el.err != nil {
err = el.err
w.qids[el.index] = el.qid
for i := len(w.found); i < cap(w.found); i++ {
if w.qids[i] != nil {
w.found = w.found[:i+1]
if w.count == len(w.qids) {
break Loop
if !w.session.conn.clearTag(w.tag) {
if len(w.found) == 0 {
if err != nil {
w.session.conn.Rerror(w.tag, "%s", err)
} else {
w.session.conn.Rerror(w.tag, "No such file or directory")
} else {
w.session.files.Put(w.newfid, file{name: w.path})
w.session.conn.sessionFid.Put(w.newfid, w.session)
if err := w.session.conn.Rwalk(w.tag, w.found...); err != nil {
panic(err) // should never happen
// A client sends a Twalk message both to probe if a file exists, and to
// move a "cursor" within the filesystem hierarchy. In a traditional file
// system, a Twalk request is similar to using chdir to change the current
// directory. File servers are free to attach additional meaning to Twalk
// requests. For instance, a server may create directories on-demand as
// clients walk to them.
// The 9P protocol allows for clients to walk multiple directories with
// a single 9P message. The styx package translates such requests into
// multiple Twalk values, providing the following guarantees:
// - Path() will return a cleaned, absolute path
// - Consecutive, related Twalk requests will differ by at
// most 1 path element.
// The default response to a Twalk request is an Rerror message saying
// "No such file or directory".
type Twalk struct {
index int
walk *walker
func (t Twalk) WithContext(ctx context.Context) Request {
t.ctx = ctx
return t
func (t Twalk) handled() bool {
return t.walk.filled[t.index] == 1
// Rwalk signals to the client that the file named by the Twalk's
// Path method exists and is of the given mode. The permission bits of
// mode are ignored, and only the file type bits, such as os.ModeDir,
// are sent to the client. If err is non-nil, an error response is sent to the
// client instead.
func (t Twalk) Rwalk(info os.FileInfo, err error) {
var qid styxproto.Qid
var mode os.FileMode
if err == nil {
mode = info.Mode()
qid = t.session.conn.qid(t.Path(), styxfile.QidType(styxfile.Mode9P(mode)))
t.walk.filled[t.index] = 1
elem := walkElem{qid: qid, index: t.index, err: err}
select {
case t.walk.collect <- elem:
case <-t.walk.complete:
// Rerror signals to the client that the file named by the Twalk's
// Path method does not exist.
func (t Twalk) Rerror(format string, args ...interface{}) {
t.Rwalk(nil, fmt.Errorf(format, args...))
func (t Twalk) defaultResponse() {
t.Rerror("No such file or directory")