9p: snapshot *wait baseline at open time to eliminate inter-read race

The previous design read the current value at read time. If a state
change fired between one cat returning and the next cat opening, the
new cat would see the changed value as current and wait forever for
the next change.

Fix: snapshot the baseline in the fid at Topen time. Each read blocks
until the value changes from the open-time snapshot, then updates the
fid's baseline for the next read. The race window is closed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/p9/server.go

@@ -91,6 +91,7 @@ type fid struct {
 	qid      plan9.Qid
 	mode     uint8
 	writeBuf []byte
+	waitBase string // for *wait files: value snapshotted at open time
 }
 
 // connState tracks all open fids for a single 9P connection.
@@ -515,6 +516,17 @@ func (s *Server) open(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
 	}
 
 	f.mode = fc.Mode
+	// Snapshot the current value for *wait files at open time so that the
+	// baseline is fixed before any read — eliminating the race between a
+	// prior read returning and the next read opening.
+	if strings.HasSuffix(pathBase(f.path), "wait") {
+		parts := strings.SplitN(strings.TrimPrefix(f.path, "/"), "/", 3)
+		if len(parts) == 3 {
+			if store, ok := s.sessionFileStore(parts[1]); ok {
+				f.waitBase = store.CurrentWaitValue(parts[2])
+			}
+		}
+	}
 	plog.Debug("Topen fid=%d path=%q mode=%d", fc.Fid, f.path, fc.Mode)
 	return &plan9.Fcall{Type: plan9.Ropen, Tag: fc.Tag, Qid: f.qid}
 }
@@ -751,10 +763,25 @@ func (s *Server) read(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
 			}
 			// *wait files block until a value changes; use connection context.
 			if strings.HasSuffix(parts[2], "wait") {
-				content, err := store.Wait(cs.ctx, parts[2])
+				cs.mu.RLock()
+				f, fidOK := cs.fids[fc.Fid]
+				var base string
+				if fidOK {
+					base = f.waitBase
+				}
+				cs.mu.RUnlock()
+				content, err := store.Wait(cs.ctx, parts[2], base)
 				if err != nil {
 					return errFcall(fc, err.Error())
 				}
+				// Update the fid's baseline to the returned value for subsequent reads.
+				if content != nil {
+					cs.mu.Lock()
+					if f, ok := cs.fids[fc.Fid]; ok {
+						f.waitBase = strings.TrimSuffix(string(content), "\n")
+					}
+					cs.mu.Unlock()
+				}
 				return s.readSlice(fc, content)
 			}
 			content, err := store.Get(parts[2])

internal/p9/sessionfilestore.go

@@ -272,30 +272,49 @@ func (s *SessionFileStore) content(name string) string {
 	return ""
 }
 
-// Wait blocks until the named *wait file's underlying value changes, then
-// returns the new value. Unblocks when connCtx or the session context is
-// cancelled, returning nil with no error (caller returns EOF to client).
-func (s *SessionFileStore) Wait(connCtx context.Context, name string) ([]byte, error) {
+// CurrentWaitValue returns the current value for the named *wait file.
+func (s *SessionFileStore) CurrentWaitValue(name string) string {
+	switch name {
+	case "statewait":
+		return s.sess.core.State()
+	case "usagewait":
+		return s.sess.core.Usage()
+	case "ctxszwait":
+		return s.sess.core.CtxSz()
+	case "cwdwait":
+		return s.sess.core.CWD()
+	}
+	return ""
+}
+
+// Wait blocks until the named *wait file's underlying value changes from base,
+// then returns the new value. If base is empty, the current value is used.
+// Unblocks when connCtx or the session context is cancelled.
+func (s *SessionFileStore) Wait(connCtx context.Context, name, base string) ([]byte, error) {
 	ctx, cancel := context.WithCancel(connCtx)
 	defer cancel()
 	// also unblock when the session itself is killed.
 	context.AfterFunc(s.sess.ctx, cancel)
 
-	var field, current string
+	var field string
 	switch name {
 	case "statewait":
-		field, current = agent.WatchState, s.sess.core.State()
+		field = agent.WatchState
 	case "usagewait":
-		field, current = agent.WatchUsage, s.sess.core.Usage()
+		field = agent.WatchUsage
 	case "ctxszwait":
-		field, current = agent.WatchCtxSz, s.sess.core.CtxSz()
+		field = agent.WatchCtxSz
 	case "cwdwait":
-		field, current = agent.WatchCWD, s.sess.core.CWD()
+		field = agent.WatchCWD
 	default:
 		return nil, fmt.Errorf("%s: not a wait file", name)
 	}
 
-	v, ok := s.sess.core.WaitChange(ctx, field, current)
+	if base == "" {
+		base = s.CurrentWaitValue(name)
+	}
+
+	v, ok := s.sess.core.WaitChange(ctx, field, base)
 	if !ok {
 		return nil, nil
 	}