// 9P filesystem server for ollie sessions.
// See doc comment below for filesystem layout.
package server

import (
	"context"
	"encoding/json"
	"fmt"
	"hash/fnv"
	"io"
	"net"
	"os"
	"os/user"
	"strings"
	"sync"
	"time"

	"ollie/backend"
	"ollie/tools"
	"ollie/elevate"
	olog "ollie/log"
	"ollie/paths"
	"olliesrv/fs"
	"olliesrv/session"

	"9fans.net/go/plan9"
)

// Re-export fs types for use in server.go.
type (
	File     = fs.File
	FileTree = fs.FileTree

	syntheticFileInfo = fs.SyntheticFileInfo
)

const (
	QTDir  = plan9.QTDIR
	QTFile = plan9.QTFILE
)

// fid tracks per-descriptor state for a single 9P connection.
type fid struct {
	path           string
	qid            plan9.Qid
	mode           uint8
	writeBuf       []byte
	waitBase       string // for *wait files: value snapshotted at open time
	dirCache       []byte // cached serialized dir entries for readdir
	completeResult []byte // for /complete: result of last write (per-fid state)
	generateResult []byte // for /generate: result of last write (per-fid state)
	routeResult    []byte // for /route: result of last write (per-fid state)
}

// connState tracks all open fids for a single 9P connection.
type connState struct {
	mu      sync.RWMutex
	fids    map[uint32]*fid
	ctx     context.Context
	cancel  context.CancelFunc
	pending map[uint16]context.CancelFunc // in-flight request cancels, keyed by tag
	uname   string                        // user principal from Tattach
}

// Server is the 9P server for ollie sessions.
type Server struct {
	mu               sync.RWMutex
	conns            []*connState
	log              *olog.Logger
	sink             *olog.Sink
	sessionMgr       *session.Manager
	rootStore        *fs.Tree
	elevateTree      *elevateTree
	groups           map[string]map[string]bool // group → set of members
	invalidateModels func()
	toolPrompt       func(string) string
	toolRegistry     *tools.Registry
}

// Config holds the pre-built trees and manager for the server.
type Config struct {
	Sink             *olog.Sink
	SessionMgr       *session.Manager
	RootStore        *fs.Tree
	ElevateBroker    *elevate.Broker
	InvalidateModels func()
	ToolPrompt       func(string) string
	// ToolRegistry is the shared tool registry for lazy tool promotion.
	ToolRegistry *tools.Registry
}

// Option configures the 9P server.
type Option func(*Server)

// New creates a new Server from a pre-built Config.
func New(cfg Config) *Server {
	s := &Server{
		log:              cfg.Sink.Logger("9p", olog.LevelDebug),
		sink:             cfg.Sink,
		sessionMgr:       cfg.SessionMgr,
		rootStore:        cfg.RootStore,
		groups:           make(map[string]map[string]bool),
		invalidateModels: cfg.InvalidateModels,
		toolPrompt:       cfg.ToolPrompt,
		toolRegistry:     cfg.ToolRegistry,
	}
	if cfg.ElevateBroker != nil {
		s.elevateTree = newElevateTree(cfg.ElevateBroker)
	}
	return s
}

// fsRoute maps a path prefix to its backing fs and directory permissions.
type fsRoute struct {
	prefix  string
	dirMode os.FileMode
	tree    func() FileTree
}

// routes returns the route table for the file tree namespace.
// Directory permissions come from the fs.Perms registry.
func (s *Server) routes() []fsRoute {
	routes := []fsRoute{
		{fs.PathSessions, fs.Perms[fs.PathSessions].DirMode, func() FileTree { return s.sessionMgr.Tree() }},
	}
	if s.elevateTree != nil {
		routes = append(routes, fsRoute{fs.PathElevate, fs.Perms[fs.PathElevate].DirMode, func() FileTree { return s.elevateTree }})
	}
	return routes
}

// route returns the tree and entry name for a path, or nil if no route matches.
func (s *Server) route(path string) (FileTree, string) {
	for _, r := range s.routes() {
		if strings.HasPrefix(path, r.prefix) {
			return r.tree(), strings.TrimPrefix(path, r.prefix)
		}
	}
	// Root-level files (e.g. /backends, /help).
	name := strings.TrimPrefix(path, "/")
	if !strings.Contains(name, "/") {
		if _, err := s.rootStore.Stat(name); err == nil {
			return s.rootStore, name
		}
	}
	return nil, ""
}

// openEntry opens a File for the given path.
func (s *Server) openEntry(path string) (File, error) {
	// Intercept s/{id}/elevate — session policy via broker
	if s.elevateTree != nil && strings.HasPrefix(path, fs.PathSessions) {
		rel := strings.TrimPrefix(path, fs.PathSessions)
		if parts := strings.SplitN(rel, "/", 2); len(parts) == 2 && parts[1] == "elevate" {
			sessionID := parts[0]
			return &sessionElevateFile{broker: s.elevateTree.broker, sessionID: sessionID}, nil
		}
	}

	st, name := s.route(path)
	if st == nil {
		return nil, fmt.Errorf("%s: not found", path)
	}
	return st.Open(name)
}

// routeDir returns the tree for a directory path (e.g. "/a" → agentStore).
func (s *Server) routeDir(path string) FileTree {
	for _, r := range s.routes() {
		if path+"/" == r.prefix {
			return r.tree()
		}
	}
	return nil
}

// dirMode returns the directory permission for a path.
// Top-level directories get their mode from the route table.
func (s *Server) dirMode(path string) plan9.Perm {
	for _, r := range s.routes() {
		if path+"/" == r.prefix {
			return plan9.Perm(r.dirMode)
		}
		if strings.HasPrefix(path, r.prefix) {
			return plan9.Perm(r.dirMode)
		}
	}
	return plan9.Perm(fs.Perms[fs.PathRoot].DirMode)
}

// filePerm returns the file permission for a path from the permission registry.
func (s *Server) filePerm(path string) os.FileMode {
	// Session files have per-file overrides.
	if strings.HasPrefix(path, fs.PathSessions) {
		// Peer/proc file permissions.
		name := strings.TrimPrefix(path, fs.PathSessions)
		parts := strings.SplitN(name, "/", 3)
		if len(parts) == 3 && parts[1] == "peer" {
			return 0666
		}
		if len(parts) == 3 && parts[1] == "proc" {
			return 0444
		}
		base := pathBase(path)
		if m, ok := fs.Perms[fs.PathSessionFile].Files[base]; ok {
			return m
		}
		if m, ok := fs.Perms[fs.PathSessions].Files[base]; ok {
			return m
		}
		return fs.Perms[fs.PathSessions].FileMode
	}
	for _, r := range s.routes() {
		if strings.HasPrefix(path, r.prefix) {
			if p, ok := fs.Perms[r.prefix]; ok {
				return p.FileMode
			}
		}
	}
	// Root-level files: query the rootStore for actual permission
	name := strings.TrimPrefix(path, "/")
	if !strings.Contains(name, "/") {
		if info, err := s.rootStore.Stat(name); err == nil {
			return info.Mode()
		}
	}
	return fs.PermIdx
}

// AddGroup adds a user to a group.
func (s *Server) AddGroup(group, user string) {
	s.mu.Lock()
	if s.groups[group] == nil {
		s.groups[group] = make(map[string]bool)
	}
	s.groups[group][user] = true
	s.mu.Unlock()
}

// RemoveGroup removes a user from a group.
func (s *Server) RemoveGroup(group, user string) {
	s.mu.Lock()
	if m := s.groups[group]; m != nil {
		delete(m, user)
		if len(m) == 0 {
			delete(s.groups, group)
		}
	}
	s.mu.Unlock()
}

// InGroup returns true if user is a member of group.
func (s *Server) InGroup(group, user string) bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.groups[group][user]
}

// fileOwnerGroup returns the uid and gid for a given path.
// Session namespace (/session/{sid}/**) is owned by the agent principal with group "agent".
// Everything else is owned by the current system user and their primary group.
func (s *Server) fileOwnerGroup(path string) (uid, gid string) {
	if strings.HasPrefix(path, fs.PathSessions) {
		parts := strings.SplitN(strings.TrimPrefix(path, fs.PathSessions), "/", 2)
		if len(parts) >= 1 && parts[0] != "new" && !isSessionFile(path) {
			if sess := s.sessionMgr.Session(parts[0]); sess != nil {
				return sess.Uname(), "agent"
			}
		}
	}

	if u, err := user.Current(); err == nil {
		return u.Username, "agent"
	}
	return "ollie", "agent"
}

// checkPerm verifies that uname has the requested access (mode) to path.
// mode is the 9P open mode: OREAD=0, OWRITE=1, ORDWR=2, OEXEC=3.
// Enforced on all paths.
func (s *Server) checkPerm(uname, path string, mode uint8) error {
	uid, gid := s.fileOwnerGroup(path)
	dir := s.makeStat(path)
	perm := uint32(dir.Mode) & 0777

	// Determine which permission bits apply.
	var bits uint32
	if uname == uid {
		bits = (perm >> 6) & 7
	} else if s.InGroup(gid, uname) {
		bits = (perm >> 3) & 7
	} else {
		bits = perm & 7
	}

	// Map 9P open mode to required permission bit.
	omode := mode & 3
	switch omode {
	case plan9.OREAD:
		if bits&4 == 0 {
			return fmt.Errorf("permission denied")
		}
	case plan9.OWRITE:
		if bits&2 == 0 {
			return fmt.Errorf("permission denied")
		}
	case plan9.ORDWR:
		if bits&6 != 6 {
			return fmt.Errorf("permission denied")
		}
	case plan9.OEXEC:
		if bits&1 == 0 {
			return fmt.Errorf("permission denied")
		}
	}
	return nil
}

// readFile opens an entry in a tree and reads it.
func readFile(s fs.FileTree, name string) ([]byte, error) {
	e, err := s.Open(name)
	if err != nil {
		return nil, err
	}
	return e.Read()
}

// writeFile opens an entry in a tree and writes to it.
func writeFile(s fs.FileTree, name string, data []byte) error {
	e, err := s.Open(name)
	if err != nil {
		return err
	}
	return e.Write(data)
}

// readTimeout is the server-side deadline for all non-blocking reads.
const readTimeout = 10 * time.Second

// Serve handles a single 9P connection. Each request is dispatched to its own
// goroutine so blocking reads (e.g. *wait files) do not stall the serve loop.
func (s *Server) Serve(conn net.Conn) {
	defer conn.Close()
	connCtx, connCancel := context.WithCancel(context.Background())
	cs := &connState{
		fids:    make(map[uint32]*fid),
		ctx:     connCtx,
		cancel:  connCancel,
		pending: make(map[uint16]context.CancelFunc),
	}
	s.mu.Lock()
	s.conns = append(s.conns, cs)
	s.mu.Unlock()
	defer func() {
		connCancel()
		s.mu.Lock()
		for i, c := range s.conns {
			if c == cs {
				s.conns = append(s.conns[:i], s.conns[i+1:]...)
				break
			}
		}
		s.mu.Unlock()
	}()

	responses := make(chan *plan9.Fcall, 16)
	var wg sync.WaitGroup

	// writer: serialises responses back onto the connection.
	go func() {
		for resp := range responses {
			plan9.WriteFcall(conn, resp) //nolint:errcheck
		}
	}()

	for {
		fc, err := plan9.ReadFcall(conn)
		if err != nil {
			if err != io.EOF {
				s.log.Error("read: %v", err)
			}
			break
		}
		reqCtx, reqCancel := context.WithCancel(connCtx)
		cs.mu.Lock()
		cs.pending[fc.Tag] = reqCancel
		cs.mu.Unlock()

		wg.Add(1)
		go func(fc *plan9.Fcall, ctx context.Context) {
			defer func() {
				reqCancel()
				cs.mu.Lock()
				delete(cs.pending, fc.Tag)
				cs.mu.Unlock()
				wg.Done()
			}()
			responses <- s.handle(cs, fc, ctx)
		}(fc, reqCtx)
	}

	wg.Wait()
	close(responses)
}

func (s *Server) handle(cs *connState, fc *plan9.Fcall, ctx context.Context) *plan9.Fcall {
	start := time.Now()
	defer func() {
		s.log.Debug("  -> %s %dµs", fcallTypeName(fc.Type), time.Since(start).Microseconds())
	}()
	switch fc.Type {
	case plan9.Tversion:
		msize := fc.Msize
		if msize > 65536 {
			msize = 65536
		}
		s.log.Debug("Tversion msize=%d", msize)
		return &plan9.Fcall{Type: plan9.Rversion, Tag: fc.Tag, Msize: msize, Version: "9P2000"}
	case plan9.Tauth:
		return errFcall(fc, "no auth required")
	case plan9.Tattach:
		s.log.Debug("Tattach fid=%d", fc.Fid)
		return s.attach(cs, fc)
	case plan9.Twalk:
		return s.walk(cs, fc)
	case plan9.Topen:
		return s.open(cs, fc)
	case plan9.Tcreate:
		return s.create(cs, fc)
	case plan9.Tread:
		return s.read(cs, fc, ctx)
	case plan9.Twrite:
		return s.write(cs, fc)
	case plan9.Tstat:
		return s.stat(cs, fc)
	case plan9.Twstat:
		return s.wstat(cs, fc)
	case plan9.Tflush:
		cs.mu.Lock()
		if cancel, ok := cs.pending[fc.Oldtag]; ok {
			cancel()
		}
		cs.mu.Unlock()
		return &plan9.Fcall{Type: plan9.Rflush, Tag: fc.Tag}
	case plan9.Tclunk:
		return s.clunk(cs, fc)
	case plan9.Tremove:
		return s.remove(cs, fc)
	default:
		return errFcall(fc, "unsupported operation")
	}
}

func fcallTypeName(t uint8) string {
	switch t {
	case plan9.Tversion:
		return "Tversion"
	case plan9.Tauth:
		return "Tauth"
	case plan9.Tattach:
		return "Tattach"
	case plan9.Twalk:
		return "Twalk"
	case plan9.Topen:
		return "Topen"
	case plan9.Tcreate:
		return "Tcreate"
	case plan9.Tread:
		return "Tread"
	case plan9.Twrite:
		return "Twrite"
	case plan9.Tstat:
		return "Tstat"
	case plan9.Twstat:
		return "Twstat"
	case plan9.Tflush:
		return "Tflush"
	case plan9.Tclunk:
		return "Tclunk"
	case plan9.Tremove:
		return "Tremove"
	default:
		return fmt.Sprintf("T%d", t)
	}
}

// isSessionFile reports whether path is a fixed file directly under /session/
// (i.e. /session/<name> where name is in sessionStoreFiles).
func isSessionFile(path string) bool {
	name, ok := strings.CutPrefix(path, fs.PathSessions)
	if !ok || strings.Contains(name, "/") {
		return false
	}
	_, ok = session.FileMode(name)
	return ok
}

func errFcall(fc *plan9.Fcall, msg string) *plan9.Fcall {
	return &plan9.Fcall{Type: plan9.Rerror, Tag: fc.Tag, Ename: msg}
}

// qidPath returns a stable numeric path for use in Qid structs.
func qidPath(path string) uint64 {
	if path == "/" {
		return 0
	}
	h := fnv.New64a()
	h.Write([]byte(path))
	return h.Sum64()
}

// pathType returns "dir", "file", or "" (not found) for a logical path.
func (s *Server) pathType(path string) string {
	if path == "/" {
		return "dir"
	}

	// Check route table: directory match (e.g. "/a") or file match (e.g. "/a/foo").
	if st := s.routeDir(path); st != nil {
		return "dir"
	}

	// Session paths: determine type from path structure without calling Stat
	// (which triggers expensive content reads).
	if strings.HasPrefix(path, fs.PathSessions) {
		name := strings.TrimPrefix(path, fs.PathSessions)
		if !strings.Contains(name, "/") {
			// /session/{name} — could be a session dir or a fixed file (new, idx, ls, etc.)
			if isSessionFile(path) {
				return "file"
			}
			return "dir"
		}
		// /session/{id}/peer or /session/{id}/proc — directory
		parts := strings.SplitN(name, "/", 3)
		if len(parts) == 2 && (parts[1] == "peer" || parts[1] == "proc") {
			return "dir"
		}
		// /session/{id}/peer/{peer-id} — verify peer exists
		if len(parts) == 3 && parts[1] == "peer" {
			sess := s.sessionMgr.Session(parts[0])
			if sess == nil {
				return ""
			}
			if sess.HasPeer(parts[2]) {
				return "file"
			}
			return ""
		}
		// /session/{id}/proc/{pid} — verify PID exists in detached list
		if len(parts) == 3 && parts[1] == "proc" {
			sess := s.sessionMgr.Session(parts[0])
			if sess == nil {
				return ""
			}
			for _, p := range sess.Core.ListDetached() {
				if fmt.Sprintf("%d", p.PID) == parts[2] {
					return "file"
				}
			}
			return ""
		}
		// /session/{id}/{file} — file
		return "file"
	}

	if st, name := s.route(path); st != nil {
		if info, err := st.Stat(name); err == nil {
			if info.IsDir() {
				return "dir"
			}
			return "file"
		}
		return ""
	}
	return ""
}

func pathParent(path string) string {
	if path == "/" {
		return "/"
	}
	i := strings.LastIndex(path, "/")
	if i == 0 {
		return "/"
	}
	return path[:i]
}

func pathJoin(parent, name string) string {
	if parent == "/" {
		return "/" + name
	}
	return parent + "/" + name
}

func pathBase(path string) string {
	i := strings.LastIndex(path, "/")
	if i < 0 {
		return path
	}
	return path[i+1:]
}

func boolToDir(isDir bool) uint32 {
	if isDir {
		return uint32(plan9.DMDIR)
	}
	return 0
}

func (s *Server) attach(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	defer cs.mu.Unlock()
	cs.uname = fc.Uname
	qid := plan9.Qid{Type: QTDir, Path: 0}
	cs.fids[fc.Fid] = &fid{path: "/", qid: qid}
	s.log.Debug("Tattach uname=%q", fc.Uname)
	// All clients get agent group membership so they can access shared
	// directories. Owner bits handle user-vs-agent distinction.
	if fc.Uname != "" {
		s.AddGroup("agent", fc.Uname)
	}
	return &plan9.Fcall{Type: plan9.Rattach, Tag: fc.Tag, Qid: qid}
}

func (s *Server) walk(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	f, ok := cs.fids[fc.Fid]
	if !ok {
		return errFcall(fc, "bad fid")
	}

	// Clone: copy the fid to newfid (even when wnames is empty).
	newf := &fid{path: f.path, qid: f.qid}

	if len(fc.Wname) == 0 {
		cs.fids[fc.Newfid] = newf
		return &plan9.Fcall{Type: plan9.Rwalk, Tag: fc.Tag, Wqid: []plan9.Qid{}}
	}

	s.log.Debug("Twalk fid=%d newfid=%d from=%q wnames=%v", fc.Fid, fc.Newfid, f.path, fc.Wname)

	wqids := make([]plan9.Qid, 0, len(fc.Wname))
	cur := f.path

	for _, name := range fc.Wname {
		var next string
		if name == ".." {
			next = pathParent(cur)
		} else {
			next = pathJoin(cur, name)
		}

		t := s.pathType(next)
		if t == "" {
			if len(wqids) == 0 {
				return errFcall(fc, name+": file not found")
			}
			break
		}

		q := plan9.Qid{Path: qidPath(next)}
		if t == "dir" {
			q.Type = QTDir
		}
		wqids = append(wqids, q)
		cur = next
	}

	if len(wqids) == len(fc.Wname) {
		newf.path = cur
		newf.qid = wqids[len(wqids)-1]
		cs.fids[fc.Newfid] = newf
	}

	return &plan9.Fcall{Type: plan9.Rwalk, Tag: fc.Tag, Wqid: wqids}
}

func (s *Server) open(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	f, ok := cs.fids[fc.Fid]
	if !ok {
		return errFcall(fc, "bad fid")
	}

	if err := s.checkPerm(cs.uname, f.path, fc.Mode); err != nil {
		return errFcall(fc, err.Error())
	}

	f.mode = fc.Mode
	s.log.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}
}

func (s *Server) create(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	defer cs.mu.Unlock()

	f, ok := cs.fids[fc.Fid]
	if !ok {
		return errFcall(fc, "bad fid")
	}

	newPath := pathJoin(f.path, fc.Name)

	if fc.Perm&plan9.DMDIR != 0 {
		// mkdir: delegate to tree if it supports MkdirAll.
		type dirCreator interface {
			MkdirAll(string) error
		}
		st, name := s.route(newPath)
		if st == nil {
			return errFcall(fc, "mkdir not supported")
		}
		if dc, ok := st.(dirCreator); ok {
			if err := dc.MkdirAll(name); err != nil {
				return errFcall(fc, err.Error())
			}
		} else {
			return errFcall(fc, "mkdir not supported")
		}
		qid := plan9.Qid{Type: QTDir, Path: qidPath(newPath)}
		f.path = newPath
		f.qid = qid
		f.mode = fc.Mode
		return &plan9.Fcall{Type: plan9.Rcreate, Tag: fc.Tag, Qid: qid}
	}

	s.log.Debug("Tcreate parent=%q name=%q", f.path, fc.Name)

	// File create: route to the parent directory's fs.
	if st := s.routeDir(f.path); st != nil {
		if err := st.Create(fc.Name); err != nil {
			return errFcall(fc, err.Error())
		}
	} else if st, parentName := s.route(f.path); st != nil {
		// Parent is a subdirectory within a tree; create relative to it.
		type subCreator interface {
			Create(string) error
		}
		if sc, ok := st.(subCreator); ok {
			if err := sc.Create(parentName + "/" + fc.Name); err != nil {
				return errFcall(fc, err.Error())
			}
		}
	}

	qid := plan9.Qid{Path: qidPath(newPath)}
	f.path = newPath
	f.qid = qid
	f.mode = fc.Mode
	return &plan9.Fcall{Type: plan9.Rcreate, Tag: fc.Tag, Qid: qid}
}

func (s *Server) read(cs *connState, fc *plan9.Fcall, ctx context.Context) *plan9.Fcall {
	cs.mu.RLock()
	f, ok := cs.fids[fc.Fid]
	if !ok {
		cs.mu.RUnlock()
		return errFcall(fc, "bad fid")
	}
	path := f.path
	isDir := f.qid.Type&QTDir != 0
	cs.mu.RUnlock()

	if isDir {
		s.log.Debug("Tread dir path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
		// Use cached dir data if available; rebuild on offset=0 (new listing).
		if fc.Offset == 0 || f.dirCache == nil {
			f.dirCache = s.buildDirData(path)
		}
		data := s.sliceDirData(f.dirCache, fc.Offset, fc.Count)
		return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
	}

	// /complete: return per-fid result from previous write (offset 0-based)
	if path == "/complete" {
		s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
		return s.readDNS(fc, &f.completeResult)
	}

	// /generate: return per-fid result from previous write (offset 0-based)
	if path == "/generate" {
		s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
		return s.readDNS(fc, &f.generateResult)
	}

	// /route: return per-fid result from previous write (offset 0-based)
	if path == "/route" {
		s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
		return s.readDNS(fc, &f.routeResult)
	}

	s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
	entry, err := s.openEntry(path)
	if err != nil {
		return errFcall(fc, err.Error())
	}

	// OneShot entries yield data once per open; offset>0 means EOF.
	if entry.OneShot() && fc.Offset > 0 {
		return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: 0}
	}

	// Blocking entries use BlockingRead with timeout and waitBase tracking.
	if entry.IsBlocking() {
		cs.mu.RLock()
		f, fidOK := cs.fids[fc.Fid]
		var base string
		if fidOK {
			base = f.waitBase
		}
		cs.mu.RUnlock()
		waitCtx, waitCancel := context.WithTimeout(ctx, 5*time.Second)
		defer waitCancel()
		content, nextBase, err := entry.BlockingRead(waitCtx, base)
		if err != nil {
			return errFcall(fc, err.Error())
		}
		if nextBase != "" {
			cs.mu.Lock()
			if f, ok := cs.fids[fc.Fid]; ok {
				f.waitBase = nextBase
			}
			cs.mu.Unlock()
		}
		return s.readSlice(fc, content)
	}

	// Normal read with timeout.
	type result struct {
		data []byte
		err  error
	}
	ch := make(chan result, 1)
	go func() {
		data, err := entry.Read()
		ch <- result{data, err}
	}()
	timer := time.NewTimer(readTimeout)
	defer timer.Stop()
	select {
	case r := <-ch:
		if r.err != nil {
			return errFcall(fc, r.err.Error())
		}
		return s.readSlice(fc, r.data)
	case <-ctx.Done():
		return errFcall(fc, ctx.Err().Error())
	case <-timer.C:
		return errFcall(fc, "read timeout")
	}
}

// readSlice serves a byte slice at the requested offset/count.
func (s *Server) readSlice(fc *plan9.Fcall, content []byte) *plan9.Fcall {
	var data []byte
	off := int(fc.Offset)
	if off < len(content) {
		end := off + int(fc.Count)
		if end > len(content) {
			end = len(content)
		}
		data = content[off:end]
	}
	return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
}

// readDNS implements the Plan 9 /net/dns read pattern: the result is always
// read from offset 0 regardless of the client's file position (which advances
// after a write). Returns the content once, then returns EOF on subsequent reads.
func (s *Server) readDNS(fc *plan9.Fcall, content *[]byte) *plan9.Fcall {
	var data []byte
	if len(*content) > 0 {
		end := int(fc.Count)
		if end > len(*content) {
			end = len(*content)
		}
		data = (*content)[:end]
		*content = nil // consumed; next read returns EOF
	}
	return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
}

func (s *Server) helpPath() string {
	return paths.CfgDir() + "/help.md"
}

func (s *Server) write(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	f, ok := cs.fids[fc.Fid]
	if !ok {
		cs.mu.Unlock()
		return errFcall(fc, "bad fid")
	}
	s.log.Debug("Twrite fid=%d path=%q offset=%d len=%d", fc.Fid, f.path, fc.Offset, len(fc.Data))

	// /complete: synchronous request-response (per-fid state, like /net/dns)
	if f.path == "/complete" {
		cs.mu.Unlock()
		var req struct {
			CWD          string `json:"cwd"`
			FilePath     string `json:"file"`
			Prefix       string `json:"prefix"`
			Suffix       string `json:"suffix"`
			ExtraContext string `json:"context"`
		}
		if err := json.Unmarshal(fc.Data, &req); err != nil {
			// If not JSON, treat entire write as prefix (simple mode)
			req.Prefix = strings.TrimSpace(string(fc.Data))
		}
		result, err := s.sessionMgr.Complete(req.CWD, req.FilePath, req.Prefix, req.Suffix, req.ExtraContext)
		if err != nil {
			return errFcall(fc, err.Error())
		}
		f.completeResult = []byte(result)
		return &plan9.Fcall{Type: plan9.Rwrite, Tag: fc.Tag, Count: uint32(len(fc.Data))}
	}

	// /generate: stateless one-shot generation (per-fid state, like /net/dns)
	if f.path == "/generate" {
		cs.mu.Unlock()
		var req backend.GenerateRequest
		if err := json.Unmarshal(fc.Data, &req); err != nil {
			req.Prompt = strings.TrimSpace(string(fc.Data))
		}
		result, err := backend.Generate(context.Background(), req)
		if err != nil {
			return errFcall(fc, err.Error())
		}
		f.generateResult = []byte(result)
		return &plan9.Fcall{Type: plan9.Rwrite, Tag: fc.Tag, Count: uint32(len(fc.Data))}
	}

	// /route: classify task and select appropriate backend+model (per-fid state, like /net/dns)
	if f.path == "/route" {
		cs.mu.Unlock()
		var req backend.RouteRequest
		if err := json.Unmarshal(fc.Data, &req); err != nil {
			req.Task = strings.TrimSpace(string(fc.Data))
		}
		result, err := backend.Route(context.Background(), req)
		if err != nil {
			return errFcall(fc, err.Error())
		}
		f.routeResult = []byte(fmt.Sprintf("backend=%s model=%s", result.Backend, result.Model))
		return &plan9.Fcall{Type: plan9.Rwrite, Tag: fc.Tag, Count: uint32(len(fc.Data))}
	}

	// Accumulate; the 9P client may split large writes across multiple Twrite messages.
	end := int(fc.Offset) + len(fc.Data)
	if end > len(f.writeBuf) {
		grown := make([]byte, end)
		copy(grown, f.writeBuf)
		f.writeBuf = grown
	}
	copy(f.writeBuf[fc.Offset:], fc.Data)
	cs.mu.Unlock()
	return &plan9.Fcall{Type: plan9.Rwrite, Tag: fc.Tag, Count: uint32(len(fc.Data))}
}

func (s *Server) stat(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.RLock()
	f, ok := cs.fids[fc.Fid]
	cs.mu.RUnlock()
	if !ok {
		return errFcall(fc, "bad fid")
	}
	dir := s.makeStat(f.path)
	s.log.Debug("Tstat path=%q mode=%o len=%d", f.path, dir.Mode, dir.Length)
	stat, err := dir.Bytes()
	if err != nil {
		return errFcall(fc, err.Error())
	}
	return &plan9.Fcall{Type: plan9.Rstat, Tag: fc.Tag, Stat: stat}
}

func (s *Server) wstat(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	f, ok := cs.fids[fc.Fid]
	cs.mu.Unlock()
	if !ok {
		return errFcall(fc, "bad fid")
	}

	// Parse the new Dir from the stat bytes.
	newDir, err := plan9.UnmarshalDir(fc.Stat)
	if err != nil {
		// Some clients send minimal wstat (e.g. truncate); accept silently.
		return &plan9.Fcall{Type: plan9.Rwstat, Tag: fc.Tag}
	}

	oldName := pathBase(f.path)
	s.log.Debug("Twstat path=%q oldName=%q newName=%q", f.path, oldName, newDir.Name)
	if newDir.Name == "" || newDir.Name == oldName {
		return &plan9.Fcall{Type: plan9.Rwstat, Tag: fc.Tag}
	}

	st, relPath := s.route(f.path)
	if st == nil {
		return &plan9.Fcall{Type: plan9.Rwstat, Tag: fc.Tag}
	}
	if err := st.Rename(relPath, newDir.Name); err != nil {
		return errFcall(fc, err.Error())
	}
	// Rewrite all fids that share the old path prefix.
	for _, r := range s.routes() {
		if strings.HasPrefix(f.path, r.prefix) {
			oldPath := f.path
			newPath := r.prefix + relPath[:len(relPath)-len(oldName)] + newDir.Name
			s.mu.RLock()
			conns := s.conns
			s.mu.RUnlock()
			for _, c := range conns {
				c.mu.Lock()
				for _, fid := range c.fids {
					if fid.path == oldPath || strings.HasPrefix(fid.path, oldPath+"/") {
						fid.path = newPath + fid.path[len(oldPath):]
						fid.qid.Path = qidPath(fid.path)
					}
				}
				c.mu.Unlock()
			}
			// Update group membership for session renames.
			if r.prefix == fs.PathSessions && !strings.Contains(relPath, "/") {
				s.RemoveGroup("agent", oldName)
				s.AddGroup("agent", newDir.Name)
			}
			break
		}
	}
	return &plan9.Fcall{Type: plan9.Rwstat, Tag: fc.Tag}
}

func (s *Server) clunk(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	f, ok := cs.fids[fc.Fid]
	var path string
	var data []byte
	var writable bool
	if ok {
		if m := f.mode & 3; m == plan9.OWRITE || m == plan9.ORDWR {
			path = f.path
			writable = true
			data = make([]byte, len(f.writeBuf))
			copy(data, f.writeBuf)
		}
		delete(cs.fids, fc.Fid)
	}
	uname := cs.uname
	cs.mu.Unlock()
	if writable {
		s.log.Debug("Tclunk flush path=%q writeBuf=%d uname=%q", path, len(data), uname)
		input := strings.TrimSpace(string(data))
		async := false
		if entry, err := s.openEntry(path); err == nil {
			async = entry.Async()
		}
		if async {
			go s.handleWrite(path, input, uname) //nolint:errcheck
		} else if err := s.handleWrite(path, input, uname); err != nil {
			s.log.Debug("Tclunk handleWrite err=%v", err)
			return errFcall(fc, err.Error())
		}
	} else {
		s.log.Debug("Tclunk fid=%d path=%q (no write)", fc.Fid, path)
	}
	return &plan9.Fcall{Type: plan9.Rclunk, Tag: fc.Tag}
}

func (s *Server) remove(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
	cs.mu.Lock()
	f, ok := cs.fids[fc.Fid]
	if ok {
		delete(cs.fids, fc.Fid)
	}
	cs.mu.Unlock()
	if !ok {
		return errFcall(fc, "bad fid")
	}

	path := f.path
	s.log.Debug("Tremove path=%q", path)

	st, name := s.route(path)
	if st == nil {
		return errFcall(fc, "remove not supported")
	}
	if err := st.Delete(name); err != nil {
		return errFcall(fc, err.Error())
	}
	return &plan9.Fcall{Type: plan9.Rremove, Tag: fc.Tag}
}

// handleWrite processes a fully-assembled write payload for the given path.
// Called synchronously from clunk; prompt writes are the exception (spawned
// as a goroutine because they block for the entire agent turn).
func (s *Server) handleWrite(path, input, uname string) error {
	s.log.Debug("handleWrite path=%q input_len=%d uname=%q", path, len(input), uname)

	// Handle /tools/load - promote a tool for the current session
	st, name := s.route(path)
	if st == nil {
		return nil
	}
	return writeFile(st, name, []byte(input))
}

// Shutdown kills all active sessions and batch jobs.
func (s *Server) Shutdown() {
	s.sessionMgr.Shutdown()
}

// InterruptAll cancels any in-progress agent turn on every active session.
func (s *Server) InterruptAll() {
	s.sessionMgr.InterruptAll()
}

// buildDirData serializes all directory entries for the given path into a byte slice.
func (s *Server) buildDirData(path string) []byte {
	var dirs []plan9.Dir

	if path == "/" {
		rootEntries := []string{"agents", "backends", "complete", "ctl", "generate", "help", "models", "route", "session"}
		for _, name := range rootEntries {
			fpath := "/" + name
			st := s.makeStat(fpath)
			dirs = append(dirs, st)
		}
	} else if st := s.routeDir(path); st != nil {
		// Top-level tree directory (e.g. /a, /s, /m).
		entries, _ := st.List()
		for _, e := range entries {
			fpath := path + "/" + e.Name()
			d := s.makeStat(fpath)
			if info, err := e.Info(); err == nil {
				d.Atime = uint32(info.ModTime().Unix())
				d.Mtime = uint32(info.ModTime().Unix())
			}
			dirs = append(dirs, d)
		}
	} else if st, name := s.route(path); st != nil {
		// Subdirectory within a tree (e.g. /session/mysession, /session/mysession/t).
		type dirLister interface {
			Readdir(string) ([]os.DirEntry, error)
		}
		if dl, ok := st.(dirLister); ok {
			entries, _ := dl.Readdir(name)
			for _, e := range entries {
				fpath := path + "/" + e.Name()
				d := s.makeStat(fpath)
				dirs = append(dirs, d)
			}
		}
	}

	// Serialize all entries to a byte slice.
	var allData []byte
	for _, d := range dirs {
		b, err := d.Bytes()
		if err != nil {
			continue
		}
		allData = append(allData, b...)
	}
	return allData
}

// sliceDirData extracts complete directory entries from allData at the given offset,
// up to count bytes.
func (s *Server) sliceDirData(allData []byte, offset uint64, count uint32) []byte {
	if offset >= uint64(len(allData)) {
		return nil
	}
	remaining := allData[offset:]
	var result []byte
	for len(remaining) >= 2 {
		entrySize := int(remaining[0]) | int(remaining[1])<<8
		total := entrySize + 2
		if total > len(remaining) {
			break
		}
		if uint32(len(result)+total) > count {
			break
		}
		result = append(result, remaining[:total]...)
		remaining = remaining[total:]
	}
	return result
}

// makeStat builds a plan9.Dir for a logical path.
func (s *Server) makeStat(path string) plan9.Dir {
	base := pathBase(path)
	if path == "/" {
		base = "."
	}

	t := s.pathType(path)
	isDir := t == "dir"

	qid := plan9.Qid{Path: qidPath(path)}
	var mode plan9.Perm
	if isDir {
		qid.Type = QTDir
		mode = plan9.DMDIR | s.dirMode(path)
	} else {
		// Use the permission registry for file mode rather than os.Stat
		// (which is affected by umask and may not reflect intended perms).
		mode = plan9.Perm(s.filePerm(path))
	}

	uid, gid := s.fileOwnerGroup(path)
	dir := plan9.Dir{
		Qid:  qid,
		Mode: mode,
		Name: base,
		Uid:  uid,
		Gid:  gid,
		Muid: uid,
	}

	// For chat and tailable mutable files, report actual size and
	// Qid version so polling tools (tail -f) can detect changes via stat.
	// Path format: /session/{sessid}/{file}
	if strings.HasPrefix(path, fs.PathSessions) {
		parts := strings.SplitN(strings.TrimPrefix(path, "/"), "/", 3)
		if len(parts) == 3 && parts[0] == "session" {
			if sess := s.sessionMgr.Session(parts[1]); sess != nil {
				if base == "chat" {
					length, vers := sess.LogInfo()
					dir.Length = uint64(length)
					dir.Qid.Vers = vers
				}
			}
		}
	}

	// For all other readable files, compute content length so clients
	// that check stat before reading (cat, 9pfuse, etc.) see non-zero size.
	if dir.Length == 0 && !isDir {
		switch {
		case isSessionFile(path):
			if content, err := readFile(s.sessionMgr.Tree(), base); err == nil {
				dir.Length = uint64(len(content))
			}
		case path == "/backends":
			dir.Length = uint64(len(strings.Join(backend.Backends(), "\n") + "\n"))
		case path == "/models":
			if content, err := readFile(s.rootStore, "models"); err == nil {
				dir.Length = uint64(len(content))
			}
		case path == "/help":
			if info, err := os.Stat(s.helpPath()); err == nil {
				dir.Length = uint64(info.Size())
			}
		case strings.HasPrefix(path, fs.PathSessions):
			// HACK: 9pfuse trusts stat Length — if we report 0, the kernel
			// never issues a read. Compute size by reading the file content.
			if entry, err := s.openEntry(path); err == nil {
				if data, err := entry.Read(); err == nil {
					dir.Length = uint64(len(data))
				}
			}
		}
	}

	return dir
}