// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package nfs4server

import (
	crand "crypto/rand"
	"encoding/binary"
	"encoding/hex"
	"encoding/json"
	"os"
	"path/filepath"
	"sync"

	"sourcedock.dev/petrbalvin/nfs/internal/nfs4"
	"sourcedock.dev/petrbalvin/nfs/internal/nfsfs"
)

// randCounter draws a random starting value for a state counter, so the
// other field of a stateid this server mints is difficult to guess,
// RFC 8881 section 8.2.2. A counter that started at one would let a
// client walk another client's stateids by enumeration.
func randCounter() uint64 {
	var b [8]byte
	if _, err := crand.Read(b[:]); err != nil {
		panic("nfs4server: the random source failed: " + err.Error())
	}
	return binary.BigEndian.Uint64(b[:])
}

var dbgMu sync.Mutex

// stateidSeq answers the sequence field of a stateid as the 32 bit
// big-endian word the wire carries, RFC 7863.
func stateidSeq(st nfs4.Stateid) uint32 {
	return binary.BigEndian.Uint32(st[0:4])
}

// setStateidSeq writes the sequence field of a stateid as a big-endian
// word.
func setStateidSeq(st *nfs4.Stateid, seq uint32) {
	binary.BigEndian.PutUint32(st[0:4], seq)
}

// shareConflict reports whether a new open with the given access and deny
// bits collides with an existing one on the same file. A requested access
// fights an existing deny of the same kind, and a requested deny fights an
// existing access.
func shareConflicts(access, deny uint32, existing *openEntry) bool {
	if access&nfs4.ShareAccessRead != 0 && existing.deny&nfs4.ShareDenyRead != 0 {
		return true
	}
	if access&nfs4.ShareAccessWrite != 0 && existing.deny&nfs4.ShareDenyWrite != 0 {
		return true
	}
	if deny&nfs4.ShareDenyRead != 0 && existing.access&nfs4.ShareAccessRead != 0 {
		return true
	}
	if deny&nfs4.ShareDenyWrite != 0 && existing.access&nfs4.ShareAccessWrite != 0 {
		return true
	}
	return false
}

// An openEntry is one live OPEN: the stateid the client holds, the share
// reservation it made, and the file it points at. A recovered open is
// one the store loaded back after a server restart: it still carries
// the client id of its past life, which a CLAIM_PREVIOUS in the grace
// window rebinds to the client that comes back for it.
type openEntry struct {
	stateid   nfs4.Stateid
	clientID  uint64
	owner     []byte
	fh        nfsfs.Handle
	fileKey   string
	access    uint32
	deny      uint32
	delegSt   *nfs4.Stateid
	recovered bool
}

// stateStore keeps the OPEN state of the server: every live open, the
// share reservations grouped per file, and the tombstones of closed
// stateids, which turn a reuse of an old stateid into NFS4ERR_OLD_STATEID
// instead of the vaguer NFS4ERR_BAD_STATEID.
type stateStore struct {
	mu     sync.Mutex
	next   uint64
	dir    string // when set, live opens persist here across restarts
	opens  map[string]*openEntry
	byFile map[string][]*openEntry
	closed map[string]uint32
}

func newStateStore(dir string) *stateStore {
	s := &stateStore{
		next:   randCounter(),
		dir:    dir,
		opens:  make(map[string]*openEntry),
		byFile: make(map[string][]*openEntry),
		closed: make(map[string]uint32),
	}
	if dir != "" {
		s.load(dir)
	}
	return s
}

// persist writes the live opens into dir, atomically.
func (s *stateStore) persist(dir string) {
	if dir == "" {
		return
	}
	s.mu.Lock()
	list := make([]persistedOpen, 0, len(s.opens))
	for other, e := range s.opens {
		list = append(list, persistedOpen{
			Other:    hex.EncodeToString([]byte(other)),
			Seqid:    stateidSeq(e.stateid),
			ClientID: e.clientID,
			FileKey:  hex.EncodeToString([]byte(e.fileKey)),
			Access:   e.access,
			Deny:     e.deny,
		})
	}
	s.mu.Unlock()
	image, err := json.Marshal(map[string][]persistedOpen{"opens": list})
	if err != nil {
		return
	}
	tmp := filepath.Join(dir, "opens.json.tmp")
	if err := os.WriteFile(tmp, image, 0o600); err != nil {
		return
	}
	_ = os.Rename(tmp, filepath.Join(dir, "opens.json"))
}

// loadDir reads the persisted opens from dir into the store.
func (s *stateStore) load(dir string) {
	data, err := os.ReadFile(filepath.Join(dir, "opens.json"))
	if err != nil {
		return
	}
	var image struct {
		Opens []persistedOpen `json:"opens"`
	}
	if json.Unmarshal(data, &image) != nil {
		return
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, p := range image.Opens {
		other, derr := hex.DecodeString(p.Other)
		if derr != nil || len(other) != 12 {
			continue
		}
		fileKeyBytes, derr := hex.DecodeString(p.FileKey)
		if derr != nil {
			continue
		}
		var st nfs4.Stateid
		setStateidSeq(&st, p.Seqid)
		copy(st[4:], other)
		e := &openEntry{
			stateid:   st,
			clientID:  p.ClientID,
			fileKey:   string(fileKeyBytes),
			access:    p.Access,
			deny:      p.Deny,
			recovered: true,
		}
		s.opens[string(other)] = e
		s.byFile[e.fileKey] = append(s.byFile[e.fileKey], e)
	}
}

// fileKey names one file across all its handles: the backend handle bytes
// already encode the stable identity of the file.
func fileKey(fh nfsfs.Handle) string {
	return string(fh)
}

// open registers a new OPEN of a file, enforcing the share reservations of
// the opens already live on it. It answers the stateid of the new open.
func (s *stateStore) open(fh nfsfs.Handle, clientid uint64, owner []byte, access, deny uint32) (nfs4.Stateid, uint32) {
	key := fileKey(fh)
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, e := range s.byFile[key] {
		if shareConflicts(access, deny, e) {
			return nfs4.Stateid{}, nfs4.ErrShareDenied
		}
	}
	s.next++
	var other [12]byte
	copy(other[:4], []byte("OPEN"))
	otherUint := s.next
	other[4] = byte(otherUint >> 56)
	other[5] = byte(otherUint >> 48)
	other[6] = byte(otherUint >> 40)
	other[7] = byte(otherUint >> 32)
	other[8] = byte(otherUint >> 24)
	other[9] = byte(otherUint >> 16)
	other[10] = byte(otherUint >> 8)
	other[11] = byte(otherUint)
	st := nfs4.Stateid{}
	setStateidSeq(&st, 1) // the stateid of the first state change
	copy(st[4:], other[:])

	e := &openEntry{
		stateid:  st,
		clientID: clientid,
		owner:    owner,
		fh:       fh,
		fileKey:  key,
		access:   access,
		deny:     deny,
	}
	s.opens[string(other[:])] = e
	s.byFile[key] = append(s.byFile[key], e)
	return st, nfs4.ErrOK
}

// close releases an OPEN by its stateid. Only the client the open
// belongs to may close it. A stateid older than the live one answers
// NFS4ERR_OLD_STATEID, an unknown one NFS4ERR_BAD_STATEID.
func (s *stateStore) close(st nfs4.Stateid, clientid uint64) (nfs4.Stateid, uint32) {
	other := st[4:]
	s.mu.Lock()
	defer s.mu.Unlock()
	e, ok := s.opens[string(other)]
	if !ok {
		if last, was := s.closed[string(other)]; was && stateidSeq(st) <= last {
			return nfs4.Stateid{}, nfs4.ErrOldStateid
		}
		return nfs4.Stateid{}, nfs4.ErrBadStateid
	}
	if e.clientID != clientid {
		return nfs4.Stateid{}, nfs4.ErrBadStateid
	}
	switch {
	case stateidSeq(st) == 0: // a cleared sequence names the current version
	case stateidSeq(st) < stateidSeq(e.stateid):
		return nfs4.Stateid{}, nfs4.ErrOldStateid
	case stateidSeq(st) > stateidSeq(e.stateid):
		return nfs4.Stateid{}, nfs4.ErrBadStateid
	}
	closed := e.stateid
	setStateidSeq(&closed, stateidSeq(e.stateid)+1) // CLOSE answers a dead stateid

	delete(s.opens, string(other))
	s.closed[string(other)] = stateidSeq(closed)
	list := s.byFile[e.fileKey]
	for i, cand := range list {
		if cand == e {
			s.byFile[e.fileKey] = append(list[:i], list[i+1:]...)
			break
		}
	}
	return closed, nfs4.ErrOK
}

// currentStateid is the special CURRENT_STATEID of RFC 8881 section
// 8.2.3: sequence one and an empty other field name the most recent
// stateid the caller holds on the file.
func currentStateid() nfs4.Stateid {
	var st nfs4.Stateid
	setStateidSeq(&st, 1)
	return st
}

// checkStateid resolves a stateid handed to READ, WRITE or SETATTR. The
// anonymous forms pass through with no entry; CURRENT_STATEID resolves
// to the caller's open of the file; anything else must name a live open
// of the same file that belongs to the asking client.
func (s *stateStore) checkStateid(st nfs4.Stateid, fh nfsfs.Handle, clientid uint64) (uint32, uint32) {
	allOnes := nfs4.Stateid{}
	for i := range allOnes {
		allOnes[i] = 0xff
	}
	if st == allOnes || st == (nfs4.Stateid{}) {
		return 0, nfs4.ErrOK
	}
	other := st[4:]
	s.mu.Lock()
	defer s.mu.Unlock()
	var e *openEntry
	if st == currentStateid() {
		// The current stateid names the caller's own open of this file.
		for _, cand := range s.byFile[fileKey(fh)] {
			if cand.clientID == clientid {
				e = cand
				break
			}
		}
		if e == nil {
			return 0, nfs4.ErrBadStateid
		}
	} else {
		var ok bool
		e, ok = s.opens[string(other)]
		if !ok {
			if last, was := s.closed[string(other)]; was && stateidSeq(st) <= last {
				return 0, nfs4.ErrOldStateid
			}
			return 0, nfs4.ErrBadStateid
		}
	}
	if e.clientID != clientid {
		return 0, nfs4.ErrBadStateid
	}
	// A zero sequence names whatever version is current, RFC 8881
	// section 8.2.2: conformant clients present stateids with the
	// sequence field cleared, and the server honours them as the live
	// version.
	if st != currentStateid() && stateidSeq(st) != 0 {
		if stateidSeq(st) < stateidSeq(e.stateid) {
			return 0, nfs4.ErrOldStateid
		}
		if stateidSeq(st) > stateidSeq(e.stateid) {
			return 0, nfs4.ErrBadStateid
		}
	}
	if e.fileKey != fileKey(fh) {
		return 0, nfs4.ErrBadStateid
	}
	return e.access, nfs4.ErrOK
}

// lookupOpen resolves a stateid to its live open entry, checking that
// the stateid names the file the caller says it does and belongs to the
// asking client. The anonymous forms never resolve here.
func (s *stateStore) lookupOpen(st nfs4.Stateid, fh nfsfs.Handle, clientid uint64) (*openEntry, uint32) {
	other := st[4:]
	s.mu.Lock()
	defer s.mu.Unlock()
	e, ok := s.opens[string(other)]
	if !ok {
		return nil, nfs4.ErrBadStateid
	}
	if e.fileKey != fileKey(fh) || e.clientID != clientid {
		return nil, nfs4.ErrBadStateid
	}
	return e, nfs4.ErrOK
}

// downgrade reduces the share access and deny bits of a live open. The
// stateid sequence moves one up; an older stateid is OLD_STATEID, an
// unknown one BAD_STATEID. Only the client the open belongs to may
// narrow it.
func (s *stateStore) downgrade(st nfs4.Stateid, clientid uint64, access, deny uint32) (nfs4.Stateid, uint32) {
	other := st[4:]
	s.mu.Lock()
	defer s.mu.Unlock()
	e, ok := s.opens[string(other)]
	if !ok {
		if last, was := s.closed[string(other)]; was && stateidSeq(st) <= last {
			return nfs4.Stateid{}, nfs4.ErrOldStateid
		}
		return nfs4.Stateid{}, nfs4.ErrBadStateid
	}
	if e.clientID != clientid {
		return nfs4.Stateid{}, nfs4.ErrBadStateid
	}
	if stateidSeq(st) != stateidSeq(e.stateid) && stateidSeq(st) != 0 {
		// A cleared sequence names the current version, RFC 8881
		// section 8.2.2.
		if stateidSeq(st) < stateidSeq(e.stateid) {
			return nfs4.Stateid{}, nfs4.ErrOldStateid
		}
		return nfs4.Stateid{}, nfs4.ErrBadStateid
	}
	// The narrowed bits must not collide with the other opens of the file.
	for _, cand := range s.byFile[e.fileKey] {
		if cand == e {
			continue
		}
		if shareConflicts(access, deny, cand) {
			return nfs4.Stateid{}, nfs4.ErrShareDenied
		}
	}
	e.access = access
	e.deny = deny
	setStateidSeq(&e.stateid, stateidSeq(e.stateid)+1)
	return e.stateid, nfs4.ErrOK
}

// dropClient releases every OPEN of the client, which is what
// DESTROY_CLIENTID and lease expiry require.
func (s *stateStore) dropClient(clientid uint64) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for other, e := range s.opens {
		if e.clientID == clientid {
			delete(s.opens, other)
			list := s.byFile[e.fileKey]
			for i, cand := range list {
				if cand == e {
					s.byFile[e.fileKey] = append(list[:i], list[i+1:]...)
					break
				}
			}
		}
	}
}

// A delegation is one granted OPEN delegation: the stateid the client
// holds, the session its recalls travel over, and the file it names.
type delegation struct {
	stateid  nfs4.Stateid
	sessID   nfs4.SessionID
	fileKey  string
	clientID uint64
	kind     uint32 // nfs4.OpenDelegRead or nfs4.OpenDelegWrite
}

// delegStore tracks the live delegations. A file carries at most one; the
// store only answers whether a grant is possible and who holds what;
// the recall travels over the holder's back channel.
type delegStore struct {
	mu    sync.Mutex
	next  uint64
	byKey map[string]*delegation // one per file key
}

func newDelegStore() *delegStore {
	return &delegStore{next: randCounter(), byKey: make(map[string]*delegation)}
}

// grant registers a delegation of the file for the client and answers the
// delegation stateid. A file already delegated to somebody else is
// refused, which keeps the grants exclusive.
func (s *delegStore) grant(sessID nfs4.SessionID, clientid uint64, key string, kind uint32) (nfs4.Stateid, uint32) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if _, ok := s.byKey[key]; ok {
		return nfs4.Stateid{}, nfs4.ErrDenied
	}
	s.next++
	var other [12]byte
	copy(other[:4], []byte("DELE"))
	be := uint64(s.next)
	for i := range 8 {
		other[11-i] = byte(be >> (8 * i))
	}
	var st nfs4.Stateid
	setStateidSeq(&st, 1)
	copy(st[4:], other[:])
	d := &delegation{stateid: st, sessID: sessID, fileKey: key, clientID: clientid, kind: kind}
	s.byKey[key] = d
	return st, nfs4.ErrOK
}

// holder returns the live delegation of a file, if any.
func (s *delegStore) holder(key string) (*delegation, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	d, ok := s.byKey[key]
	return d, ok
}

// revoke drops the delegation of a file.
func (s *delegStore) revoke(key string) {
	s.mu.Lock()
	delete(s.byKey, key)
	s.mu.Unlock()
}

// dropClient releases every delegation the client holds.
func (s *delegStore) dropClient(clientid uint64) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for key, d := range s.byKey {
		if d.clientID == clientid {
			delete(s.byKey, key)
		}
	}
}

// countOpens reports how many live opens the file carries.
func (s *stateStore) countOpens(fh nfsfs.Handle) int {
	key := fileKey(fh)
	s.mu.Lock()
	defer s.mu.Unlock()
	return len(s.byFile[key])
}

// bindDelegation records the delegation stateid on the open, so a CLOSE
// of the open revokes the delegation with it.
func (s *stateStore) bindDelegation(open nfs4.Stateid, deleg nfs4.Stateid) {
	other := open[4:]
	s.mu.Lock()
	defer s.mu.Unlock()
	if e, ok := s.opens[string(other)]; ok {
		e.delegSt = &deleg
	}
}

// delegOf reports the delegation bound to an open, if any.
func (s *stateStore) delegOf(open nfs4.Stateid) (nfs4.Stateid, bool) {
	other := open[4:]
	s.mu.Lock()
	defer s.mu.Unlock()
	e, ok := s.opens[string(other)]
	if !ok || e.delegSt == nil {
		return nfs4.Stateid{}, false
	}
	return *e.delegSt, true
}

// A persistedOpen is the on disk image of one live OPEN: enough to
// re-register the state after a server restart, so the handles clients
// hold keep their state across the restart.
type persistedOpen struct {
	Other    string `json:"other"` // hex of the 12 byte other field
	Seqid    uint32 `json:"seqid"`
	ClientID uint64 `json:"clientid"`
	FileKey  string `json:"file_key"` // hex of the backend file key
	Access   uint32 `json:"access"`
	Deny     uint32 `json:"deny"`
}

// reclaimOpen resolves a CLAIM_PREVIOUS open: the pre restart open of
// the file whose handle the client presented. The client's own open
// answers first; otherwise a recovered open of the file, one the store
// loaded back after the restart, is rebound to the claiming client,
// because client ids do not survive a restart and the grace window is
// the only gate. It answers the open's stateid by value, so the caller
// holds no pointer into the store. When nothing answers it returns
// BAD_STATEID.
func (s *stateStore) reclaimOpen(fh nfsfs.Handle, clientid uint64) (nfs4.Stateid, uint32) {
	key := fileKey(fh)
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, e := range s.byFile[key] {
		if e.clientID == clientid {
			e.recovered = false
			return e.stateid, nfs4.ErrOK
		}
	}
	for _, e := range s.byFile[key] {
		if e.recovered {
			e.clientID = clientid
			e.recovered = false
			return e.stateid, nfs4.ErrOK
		}
	}
	return nfs4.Stateid{}, nfs4.ErrBadStateid
}

// dropStateid removes the delegation the stateid names, when it belongs
// to the asking client, and reports whether there was one, which
// DELEGRETURN requires.
func (s *delegStore) dropStateid(st nfs4.Stateid, clientid uint64) bool {
	s.mu.Lock()
	defer s.mu.Unlock()
	for key, d := range s.byKey {
		if d.stateid == st {
			if d.clientID != clientid {
				return false
			}
			delete(s.byKey, key)
			return true
		}
	}
	return false
}

// revokeIf drops the delegation of the file when it is still the one the
// recall named, so a recall that completes after a fresh grant never
// kills the new holder.
func (s *delegStore) revokeIf(key string, st nfs4.Stateid) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if d, ok := s.byKey[key]; ok && d.stateid == st {
		delete(s.byKey, key)
	}
}

// hasStateid reports whether the delegation stateid is live.
func (s *delegStore) hasStateid(st nfs4.Stateid) bool {
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, d := range s.byKey {
		if d.stateid == st {
			return true
		}
	}
	return false
}

// checkDataStateid validates a stateid handed to a data operation
// against the delegation store: RFC 8881 sections 8.2.3 and 10.3 let a
// client present the delegation stateid of the file to READ, WRITE and
// their kin. The stateid must be live, name the asking client and name
// this very file.
func (s *delegStore) checkDataStateid(st nfs4.Stateid, fh nfsfs.Handle, clientid uint64) uint32 {
	key := fileKey(fh)
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, d := range s.byKey {
		// The presented sequence is irrelevant: a conformant client
		// may present the stateid with the sequence field cleared,
		// RFC 8881 section 8.2.2.
		if string(d.stateid[4:]) == string(st[4:]) {
			if d.clientID != clientid || d.fileKey != key {
				return nfs4.ErrBadStateid
			}
			return nfs4.ErrOK
		}
	}
	return nfs4.ErrBadStateid
}

// testStateid reports the status of one stateid against the open store:
// the answer TEST_STATEID hands back without touching any state.
func (s *stateStore) testStateid(st nfs4.Stateid) uint32 {
	allOnes := nfs4.Stateid{}
	for i := range allOnes {
		allOnes[i] = 0xff
	}
	if st == allOnes || st == (nfs4.Stateid{}) {
		return nfs4.ErrOK
	}
	other := st[4:]
	s.mu.Lock()
	defer s.mu.Unlock()
	if e, ok := s.opens[string(other)]; ok {
		if stateidSeq(st) < stateidSeq(e.stateid) {
			return nfs4.ErrOldStateid
		}
		if stateidSeq(st) > stateidSeq(e.stateid) {
			return nfs4.ErrBadStateid
		}
		return nfs4.ErrOK
	}
	if last, was := s.closed[string(other)]; was {
		if stateidSeq(st) <= last {
			return nfs4.ErrOldStateid
		}
	}
	return nfs4.ErrBadStateid
}