// Package instanceguard 强制同一 SQLite 数据文件只有一个活动服务。
// 锁在打开数据库（迁移发生在打开内部）之前取得。账户 CLI 不使用本包。
package instanceguard

import (
	"crypto/rand"
	"encoding/hex"
	"errors"
	"strconv"
	"strings"

	"agent-system-backend/internal/store"
)

var (
	ErrBusy             = errors.New("instance guard: sqlite service already running")
	ErrHardlink         = errors.New("instance guard: sqlite database has a hardlink alias")
	ErrDBReplaced       = errors.New("instance guard: sqlite database replaced")
	ErrDBMissing        = errors.New("instance guard: recorded sqlite database is missing")
	ErrLockRebuilt      = errors.New("instance guard: lock file was rebuilt")
	ErrLockCorrupt      = errors.New("instance guard: lock record is corrupt")
	ErrInstanceMismatch = errors.New("instance guard: instance id mismatch")
	ErrUnsupported      = errors.New("instance guard: sqlite activity lock is not available on this platform")
	ErrLockAliased      = errors.New("instance guard: lock file aliases the sqlite database")
)

// Record 是锁文件里记下的数据库身份。没有记录表示锁文件是新建的。
type Record struct {
	DBDev      uint64
	DBIno      uint64
	InstanceID string
	LockGen    string
}

// Lease 持有活动服务锁，直到 Release。关闭顺序由调用方保证：先关存储，再 Release。
type Lease struct {
	fd       int
	dbPath   string
	lockDev  uint64
	lockIno  uint64
	dbDev    uint64
	dbIno    uint64
	dbExists bool
	record   *Record
	parseErr error
	release  func()
}

// Release 放开锁并关闭锁文件描述符。锁文件留在磁盘上。
func (l *Lease) Release() {
	if l == nil || l.release == nil {
		return
	}
	l.release()
	l.release = nil
}

// DBPath 是解析后的数据库路径。符号链接指向真实文件。
func (l *Lease) DBPath() string {
	if l == nil {
		return ""
	}
	return l.dbPath
}

// AutomaticRefusal 在打开数据库之前调用。
// 尚无记录时返回 nil（首次建立，含旧库升级）。硬链接在 Acquire 阶段已经拒绝。
func (l *Lease) AutomaticRefusal() error {
	if l.parseErr != nil {
		return l.parseErr
	}
	if l.record == nil {
		return nil
	}
	if !l.dbExists {
		return ErrDBMissing
	}
	if l.record.DBDev != l.dbDev || l.record.DBIno != l.dbIno {
		return ErrDBReplaced
	}
	return nil
}

// BindOptions 控制守卫建立后的比对。
type BindOptions struct {
	Adopt      bool
	InstanceID string
}

// Bind 在数据库已经打开并完成迁移之后调用。
// 锁文件 inode 与表不一致则拒绝，除非 Adopt。通过时重写锁文件内容与表。
func Bind(st *store.Store, lease *Lease, opt BindOptions) error {
	if lease == nil {
		return errors.New("instance guard: nil lease")
	}
	marker, ok, err := st.LoadInstanceGuard()
	if err != nil {
		return err
	}
	if ok && !opt.Adopt {
		inodeMismatch := marker.LockDev != lease.lockDev || marker.LockIno != lease.lockIno
		genMismatch := marker.LockGen != "" && (lease.record == nil || lease.record.LockGen != marker.LockGen)
		if inodeMismatch || genMismatch {
			return ErrLockRebuilt
		}
	}
	id := opt.InstanceID
	if ok && marker.InstanceID != "" && id != "" && marker.InstanceID != id && !opt.Adopt {
		return ErrInstanceMismatch
	}
	if id == "" && ok {
		id = marker.InstanceID
	}
	dev, ino, exists, err := lease.currentDB()
	if err != nil {
		return err
	}
	if !exists {
		return ErrDBMissing
	}
	lease.dbDev, lease.dbIno, lease.dbExists = dev, ino, true
	gen := ""
	if ok && marker.LockGen != "" && !opt.Adopt {
		gen = marker.LockGen
	} else {
		gen, err = newLockGen()
		if err != nil {
			return err
		}
	}
	next := store.InstanceGuardMarker{
		DBDev: dev, DBIno: ino, LockDev: lease.lockDev, LockIno: lease.lockIno,
		InstanceID: id, LockGen: gen,
	}
	if err := lease.writeRecord(dev, ino, id, gen); err != nil {
		return err
	}
	if !ok || opt.Adopt || marker != next {
		if err := st.SaveInstanceGuard(next); err != nil {
			return err
		}
	}
	return nil
}

func newLockGen() (string, error) {
	buf := make([]byte, 16)
	if _, err := rand.Read(buf); err != nil {
		return "", err
	}
	return hex.EncodeToString(buf), nil
}

func parseRecord(raw string) (*Record, error) {
	raw = strings.TrimSpace(raw)
	if raw == "" {
		return nil, nil
	}
	parts := strings.Split(raw, "\t")
	if parts[0] != "v1" {
		return nil, ErrLockCorrupt
	}
	rec := &Record{}
	seen := map[string]bool{}
	for _, part := range parts[1:] {
		key, val, ok := strings.Cut(part, "=")
		if !ok || seen[key] || strings.ContainsAny(val, " \t") {
			return nil, ErrLockCorrupt
		}
		seen[key] = true
		switch key {
		case "db_dev":
			n, err := strconv.ParseUint(val, 10, 64)
			if err != nil {
				return nil, ErrLockCorrupt
			}
			rec.DBDev = n
		case "db_ino":
			n, err := strconv.ParseUint(val, 10, 64)
			if err != nil {
				return nil, ErrLockCorrupt
			}
			rec.DBIno = n
		case "instance_id":
			rec.InstanceID = val
		case "lock_gen":
			if val == "" {
				return nil, ErrLockCorrupt
			}
			rec.LockGen = val
		default:
			return nil, ErrLockCorrupt
		}
	}
	if !seen["db_dev"] || !seen["db_ino"] || !seen["instance_id"] {
		return nil, ErrLockCorrupt
	}
	return rec, nil
}