package main

// 只读文件系统（docs/design/21-knowledge-sources.md §4.2）：
// - 以 . 开头的文件和目录不出现在列表中，按路径访问也返回"不存在"；
// - 文件一律显示为所有人只读，目录为所有人可进入；
// - 每次访问都先打开源目录并核对设备号、inode 号，再用 openat2(RESOLVE_BENEATH|RESOLVE_NO_SYMLINKS|RESOLVE_NO_XDEV) 打开，
//   访问过程中目录被替换成符号链接也不会读到源目录以外的内容，也不会跨到源目录下挂载的其他文件系统；
// - 资料库不可用时根目录照常存在，读取返回"输入/输出错误"。

import (
	"context"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/hanwen/go-fuse/v2/fs"
	"github.com/hanwen/go-fuse/v2/fuse"
	"golang.org/x/sys/unix"
)

// source 一个挂载对应的源目录：真实路径及其设备号、inode 号；设备拔出、恢复时由 manager 更新。
// 不长期持有源目录的句柄，否则设备会一直"忙"，无法卸载（安全弹出、自动挂载的空闲卸载都会失败）。
type source struct {
	mu   sync.RWMutex
	path string // 为空表示不可用
	dev  uint64
	ino  uint64
	gen  uint64
	// onMiss 不可用时有人访问，通知 manager 重新检查（异步、限速）
	onMiss func()
}

func newSource() *source { return &source{} }

// set 更新源目录（path 为空表示不可用），返回是否有变化
func (s *source) set(path string, dev, ino uint64) bool {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.path == path && s.dev == dev && s.ino == ino {
		return false
	}
	s.path, s.dev, s.ino = path, dev, ino
	s.gen++
	return true
}

func (s *source) close() { s.set("", 0, 0) }

func (s *source) miss() syscall.Errno {
	if s.onMiss != nil {
		go s.onMiss()
	}
	return syscall.EIO
}

// openRoot 打开源目录并确认还是登记时的那个目录（设备拔出后，原路径可能露出挂载点下面的其他文件）
func openRoot(path string, dev, ino uint64) (int, bool) {
	how := unix.OpenHow{Flags: unix.O_PATH | unix.O_DIRECTORY | unix.O_CLOEXEC, Resolve: unix.RESOLVE_NO_SYMLINKS | unix.RESOLVE_NO_MAGICLINKS}
	fd, err := unix.Openat2(unix.AT_FDCWD, path, &how)
	if err != nil {
		return -1, false
	}
	var st unix.Stat_t
	if unix.Fstat(fd, &st) != nil || st.Dev != dev || st.Ino != ino {
		unix.Close(fd)
		return -1, false
	}
	return fd, true
}

// openat 相对源目录打开 rel（"." 表示源目录本身）
func (s *source) openat(rel string, flags int) (int, syscall.Errno) {
	s.mu.RLock()
	path, dev, ino := s.path, s.dev, s.ino
	s.mu.RUnlock()
	if path == "" {
		return -1, s.miss()
	}
	root, ok := openRoot(path, dev, ino)
	if !ok {
		return -1, s.miss()
	}
	defer unix.Close(root)
	how := unix.OpenHow{Flags: uint64(flags | unix.O_CLOEXEC | unix.O_NOFOLLOW), Resolve: unix.RESOLVE_BENEATH | unix.RESOLVE_NO_SYMLINKS | unix.RESOLVE_NO_MAGICLINKS | unix.RESOLVE_NO_XDEV}
	for {
		fd, err := unix.Openat2(root, rel, &how)
		if err == unix.EINTR || err == unix.EAGAIN {
			continue
		}
		if err != nil {
			errno := err.(syscall.Errno)
			// 路径中出现符号链接、越界或跨到其他文件系统：对调用方表现为不存在
			if errno == unix.ELOOP || errno == unix.EXDEV {
				return -1, syscall.ENOENT
			}
			return -1, errno
		}
		return fd, 0
	}
}

func (s *source) generation() uint64 {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.gen
}

type kbNode struct {
	fs.Inode
	src *source
}

var (
	_ fs.NodeLookuper   = (*kbNode)(nil)
	_ fs.NodeGetattrer  = (*kbNode)(nil)
	_ fs.NodeOpener     = (*kbNode)(nil)
	_ fs.NodeReaddirer  = (*kbNode)(nil)
	_ fs.NodeReadlinker = (*kbNode)(nil)
	_ fs.NodeStatfser   = (*kbNode)(nil)
)

func (n *kbNode) rel() string {
	p := n.Path(n.Root())
	if p == "" {
		return "."
	}
	return p
}

func (n *kbNode) child(name string) string {
	if r := n.rel(); r != "." {
		return r + "/" + name
	}
	return name
}

func hidden(name string) bool { return strings.HasPrefix(name, ".") }

// statRel 不跟随符号链接地获取属性
func (n *kbNode) statRel(rel string) (unix.Stat_t, syscall.Errno) {
	var st unix.Stat_t
	fd, errno := n.src.openat(rel, unix.O_PATH)
	if errno != 0 {
		return st, errno
	}
	defer unix.Close(fd)
	if err := unix.Fstat(fd, &st); err != nil {
		return st, err.(syscall.Errno)
	}
	return st, 0
}

func permFor(mode uint32) uint32 {
	switch mode & unix.S_IFMT {
	case unix.S_IFDIR:
		return 0o555
	case unix.S_IFLNK:
		return 0o777
	default:
		return 0o444
	}
}

// inoFor 设备号参与计算，避免源目录下挂了其他文件系统时 inode 号冲突
func inoFor(st *unix.Stat_t) uint64 {
	return st.Ino ^ (uint64(unix.Major(st.Dev))<<56 | uint64(unix.Minor(st.Dev)&0xffff)<<40)
}

func fillAttr(st *unix.Stat_t, out *fuse.Attr) {
	out.Ino = inoFor(st)
	out.Size = uint64(st.Size)
	out.Blocks = uint64(st.Blocks)
	out.Blksize = uint32(st.Blksize)
	out.Atime, out.Atimensec = uint64(st.Atim.Sec), uint32(st.Atim.Nsec)
	out.Mtime, out.Mtimensec = uint64(st.Mtim.Sec), uint32(st.Mtim.Nsec)
	out.Ctime, out.Ctimensec = uint64(st.Ctim.Sec), uint32(st.Ctim.Nsec)
	out.Nlink = uint32(st.Nlink)
	out.Mode = st.Mode&unix.S_IFMT | permFor(st.Mode)
	out.Uid, out.Gid = 0, 0
}

func (n *kbNode) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (*fs.Inode, syscall.Errno) {
	if hidden(name) {
		return nil, syscall.ENOENT
	}
	st, errno := n.statRel(n.child(name))
	if errno != 0 {
		return nil, errno
	}
	fillAttr(&st, &out.Attr)
	ch := n.NewInode(ctx, &kbNode{src: n.src}, fs.StableAttr{Mode: st.Mode & unix.S_IFMT, Ino: inoFor(&st), Gen: n.src.generation()})
	return ch, 0
}

func (n *kbNode) Getattr(ctx context.Context, fh fs.FileHandle, out *fuse.AttrOut) syscall.Errno {
	if f, ok := fh.(*kbFile); ok {
		return f.Getattr(ctx, out)
	}
	st, errno := n.statRel(n.rel())
	if errno != 0 {
		if n.IsRoot() {
			// 不可用时根目录照常存在
			now := uint64(time.Now().Unix())
			out.Mode = unix.S_IFDIR | 0o555
			out.Nlink = 2
			out.Atime, out.Mtime, out.Ctime = now, now, now
			return 0
		}
		return errno
	}
	fillAttr(&st, &out.Attr)
	return 0
}

func (n *kbNode) Open(ctx context.Context, flags uint32) (fs.FileHandle, uint32, syscall.Errno) {
	if flags&(unix.O_WRONLY|unix.O_RDWR|unix.O_TRUNC|unix.O_APPEND|unix.O_CREAT) != 0 {
		return nil, 0, syscall.EROFS
	}
	fd, errno := n.src.openat(n.rel(), unix.O_RDONLY)
	if errno != 0 {
		return nil, 0, errno
	}
	return &kbFile{fd: fd}, 0, 0
}

func (n *kbNode) Readdir(ctx context.Context) (fs.DirStream, syscall.Errno) {
	fd, errno := n.src.openat(n.rel(), unix.O_RDONLY|unix.O_DIRECTORY)
	if errno != 0 {
		return nil, errno
	}
	defer unix.Close(fd)
	var dirSt unix.Stat_t
	_ = unix.Fstat(fd, &dirSt)
	var list []fuse.DirEntry
	buf := make([]byte, 32*1024)
	for {
		nread, err := unix.Getdents(fd, buf)
		if err == unix.EINTR {
			continue
		}
		if err != nil {
			return nil, err.(syscall.Errno)
		}
		if nread <= 0 {
			break
		}
		for off := 0; off < nread; {
			reclen, ino, typ, name := parseDirent(buf[off:nread])
			if reclen == 0 {
				break
			}
			off += reclen
			if name == "" || hidden(name) {
				continue
			}
			st := unix.Stat_t{Ino: ino, Dev: dirSt.Dev}
			list = append(list, fuse.DirEntry{Name: name, Mode: direntMode(typ), Ino: inoFor(&st)})
		}
	}
	return fs.NewListDirStream(list), 0
}

func (n *kbNode) Readlink(ctx context.Context) ([]byte, syscall.Errno) {
	fd, errno := n.src.openat(n.rel(), unix.O_PATH)
	if errno != 0 {
		return nil, errno
	}
	defer unix.Close(fd)
	buf := make([]byte, 4096)
	k, err := unix.Readlinkat(fd, "", buf)
	if err != nil {
		return nil, err.(syscall.Errno)
	}
	return buf[:k], 0
}

func (n *kbNode) Statfs(ctx context.Context, out *fuse.StatfsOut) syscall.Errno {
	fd, errno := n.src.openat(".", unix.O_PATH)
	if errno != 0 {
		return 0
	}
	defer unix.Close(fd)
	var s unix.Statfs_t
	if unix.Fstatfs(fd, &s) == nil {
		out.Blocks, out.Bfree, out.Bavail = s.Blocks, 0, 0
		out.Files, out.Ffree = s.Files, 0
		out.Bsize, out.Frsize = uint32(s.Bsize), uint32(s.Frsize)
		out.NameLen = uint32(s.Namelen)
	}
	return 0
}

// kbFile 只读文件句柄
type kbFile struct {
	mu sync.Mutex
	fd int
}

var (
	_ fs.FileReader    = (*kbFile)(nil)
	_ fs.FileReleaser  = (*kbFile)(nil)
	_ fs.FileGetattrer = (*kbFile)(nil)
	_ fs.FileLseeker   = (*kbFile)(nil)
)

func (f *kbFile) Read(ctx context.Context, dest []byte, off int64) (fuse.ReadResult, syscall.Errno) {
	f.mu.Lock()
	defer f.mu.Unlock()
	return fuse.ReadResultFd(uintptr(f.fd), off, len(dest)), 0
}

func (f *kbFile) Release(ctx context.Context) syscall.Errno {
	f.mu.Lock()
	defer f.mu.Unlock()
	if f.fd >= 0 {
		unix.Close(f.fd)
		f.fd = -1
	}
	return 0
}

func (f *kbFile) Getattr(ctx context.Context, out *fuse.AttrOut) syscall.Errno {
	f.mu.Lock()
	defer f.mu.Unlock()
	var st unix.Stat_t
	if err := unix.Fstat(f.fd, &st); err != nil {
		return err.(syscall.Errno)
	}
	fillAttr(&st, &out.Attr)
	return 0
}

func (f *kbFile) Lseek(ctx context.Context, off uint64, whence uint32) (uint64, syscall.Errno) {
	f.mu.Lock()
	defer f.mu.Unlock()
	n, err := unix.Seek(f.fd, int64(off), int(whence))
	if err != nil {
		return 0, err.(syscall.Errno)
	}
	return uint64(n), 0
}

func direntMode(typ uint8) uint32 {
	switch typ {
	case unix.DT_DIR:
		return unix.S_IFDIR
	case unix.DT_LNK:
		return unix.S_IFLNK
	case unix.DT_FIFO:
		return unix.S_IFIFO
	case unix.DT_SOCK:
		return unix.S_IFSOCK
	case unix.DT_CHR:
		return unix.S_IFCHR
	case unix.DT_BLK:
		return unix.S_IFBLK
	default:
		return unix.S_IFREG
	}
}

// parseDirent 解析一条 linux_dirent64：d_ino(8) d_off(8) d_reclen(2) d_type(1) d_name
func parseDirent(b []byte) (reclen int, ino uint64, typ uint8, name string) {
	if len(b) < 19 {
		return 0, 0, 0, ""
	}
	reclen = int(uint16(b[16]) | uint16(b[17])<<8)
	if reclen < 19 || reclen > len(b) {
		return 0, 0, 0, ""
	}
	for i := 0; i < 8; i++ {
		ino |= uint64(b[i]) << (8 * i)
	}
	typ = b[18]
	nb := b[19:reclen]
	if i := indexZero(nb); i >= 0 {
		nb = nb[:i]
	}
	name = string(nb)
	if name == "." || name == ".." {
		name = ""
	}
	return
}

func indexZero(b []byte) int {
	for i, c := range b {
		if c == 0 {
			return i
		}
	}
	return -1
}

// mountFS 把源目录挂到 mountpoint
func mountFS(mountpoint, name string, src *source) (*fuse.Server, error) {
	sec := time.Second
	root := &kbNode{src: src}
	return fs.Mount(mountpoint, root, &fs.Options{
		EntryTimeout:    &sec,
		AttrTimeout:     &sec,
		NegativeTimeout: &sec,
		MountOptions: fuse.MountOptions{
			AllowOther:   true,
			FsName:       name,
			Name:         "agentkb",
			Options:      []string{"ro", "nosuid", "nodev", "noexec"},
			MaxReadAhead: 128 * 1024,
		},
	})
}