package main

// The agent's home, touched as the agent's account (novox/hq ADR 0266).
//
// On a machine that names an agent account of its own — the control node — agents run as that account, not
// as the operator's, so that no agent there can become root without a person. Its home is its own, 0700:
// the node's tool runtime, which runs this module as the operator's account, can neither read nor write it.
// And this module must: the licence token and the account the agent is logged in as, the items registered
// at the home scope, and the home's own items the status and the removal tools read.
//
// So every path under that home is touched by a helper running as the agent's account: this module's own
// binary, as the plugin carries it under the managed directory (root's, so the agent cannot change it),
// started once through `sudo -n -u <agent>` and kept, speaking one JSON request and one JSON answer per line
// on its stdin and stdout. Once, not per file: a credentials file is looked at every five seconds, and sudo
// says every start in the journal. **The escalation goes down, never up**: the runtime's account may become
// the agent's; nothing here lets the agent's become anything.
//
// Everything else — the module's state, the managed directory, a machine whose agents run as the operator —
// is the plain filesystem, as before. Files routes each path; with no agent account of its own it routes none.

import (
	"bufio"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"os"
	"os/exec"
	"os/user"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"syscall"
	"time"
)

// HelperArg is the argument that makes this binary the helper (as the agent's account).
const HelperArg = "home-fs"

// Files is the filesystem as this module touches it: a path under the agent's home through the helper when
// the agent's account is not this process's, every other path as it is.
type Files struct {
	// Home is the agent's home and Account its login; Helper starts the helper, nil for none.
	Home    string
	Account string
	Helper  *Helper
}

// fsys is the module's: plain until the launch says whose home the agent's is.
var fsys = &Files{}

// FilesFor is the filesystem for these paths: through the helper when the agent's account is another than the
// one this process runs as.
func FilesFor(p Paths) *Files {
	if p.Agent == "" || p.Home == "" {
		return &Files{}
	}
	if me, err := user.Current(); err == nil && me.Username == p.Agent {
		return &Files{}
	}
	return &Files{Home: filepath.Clean(p.Home), Account: p.Agent, Helper: &Helper{Account: p.Agent, Exe: HelperPath()}}
}

// HelperPath is the helper's binary: the guard's copy of this module's binary in the plugin, root's and
// readable by every account, so the agent's account runs exactly what the mesh placed.
func HelperPath() string {
	return filepath.Join(ManagedDir, MarketplaceDir, "plugins", Plugin, GuardDir, "guard")
}

// through says whether a path is touched as the agent.
func (f *Files) through(path string) bool {
	if f == nil || f.Helper == nil || f.Home == "" {
		return false
	}
	path = filepath.Clean(path)
	return path == f.Home || strings.HasPrefix(path, f.Home+string(os.PathSeparator))
}

// ---- the operations ---------------------------------------------------------------------------------------

func (f *Files) ReadFile(path string) ([]byte, error) {
	if !f.through(path) {
		return os.ReadFile(path)
	}
	a, err := f.Helper.Do(Op{Op: "read", Path: path})
	return a.Data, err
}

func (f *Files) WriteFile(path string, data []byte, mode os.FileMode) error {
	if !f.through(path) {
		return os.WriteFile(path, data, mode)
	}
	_, err := f.Helper.Do(Op{Op: "write", Path: path, Data: data, Mode: uint32(mode.Perm())})
	return err
}

// CreateExclusive writes a file that must not be there yet.
func (f *Files) CreateExclusive(path string, data []byte, mode os.FileMode) error {
	if !f.through(path) {
		file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, mode)
		if err != nil {
			return err
		}
		_, werr := file.Write(data)
		return errors.Join(werr, file.Close())
	}
	_, err := f.Helper.Do(Op{Op: "create", Path: path, Data: data, Mode: uint32(mode.Perm())})
	return err
}

func (f *Files) Rename(from, to string) error {
	if f.through(from) != f.through(to) {
		return fmt.Errorf("%s cannot be moved to %s: one is the agent's home and the other is not", from, to)
	}
	if !f.through(from) {
		return os.Rename(from, to)
	}
	_, err := f.Helper.Do(Op{Op: "rename", Path: from, To: to})
	return err
}

func (f *Files) Remove(path string) error {
	if !f.through(path) {
		return os.Remove(path)
	}
	_, err := f.Helper.Do(Op{Op: "remove", Path: path})
	return err
}

func (f *Files) RemoveAll(path string) error {
	if !f.through(path) {
		return os.RemoveAll(path)
	}
	_, err := f.Helper.Do(Op{Op: "remove-all", Path: path})
	return err
}

func (f *Files) MkdirAll(path string, mode os.FileMode) error {
	if !f.through(path) {
		return os.MkdirAll(path, mode)
	}
	_, err := f.Helper.Do(Op{Op: "mkdir", Path: path, Mode: uint32(mode.Perm())})
	return err
}

func (f *Files) Chmod(path string, mode os.FileMode) error {
	if !f.through(path) {
		return os.Chmod(path, mode)
	}
	_, err := f.Helper.Do(Op{Op: "chmod", Path: path, Mode: uint32(mode.Perm())})
	return err
}

func (f *Files) Stat(path string) (fs.FileInfo, error) {
	if !f.through(path) {
		return os.Stat(path)
	}
	a, err := f.Helper.Do(Op{Op: "stat", Path: path})
	if err != nil {
		return nil, err
	}
	return a.Info.info(), nil
}

func (f *Files) Lstat(path string) (fs.FileInfo, error) {
	if !f.through(path) {
		return os.Lstat(path)
	}
	a, err := f.Helper.Do(Op{Op: "lstat", Path: path})
	if err != nil {
		return nil, err
	}
	return a.Info.info(), nil
}

func (f *Files) ReadDir(path string) ([]fs.DirEntry, error) {
	if !f.through(path) {
		return os.ReadDir(path)
	}
	a, err := f.Helper.Do(Op{Op: "readdir", Path: path})
	if err != nil {
		return nil, err
	}
	out := make([]fs.DirEntry, 0, len(a.Entries))
	for _, e := range a.Entries {
		// A name is one entry of path, never a way out of it: the helper runs as the agent's account, so what it
		// answers is the agent's word, and a name joined to path is read as the runtime's when it leaves the home.
		if e == nil || !entryName(e.Name) {
			return nil, fmt.Errorf("readdir %s as %s: the helper answered an entry that is not a name in it", path,
				f.Helper.Account)
		}
		out = append(out, fs.FileInfoToDirEntry(e.info()))
	}
	return out, nil
}

// WalkDir is filepath.WalkDir, through the helper under the agent's home: lexical order, a directory before
// what is in it, a symbolic link reported and never followed.
func (f *Files) WalkDir(root string, fn fs.WalkDirFunc) error {
	if !f.through(root) {
		return filepath.WalkDir(root, fn)
	}
	info, err := f.Lstat(root)
	if err != nil {
		err = fn(root, nil, err)
	} else {
		err = f.walk(root, fs.FileInfoToDirEntry(info), fn)
	}
	if err == filepath.SkipDir || err == filepath.SkipAll {
		return nil
	}
	return err
}

// entryName says whether name is one entry of a directory: not empty, not . or .., and no separator.
func entryName(name string) bool {
	return name != "" && name != "." && name != ".." && !strings.ContainsRune(name, '/') && !strings.ContainsRune(name, 0)
}

func (f *Files) walk(path string, d fs.DirEntry, fn fs.WalkDirFunc) error {
	if err := fn(path, d, nil); err != nil || !d.IsDir() {
		if err == filepath.SkipDir && d.IsDir() {
			err = nil
		}
		return err
	}
	entries, err := f.ReadDir(path)
	if err != nil {
		if err = fn(path, d, err); err != nil {
			if err == filepath.SkipDir {
				err = nil
			}
			return err
		}
	}
	for _, e := range entries {
		if err := f.walk(filepath.Join(path, e.Name()), e, fn); err != nil {
			if err == filepath.SkipDir {
				break
			}
			return err
		}
	}
	return nil
}

// ---- the wire ---------------------------------------------------------------------------------------------

// Op is one request to the helper.
type Op struct {
	Op   string `json:"op"`
	Path string `json:"path"`
	To   string `json:"to,omitempty"`
	Data []byte `json:"data,omitempty"`
	Mode uint32 `json:"mode,omitempty"`
}

// Answer is the helper's to one Op. Err is said by its kind, so the caller's os.IsNotExist and errors.Is
// read it as they would read the filesystem's own.
type Answer struct {
	Data    []byte  `json:"data,omitempty"`
	Info    *Info   `json:"info,omitempty"`
	Entries []*Info `json:"entries,omitempty"`
	Err     string  `json:"err,omitempty"`
	Kind    string  `json:"kind,omitempty"` // not-exist | exist | permission
}

// Info is a file's information as the wire carries it.
type Info struct {
	Name    string    `json:"name"`
	Size    int64     `json:"size"`
	Mode    uint32    `json:"mode"`
	ModTime time.Time `json:"modTime"`
}

func infoOf(i fs.FileInfo) *Info {
	return &Info{Name: i.Name(), Size: i.Size(), Mode: uint32(i.Mode()), ModTime: i.ModTime()}
}

func (i *Info) info() fs.FileInfo {
	if i == nil {
		return fileInfo{}
	}
	return fileInfo{*i}
}

type fileInfo struct{ Info }

func (i fileInfo) Name() string       { return i.Info.Name }
func (i fileInfo) Size() int64        { return i.Info.Size }
func (i fileInfo) Mode() fs.FileMode  { return fs.FileMode(i.Info.Mode) }
func (i fileInfo) ModTime() time.Time { return i.Info.ModTime }
func (i fileInfo) IsDir() bool        { return i.Mode().IsDir() }
func (i fileInfo) Sys() any           { return nil }

func errorOf(op, path string, a Answer) error {
	if a.Err == "" {
		return nil
	}
	var kind error
	switch a.Kind {
	case "not-exist":
		kind = fs.ErrNotExist
	case "exist":
		kind = fs.ErrExist
	case "permission":
		kind = fs.ErrPermission
	default:
		return errors.New(a.Err)
	}
	// The kind itself, unwrapped: os.IsNotExist compares rather than unwraps.
	return &fs.PathError{Op: op, Path: path, Err: kind}
}

// ---- the client -------------------------------------------------------------------------------------------

// Helper is the helper as this module keeps it: started at the first request, restarted once when it is gone.
type Helper struct {
	Account string
	Exe     string
	// Start is how it is started; nil is through sudo as the account. A test starts it in the process.
	Start func() (io.WriteCloser, io.Reader, func(), error)

	mu    sync.Mutex
	in    io.WriteCloser
	out   *bufio.Reader
	close func()
}

// helperAnswerWithin bounds how long one request may take. The helper's own reads cannot block (serveOne), so
// a helper this slow has gone wrong: it is stopped and the request fails, said, never retried, so no request
// holds every later one behind it.
var helperAnswerWithin = 30 * time.Second

// errHelperSlow is a helper that did not answer in time.
var errHelperSlow = errors.New("the helper did not answer in time")

// Do answers one request, starting the helper when it is not running.
func (h *Helper) Do(op Op) (Answer, error) {
	h.mu.Lock()
	defer h.mu.Unlock()
	var last error
	for try := 0; try < 2; try++ {
		if h.in == nil {
			if err := h.start(); err != nil {
				return Answer{}, err
			}
		}
		a, err := h.roundTrip(op)
		if err == nil {
			return a, errorOf(op.Op, op.Path, a)
		}
		last = err
		h.stop()
		if errors.Is(err, errHelperSlow) {
			break
		}
	}
	return Answer{}, fmt.Errorf("%s %s as %s: the helper did not answer: %w", op.Op, op.Path, h.Account, last)
}

func (h *Helper) roundTrip(op Op) (Answer, error) {
	raw, err := json.Marshal(op)
	if err != nil {
		return Answer{}, err
	}
	type read struct {
		line []byte
		err  error
	}
	done := make(chan read, 1)
	in, out := h.in, h.out
	go func() {
		if _, err := in.Write(append(raw, '\n')); err != nil {
			done <- read{err: err}
			return
		}
		line, err := readLine(out, helperAnswerMax)
		done <- read{line, err}
	}()
	var line []byte
	select {
	case r := <-done:
		if r.err != nil {
			return Answer{}, r.err
		}
		line = r.line
	case <-time.After(helperAnswerWithin):
		// The goroutine ends when stop closes the pipes and the helper goes.
		return Answer{}, fmt.Errorf("%w (%s)", errHelperSlow, helperAnswerWithin)
	}
	var a Answer
	if err := json.Unmarshal(line, &a); err != nil {
		return Answer{}, fmt.Errorf("the helper said something that is not an answer: %w", err)
	}
	return a, nil
}

// helperAnswerMax bounds one answer of the helper: far above the largest file this module reads from a home
// (an item is at most 256 KiB), so an answer past it is a helper gone wrong, not a file.
const helperAnswerMax = 16 << 20

// readLine reads one line of at most max bytes; a longer one is an error and the helper is started again.
func readLine(r *bufio.Reader, max int) ([]byte, error) {
	var line []byte
	for {
		chunk, err := r.ReadSlice('\n')
		line = append(line, chunk...)
		if len(line) > max {
			return nil, fmt.Errorf("the helper's answer is larger than %d bytes", max)
		}
		if err == bufio.ErrBufferFull {
			continue
		}
		return line, err
	}
}

func (h *Helper) start() error {
	start := h.Start
	if start == nil {
		start = h.sudo
	}
	in, out, closer, err := start()
	if err != nil {
		return err
	}
	h.in, h.out, h.close = in, bufio.NewReaderSize(out, 1<<20), closer
	return nil
}

func (h *Helper) stop() {
	if h.in != nil {
		_ = h.in.Close()
	}
	if h.close != nil {
		h.close()
	}
	h.in, h.out, h.close = nil, nil, nil
}

// sudo starts the helper as the agent's account: the runtime's account may become it (the sudo module's
// rule for the operator's account names every account), and nothing the agent's account holds lets it back.
func (h *Helper) sudo() (io.WriteCloser, io.Reader, func(), error) {
	if _, err := os.Stat(h.Exe); err != nil {
		return nil, nil, nil, fmt.Errorf("the helper that touches %s's home is not placed yet (%s): the managed "+
			"directory is written first, and the next render reaches the home", h.Account, h.Exe)
	}
	cmd := exec.Command("sudo", "-n", "-u", h.Account, "--", h.Exe, HelperArg)
	cmd.Dir = "/"
	// One language for what sudo says when it refuses, so its words are read the same on every machine.
	cmd.Env = append(os.Environ(), "LC_ALL=C", "LANG=C")
	in, err := cmd.StdinPipe()
	if err != nil {
		return nil, nil, nil, err
	}
	out, err := cmd.StdoutPipe()
	if err != nil {
		return nil, nil, nil, err
	}
	var stderr strings.Builder
	cmd.Stderr = &limited{w: &stderr, left: 4096}
	if err := cmd.Start(); err != nil {
		return nil, nil, nil, fmt.Errorf("the helper could not be started as %s: %w", h.Account, err)
	}
	return in, out, func() {
		done := make(chan struct{})
		go func() { _ = cmd.Wait(); close(done) }()
		select {
		case <-done:
		case <-time.After(2 * time.Second):
			// sudo passes a SIGTERM on to the helper; a SIGKILL would end sudo alone and leave it running.
			_ = cmd.Process.Signal(syscall.SIGTERM)
			select {
			case <-done:
			case <-time.After(2 * time.Second):
				_ = cmd.Process.Kill()
				<-done
			}
		}
		if s := strings.TrimSpace(stderr.String()); s != "" {
			say("the helper as %s said: %s", h.Account, s)
		}
	}, nil
}

type limited struct {
	w    io.Writer
	left int
}

func (l *limited) Write(p []byte) (int, error) {
	n := len(p)
	if l.left <= 0 {
		return n, nil
	}
	if len(p) > l.left {
		p = p[:l.left]
	}
	l.left -= len(p)
	_, _ = l.w.Write(p)
	return n, nil
}

// ---- the helper -------------------------------------------------------------------------------------------

// helperReadMax bounds a file the helper reads: its answer, as JSON, stays within helperAnswerMax.
const helperReadMax = 10 << 20

// readRegular reads a regular file of at most helperReadMax bytes. Opened without blocking and judged by what was
// opened, so a FIFO, a device or a link to one in the agent's home answers an error at once instead of holding
// the helper, and every request behind it, or filling its memory.
func readRegular(path string) ([]byte, error) {
	f, err := os.OpenFile(path, os.O_RDONLY|syscall.O_NONBLOCK, 0)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	info, err := f.Stat()
	if err != nil {
		return nil, err
	}
	if !info.Mode().IsRegular() {
		return nil, fmt.Errorf("%s is not a regular file (%s)", path, info.Mode().Type())
	}
	data, err := io.ReadAll(io.LimitReader(f, helperReadMax+1))
	if err != nil {
		return nil, err
	}
	if len(data) > helperReadMax {
		return nil, fmt.Errorf("%s is larger than %d bytes", path, helperReadMax)
	}
	return data, nil
}

// ServeHelper is the helper: one answer per request, until its input ends. It runs as the agent's account and
// does only what that account may, on its own home: the account's permissions are the boundary.
func ServeHelper(in io.Reader, out io.Writer) error {
	dec := json.NewDecoder(bufio.NewReaderSize(in, 1<<20))
	enc := json.NewEncoder(out)
	for {
		var op Op
		if err := dec.Decode(&op); err != nil {
			if errors.Is(err, io.EOF) {
				return nil
			}
			return err
		}
		if err := enc.Encode(serveOne(op)); err != nil {
			return err
		}
	}
}

func serveOne(op Op) Answer {
	if !filepath.IsAbs(op.Path) || (op.To != "" && !filepath.IsAbs(op.To)) {
		return Answer{Err: "a path must be absolute"}
	}
	mode := os.FileMode(op.Mode).Perm()
	var a Answer
	var err error
	switch op.Op {
	case "read":
		a.Data, err = readRegular(op.Path)
	case "write":
		err = os.WriteFile(op.Path, op.Data, mode)
	case "create":
		var f *os.File
		if f, err = os.OpenFile(op.Path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, mode); err == nil {
			_, werr := f.Write(op.Data)
			err = errors.Join(werr, f.Close())
		}
	case "rename":
		err = os.Rename(op.Path, op.To)
	case "remove":
		err = os.Remove(op.Path)
	case "remove-all":
		err = os.RemoveAll(op.Path)
	case "mkdir":
		err = os.MkdirAll(op.Path, mode)
	case "chmod":
		err = os.Chmod(op.Path, mode)
	case "stat", "lstat":
		var i fs.FileInfo
		if op.Op == "stat" {
			i, err = os.Stat(op.Path)
		} else {
			i, err = os.Lstat(op.Path)
		}
		if err == nil {
			a.Info = infoOf(i)
		}
	case "readdir":
		var entries []fs.DirEntry
		entries, err = os.ReadDir(op.Path)
		sort.Slice(entries, func(i, j int) bool { return entries[i].Name() < entries[j].Name() })
		for _, e := range entries {
			if i, ierr := e.Info(); ierr == nil {
				a.Entries = append(a.Entries, infoOf(i))
			}
		}
	default:
		return Answer{Err: "no such operation: " + op.Op}
	}
	if err != nil {
		a = Answer{Err: err.Error()}
		switch {
		case errors.Is(err, fs.ErrNotExist):
			a.Kind = "not-exist"
		case errors.Is(err, fs.ErrExist):
			a.Kind = "exist"
		case errors.Is(err, fs.ErrPermission):
			a.Kind = "permission"
		}
	}
	return a
}