// MsgSyncReq: copy files to the remote host

// Copyright (C) 2021  Simon Ruderich
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

// +build !windows

package sync

import (
	"bytes"
	"fmt"
	"io"
	"io/fs"
	"math/rand"
	"net/http"
	"os"
	"os/user"
	slashpath "path"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/ianbruene/go-difflib/difflib"
	"golang.org/x/sys/unix"

	"ruderich.org/simon/safcm"
)

// NOTE: Don't use plain os.Open, os.OpenFile, os.Remove or any other function
// which uses absolute paths. These are vulnerable to symlink attacks. Always
// use *at syscalls. See below for details.

// openReadonlyFlags are flags for open* syscalls to safely read a file or
// directory.
//
// O_NOFOLLOW prevents symlink attacks
// O_NONBLOCK is necessary to prevent blocking on FIFOs
const openReadonlyFlags = unix.O_RDONLY | unix.O_NOFOLLOW | unix.O_NONBLOCK

func (s *Sync) syncFiles() error {
	// To create random file names for symlinks
	rand.Seed(time.Now().UnixNano())

	// Sort for deterministic order and so parent directories are present
	// when files in them are created
	var files []*safcm.File
	for _, x := range s.req.Files {
		files = append(files, x)
	}
	sort.Slice(files, func(i, j int) bool {
		return files[i].Path < files[j].Path
	})

	for _, x := range files {
		var changed bool
		err := s.syncFile(x, &changed)
		if err != nil {
			return fmt.Errorf("%q: %v", x.Path, err)
		}
		if changed {
			s.queueTriggers(x)
		}
	}

	return nil
}

func (s *Sync) syncFile(file *safcm.File, changed *bool) error {
	// The general strategy is "update by rename": If any property of a
	// file changes it will be written to a temporary file and then
	// renamed "over" the original file. This is simple and prevents race
	// conditions where the file is partially readable while changes to
	// permissions or owner/group are applied. However, this strategy does
	// not work for directories which must be removed first (was
	// directory), must remove the existing file (will be directory) or
	// must be directly modified (changed permissions or owner). In the
	// first two cases the old path is removed. In the last the directory
	// is modified (carefully) in place.
	//
	// The implementation is careful not to follow any symlinks to prevent
	// possible race conditions which can be exploited and are especially
	// dangerous when running with elevated privileges (which will most
	// likely be the case). This includes not using absolute paths in
	// syscalls to prevent symlink attacks when a directory is writable by
	// other users (e.g. when syncing a file to /home/user/dir/file the
	// user could create dir as symlink to another directory and file
	// would be written there). To prevent this *at syscalls are used and
	// all symlinks in the path are rejected. This still permits the user
	// to move dir during the sync but only to places which are writable
	// by the user which cannot be prevented.

	err := s.fileResolveIds(file)
	if err != nil {
		return err
	}

	change := safcm.FileChange{
		Path: file.Path,
		New: safcm.FileChangeInfo{
			Mode:  file.Mode,
			User:  file.User,
			Uid:   file.Uid,
			Group: file.Group,
			Gid:   file.Gid,
		},
	}

	debugf := func(format string, a ...interface{}) {
		s.log.Debugf("files: %q (%s): %s",
			file.Path, file.OrigGroup, fmt.Sprintf(format, a...))
	}
	verbosef := func(format string, a ...interface{}) {
		s.log.Verbosef("files: %q (%s): %s",
			file.Path, file.OrigGroup, fmt.Sprintf(format, a...))
	}

	parentFd, baseName, err := OpenParentDirectoryNoSymlinks(file.Path)
	if err != nil {
		return err
	}
	defer unix.Close(parentFd)

	var oldStat unix.Stat_t
reopen:
	oldFh, err := OpenAtNoFollow(parentFd, baseName)
	if err != nil {
		if err == unix.ELOOP || err == unix.EMLINK {
			// ELOOP from symbolic link, this is fine
			err := unix.Fstatat(parentFd, baseName, &oldStat,
				unix.AT_SYMLINK_NOFOLLOW)
			if err != nil {
				return err
			}
			if oldStat.Mode&unix.S_IFMT != unix.S_IFLNK {
				debugf("type changed from symlink, retrying")
				goto reopen
			}
		} else if os.IsNotExist(err) {
			change.Created = true
			debugf("will create")
		} else {
			return err
		}
	} else {
		defer oldFh.Close()

		err := unix.Fstat(int(oldFh.Fd()), &oldStat)
		if err != nil {
			return err
		}
	}

	var oldData []byte
	var changeType, changePerm, changeUserOrGroup, changeData bool
	if !change.Created {
		// Manually convert to FileMode; from src/os/stat_linux.go in
		// Go's sources (stat_*.go for other UNIX systems are
		// identical, except for stat_darwin.go which has an extra
		// S_IFWHT)
		mode := fs.FileMode(oldStat.Mode & 0777)
		switch oldStat.Mode & unix.S_IFMT {
		case unix.S_IFBLK:
			mode |= fs.ModeDevice
		case unix.S_IFCHR:
			mode |= fs.ModeDevice | fs.ModeCharDevice
		case unix.S_IFDIR:
			mode |= fs.ModeDir
		case unix.S_IFIFO:
			mode |= fs.ModeNamedPipe
		case unix.S_IFLNK:
			mode |= fs.ModeSymlink
		case unix.S_IFREG:
			// nothing to do
		case unix.S_IFSOCK:
			mode |= fs.ModeSocket
		// Guard against unknown file types (e.g. on darwin); not in
		// stat_*.go
		default:
			return fmt.Errorf("unexpected file mode %v",
				oldStat.Mode&unix.S_IFMT)
		}
		if oldStat.Mode&unix.S_ISGID != 0 {
			mode |= fs.ModeSetgid
		}
		if oldStat.Mode&unix.S_ISUID != 0 {
			mode |= fs.ModeSetuid
		}
		if oldStat.Mode&unix.S_ISVTX != 0 {
			mode |= fs.ModeSticky
		}

		// Compare permissions
		change.Old.Mode = mode
		if change.Old.Mode.Type() == fs.ModeSymlink {
			// Some BSD systems permit changing permissions of
			// symlinks but ignore them on traversal. To keep it
			// simple we don't support that and always use 0777
			// for symlink permissions (the value on GNU/Linux).
			//
			// TODO: Add proper support for symlinks on BSD
			change.Old.Mode |= 0777
		}
		if change.Old.Mode != file.Mode {
			if change.Old.Mode.Type() != file.Mode.Type() {
				changeType = true
				debugf("type differs %s -> %s",
					change.Old.Mode.Type(),
					file.Mode.Type())
			} else {
				// Be careful with .Perm() which does not
				// contain the setuid/setgid/sticky bits!
				changePerm = true
				debugf("permission differs %s -> %s",
					change.Old.Mode, file.Mode)
			}
		}

		// Compare user/group
		change.Old.Uid = int(oldStat.Uid)
		change.Old.Gid = int(oldStat.Gid)
		if change.Old.Uid != file.Uid || change.Old.Gid != file.Gid {
			changeUserOrGroup = true
			debugf("uid/gid differs %d/%d -> %d/%d",
				change.Old.Uid, change.Old.Gid,
				file.Uid, file.Gid)
		}
		u, g, err := resolveIdsToNames(change.Old.Uid, change.Old.Gid)
		// Errors are not relevant as this is only used to report the
		// change. If the user/group no longer exits only the ids will
		// be reported.
		if err == nil {
			change.Old.User = u
			change.Old.Group = g
		}

		// Compare file content (if possible)
		switch change.Old.Mode.Type() {
		case 0: // regular file
			x, err := io.ReadAll(oldFh)
			if err != nil {
				return fmt.Errorf("reading old content: %v",
					err)
			}
			oldData = x
		case fs.ModeSymlink:
			buf := make([]byte, unix.PathMax)
			n, err := unix.Readlinkat(parentFd, baseName, buf)
			if err != nil {
				return fmt.Errorf("reading old content: %v",
					err)
			}
			oldData = buf[:n]
		}
		if !changeType && file.Mode.Type() != fs.ModeDir {
			if !bytes.Equal(oldData, file.Data) {
				changeData = true
				debugf("content differs")
			}
		}
	}

	// No changes
	if !change.Created && !changeType &&
		!changePerm && !changeUserOrGroup &&
		!changeData {
		debugf("unchanged")
		return nil
	}
	*changed = true

	// Don't show a diff with the full content for newly created files or
	// on type changes. This is just noise for the user as the new file
	// content is obvious. But we always want to see a diff when files are
	// replaced because this destroys data.
	if !change.Created &&
		(change.Old.Mode.Type() == 0 ||
			change.Old.Mode.Type() == fs.ModeSymlink) {
		change.DataDiff, err = diffData(oldData, file.Data)
		if err != nil {
			return err
		}
	}

	// Add change here so it is stored even when applying it fails. This
	// way the user knows exactly what was attempted.
	s.resp.FileChanges = append(s.resp.FileChanges, change)

	if change.Created {
		verbosef("creating")
	} else {
		verbosef("updating")
	}

	if s.req.DryRun {
		debugf("dry-run, skipping changes")
		return nil
	}

	// We cannot rename over directories and vice versa
	if changeType && (change.Old.Mode.IsDir() || file.Mode.IsDir()) {
		debugf("removing (due to type change)")
		// In the past os.RemoveAll() was used here. However, this is
		// difficult to implement manually with *at syscalls. To keep
		// it simple only permit removing files and empty directories
		// here. This also has the bonus of preventing data loss when
		// (accidentally) replacing a directory tree with a file.
		const msg = "will not replace non-empty directory, " +
			"please remove manually"
		err := unix.Unlinkat(parentFd, baseName, 0)
		if err != nil && !os.IsNotExist(err) {
			err2 := unix.Unlinkat(parentFd, baseName,
				AT_REMOVEDIR)
			if err2 != nil && !os.IsNotExist(err2) {
				// See src/os/file_unix.go in Go's sources
				if err2 == unix.ENOTDIR {
					return err
				} else if err2 == unix.ENOTEMPTY {
					return fmt.Errorf(msg)
				} else {
					return err2
				}
			}
		}
	}

	// Directory: create new directory, also type change to directory
	if file.Mode.IsDir() && (change.Created || changeType) {
		debugf("creating directory")
		err := unix.Mkdirat(parentFd, baseName, 0700)
		if err != nil {
			return err
		}
		// We must be careful not to chmod arbitrary files. If the
		// target directory is writable then it might have changed to
		// a symlink at this point. There's no lchmod and fchmodat is
		// incomplete so open the directory.
		debugf("chmodding %s", file.Mode)
		dh, err := OpenAtNoFollow(parentFd, baseName)
		if err != nil {
			return err
		}
		defer dh.Close()

		err = dh.Chmod(file.Mode)
		if err != nil {
			return err
		}
		// Less restrictive access is not relevant here because there
		// are no files present yet.
		debugf("chowning %d/%d", file.Uid, file.Gid)
		err = dh.Chown(file.Uid, file.Gid)
		if err != nil {
			return err
		}
		return nil
	}
	// Directory: changed permission or user/group
	if file.Mode.IsDir() {
		// We don't know if the new permission or if the new
		// user/group is more restrictive (e.g. root:root 0750 ->
		// user:group 0700; applying group first gives group
		// unexpected access). To prevent a short window where the
		// access might be too lax we temporarily deny all access.
		if changePerm && changeUserOrGroup {
			// Only drop group and other permission because user
			// has access anyway (either before or after the
			// change). This also prevents temporary errors during
			// the error when the user tries to access this
			// directory (access for the group will fail though).
			mode := change.Old.Mode & fs.ModePerm & 0700
			debugf("chmodding %#o (temporary)", mode)
			err := oldFh.Chmod(mode)
			if err != nil {
				return err
			}
		}
		if changeUserOrGroup {
			debugf("chowning %d/%d", file.Uid, file.Gid)
			err := oldFh.Chown(file.Uid, file.Gid)
			if err != nil {
				return err
			}
		}
		if changePerm {
			debugf("chmodding %s", file.Mode)
			err := oldFh.Chmod(file.Mode)
			if err != nil {
				return err
			}
		}
		return nil
	}

	dir := slashpath.Dir(file.Path) // only used in debug messages
	// Create hidden file which should be ignored by most other tools and
	// thus not affect anything during creation
	tmpBase := "." + baseName

	switch file.Mode.Type() {
	case 0: // regular file
		debugf("creating temporary file %q",
			slashpath.Join(dir, tmpBase+"*"))
		x, err := WriteTempAt(parentFd, tmpBase, file.Data,
			file.Uid, file.Gid, file.Mode)
		if err != nil {
			return err
		}
		tmpBase = x

	case fs.ModeSymlink:
		i := 0
	retry:
		x := tmpBase + strconv.Itoa(rand.Int())
		debugf("creating temporary symlink %q",
			slashpath.Join(dir, x))
		err := unix.Symlinkat(string(file.Data), parentFd, x)
		if err != nil {
			if os.IsExist(err) && i < 10000 {
				i++
				goto retry
			}
			return err
		}
		tmpBase = x

		err = unix.Fchownat(parentFd, tmpBase, file.Uid, file.Gid,
			unix.AT_SYMLINK_NOFOLLOW)
		if err != nil {
			unix.Unlinkat(parentFd, tmpBase, 0)
			return err
		}
		// Permissions are irrelevant for symlinks (on most systems)

	default:
		panic(fmt.Sprintf("invalid file type %s", file.Mode))
	}

	debugf("renaming %q", slashpath.Join(dir, tmpBase))
	err = unix.Renameat(parentFd, tmpBase, parentFd, baseName)
	if err != nil {
		unix.Unlinkat(parentFd, tmpBase, 0)
		return err
	}
	// To guarantee durability fsync must be called on a parent directory
	// after adding, renaming or removing files therein.
	//
	// Calling sync on the files itself is not enough according to POSIX;
	// man 2 fsync: "Calling fsync() does not necessarily ensure that the
	// entry in the directory containing the file has also reached disk.
	// For that an explicit fsync() on a file descriptor for the directory
	// is also needed."
	err = unix.Fsync(parentFd)
	if err != nil {
		return err
	}

	return nil
}

func (s *Sync) fileResolveIds(file *safcm.File) error {
	if file.User != "" && file.Uid != -1 {
		return fmt.Errorf("cannot set both User (%q) and Uid (%d)",
			file.User, file.Uid)
	}
	if file.Group != "" && file.Gid != -1 {
		return fmt.Errorf("cannot set both Group (%q) and Gid (%d)",
			file.Group, file.Gid)
	}

	if file.User == "" && file.Uid == -1 {
		file.User = s.defaultUser
	}
	if file.User != "" {
		x, err := user.Lookup(file.User)
		if err != nil {
			return err
		}
		id, err := strconv.Atoi(x.Uid)
		if err != nil {
			return err
		}
		file.Uid = id
	}

	if file.Group == "" && file.Gid == -1 {
		file.Group = s.defaultGroup
	}
	if file.Group != "" {
		x, err := user.LookupGroup(file.Group)
		if err != nil {
			return err
		}
		id, err := strconv.Atoi(x.Gid)
		if err != nil {
			return err
		}
		file.Gid = id
	}

	return nil
}

// OpenParentDirectoryNoSymlinks opens the dirname of path without following
// any symlinks and returns a file descriptor to it and the basename of path.
// To prevent symlink attacks in earlier path components when these are
// writable by other users it starts at the root (or current directory for
// relative paths) and uses openat (with O_NOFOLLOW) for each path component.
// If a symlink is encountered it returns an error. However, it's impossible
// to guarantee that the returned descriptor refers to the same location as
// given in path because users with write access can rename path components.
// But this is not required to prevent the mentioned attacks.
func OpenParentDirectoryNoSymlinks(path string) (int, string, error) {
	// Slash separated paths are used for the configuration
	parts := strings.Split(path, "/")

	var dir string
	if path == "/" {
		// Root: use root itself as base name because root is the
		// parent of itself
		dir = "/"
		parts = []string{"/"}
	} else if parts[0] == "" {
		// Absolute path
		dir = "/"
		parts = parts[1:]
	} else if path == "." {
		// Current directory: open parent directory and use current
		// directory name as base name
		wd, err := os.Getwd()
		if err != nil {
			return -1, "", fmt.Errorf(
				"failed to get working directory: %w", err)
		}
		dir = ".."
		parts = []string{filepath.Base(wd)}
	} else if parts[0] != "." {
		// Relative path: start at the current directory
		dir = "."
	}

	dirFd, err := unix.Openat(unix.AT_FDCWD, dir, openReadonlyFlags, 0)
	if err != nil {
		return -1, "", err
	}
	// Walk path one directory at a time to ensure there are no symlinks
	// in the path. This prevents users with write access to change the
	// path to point to arbitrary locations. O_NOFOLLOW when opening the
	// path is not enough as only the last path component is checked.
	for i, name := range parts[:len(parts)-1] {
		fd, err := unix.Openat(dirFd, name, openReadonlyFlags, 0)
		if err != nil {
			unix.Close(dirFd)
			if err == unix.ELOOP || err == unix.EMLINK {
				x := filepath.Join(append([]string{dir},
					parts[:i+1]...)...)
				return -1, "", fmt.Errorf(
					"symlink not permitted in path: %q",
					x)
			}
			return -1, "", err
		}
		unix.Close(dirFd)
		dirFd = fd
	}

	return dirFd, parts[len(parts)-1], nil
}

func resolveIdsToNames(uid, gid int) (string, string, error) {
	u, err := user.LookupId(strconv.Itoa(uid))
	if err != nil {
		return "", "", err
	}
	g, err := user.LookupGroupId(strconv.Itoa(gid))
	if err != nil {
		return "", "", err
	}
	return u.Username, g.Name, nil
}

func diffData(oldData []byte, newData []byte) (string, error) {
	oldBin := !strings.HasPrefix(http.DetectContentType(oldData), "text/")
	newBin := !strings.HasPrefix(http.DetectContentType(newData), "text/")
	if oldBin && newBin {
		return fmt.Sprintf("Binary files differ (%d -> %d bytes), "+
			"cannot show diff", len(oldData), len(newData)), nil
	}
	if oldBin {
		oldData = []byte(fmt.Sprintf("<binary content, %d bytes>\n",
			len(oldData)))
	}
	if newBin {
		newData = []byte(fmt.Sprintf("<binary content, %d bytes>\n",
			len(newData)))
	}

	// TODO: difflib shows empty context lines at the end of the file
	// which should not be there
	// TODO: difflib has issues with missing newlines in either side
	result, err := difflib.GetUnifiedDiffString(difflib.LineDiffParams{
		A:       difflib.SplitLines(string(oldData)),
		B:       difflib.SplitLines(string(newData)),
		Context: 3,
	})
	if err != nil {
		return "", err
	}
	return result, nil
}

func OpenFileNoSymlinks(path string) (*os.File, error) {
	parentFd, baseName, err := OpenParentDirectoryNoSymlinks(path)
	if err != nil {
		return nil, err
	}
	defer unix.Close(parentFd)
	return OpenAtNoFollow(parentFd, baseName)
}

func OpenAtNoFollow(dirFd int, base string) (*os.File, error) {
	fd, err := unix.Openat(dirFd, base, openReadonlyFlags, 0)
	if err != nil {
		return nil, err
	}
	return os.NewFile(uintptr(fd), ""), nil
}

func WriteTempAt(dirFd int, base string, data []byte, uid, gid int,
	mode fs.FileMode) (string, error) {

	fh, tmpBase, err := createTempAt(dirFd, base)
	if err != nil {
		return "", err
	}

	_, err = fh.Write(data)
	if err != nil {
		fh.Close()
		unix.Unlinkat(dirFd, tmpBase, 0)
		return "", err
	}
	// createTempAt() creates the file with 0600
	err = fh.Chown(uid, gid)
	if err != nil {
		fh.Close()
		unix.Unlinkat(dirFd, tmpBase, 0)
		return "", err
	}
	err = fh.Chmod(mode)
	if err != nil {
		fh.Close()
		unix.Unlinkat(dirFd, tmpBase, 0)
		return "", err
	}
	err = fh.Sync()
	if err != nil {
		fh.Close()
		unix.Unlinkat(dirFd, tmpBase, 0)
		return "", err
	}
	err = fh.Close()
	if err != nil {
		unix.Unlinkat(dirFd, tmpBase, 0)
		return "", err
	}

	return tmpBase, nil
}

// createTempAt works similar to os.CreateTemp but uses unix.Openat to create
// the temporary file.
func createTempAt(dirFd int, base string) (*os.File, string, error) {
	var tmpBase string

	i := 0
retry:
	tmpBase = base + strconv.Itoa(rand.Int())

	fd, err := unix.Openat(dirFd, tmpBase,
		unix.O_RDWR|unix.O_CREAT|unix.O_EXCL, 0600)
	if err != nil {
		if os.IsExist(err) && i < 10000 {
			i++
			goto retry
		}
		return nil, "", err
	}

	return os.NewFile(uintptr(fd), ""), tmpBase, nil
}