mesh-host/internal/apply/mesh_dirs_apply_linux.go

//go:build linux

package apply

// The apply half of mesh_dirs.go (novox/hq issue 358): a mesh directory outside every home made and given its mode
// and owner through the walk's descriptor.

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"

	"golang.org/x/sys/unix"

	"github.com/novox/mesh-host/internal/declaration"
)

// meshDirsByDescriptor says this system can open a directory component by component without following a link.
const meshDirsByDescriptor = true

// openMeshDir opens the directory at path, from /, one component at a time with O_DIRECTORY|O_NOFOLLOW; a missing
// component is made in its parent's descriptor with mode, and made is called with each one made and its descriptor.
// A link in a parent is followed only when couldReplace and linkReplacer find that root alone could have made it,
// and its target is walked again from /; any other link is a LinkInPathError, and the last component being one is
// refused as not placed. Answers the directory's descriptor, which the caller closes, with the walk: what it made
// and the path it reached the directory at.
func openMeshDir(path string, mode os.FileMode, made func(dir string, fd int) error) (int, *meshWalk, error) {
	w := &meshWalk{want: filepath.Clean(path), create: true, mode: mode, made: made}
	fd, err := w.open()
	return fd, w, err
}

// applyMeshDirectory gives a mesh directory outside every home its declared mode and owner through a descriptor
// (novox/hq issue 358): opened by openMeshDir, made there when missing, judged a directory by fstat on it, changed
// with fchmod and fchown on it, and read back from it. The parents it makes inside the owner's home are the owner's,
// as makeDirs gives them (novox/hq ADR 0182).
func applyMeshDirectory(r *declaration.Directory, out Outcome, mode os.FileMode, existed bool) (Outcome, error) {
	uid, gid, chown := 0, 0, r.Owner != ""
	var err error
	if chown {
		if uid, gid, err = idsOf(r.Owner); err != nil {
			return out, fmt.Errorf("%s should belong to %q: %w", r.Path, r.Owner, err)
		}
	}
	clean := filepath.Clean(r.Path)
	home := ""
	if chown {
		if h, herr := homeOf(r.Owner); herr == nil && h != "" {
			home = filepath.Clean(h)
		}
	}
	giveMade := func(dir string, fd int) error {
		if dir == clean || home == "" || (dir != home && !strings.HasPrefix(dir, home+string(os.PathSeparator))) {
			return nil
		}
		if err := unix.Fchown(fd, uid, gid); err != nil {
			return fmt.Errorf("cannot give %s to %q: %w", dir, r.Owner, &os.PathError{Op: "chown", Path: dir, Err: err})
		}
		return nil
	}
	fd, walk, err := openMeshDir(clean, mode, giveMade)
	if err != nil {
		var link *LinkInPathError
		if errors.As(err, &link) {
			return out, &PlacementRefusedError{Path: r.Path, Why: "it " + link.why()}
		}
		return out, err
	}
	defer unix.Close(fd)
	// Made by this walk when its last component is: at the path the walk reached it at, which root-only links in its
	// path may have moved from the path declared.
	created := len(walk.Made) > 0 && walk.Made[len(walk.Made)-1] == walk.At

	var st unix.Stat_t
	if err := unix.Fstat(fd, &st); err != nil {
		return out, &os.PathError{Op: "fstat", Path: r.Path, Err: err}
	}
	beforeMode, beforeUID, beforeGID := statDirMode(st.Mode), int(st.Uid), int(st.Gid)
	// Set explicitly even when it existed: a mode given at creation is not a mode maintained — its setgid bit
	// with it (novox/hq ADR 0334 §1).
	if err := unix.Fchmod(fd, unixDirMode(mode)); err != nil {
		return out, &os.PathError{Op: "chmod", Path: r.Path, Err: err}
	}
	ownedAlready := !chown || (beforeUID == uid && beforeGID == gid)
	if !ownedAlready {
		if err := unix.Fchown(fd, uid, gid); err != nil {
			return out, fmt.Errorf("cannot give %s to %q: %w", r.Path, r.Owner, &os.PathError{Op: "chown", Path: r.Path, Err: err})
		}
		// A chown may take the setgid bit: the mode is set again after it.
		if err := unix.Fchmod(fd, unixDirMode(mode)); err != nil {
			return out, &os.PathError{Op: "chmod", Path: r.Path, Err: err}
		}
	}

	// Read back, from the descriptor that was changed.
	if err := unix.Fstat(fd, &st); err != nil {
		return out, fmt.Errorf("made %s and cannot stat it: %w", r.Path, err)
	}
	if st.Mode&unix.S_IFMT != unix.S_IFDIR {
		return out, fmt.Errorf("%s is not a directory after applying", r.Path)
	}
	if got := statDirMode(st.Mode); got != dirBits(mode) {
		return out, fmt.Errorf("%s is mode %s after setting %s", r.Path, modeText(got), modeText(mode))
	}

	out.Action = "unchanged"
	switch {
	case created || !existed:
		out.Action = "created"
	case !ownedAlready:
		out.Action = "updated"
	case beforeMode != dirBits(mode):
		out.Action = "updated"
		out.Detail = fmt.Sprintf("mode %s to %s", modeText(beforeMode), modeText(mode))
	}
	return out, nil
}