playlink/internal/fsops/fsops.go

// Package fsops contains the low-level, safety-critical filesystem primitives
// used by playlink: confined path resolution, byte comparison, atomic
// hardlink replacement and directory fsyncs.
//
// Every mutating primitive in this package follows the same rule: a path that
// holds user data is never left missing. Replacements are staged as a
// temporary hardlink in the destination directory and then atomically renamed
// over the target.
package fsops

import (
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"os"
	"path/filepath"
	"strings"
	"syscall"
)

// TmpPrefix marks staging files created by playlink. Anything carrying this
// prefix is disposable: it is either a hardlink to a master file or a copy that
// has not been renamed into place yet.
const TmpPrefix = ".playlink-tmp-"

var (
	ErrOutsideRoot  = errors.New("path escapes library root")
	ErrNotRegular   = errors.New("not a regular file")
	ErrCrossDevice  = errors.New("files are on different filesystems; hardlinks are impossible")
	ErrChanged      = errors.New("file changed since it was scanned; rescan first")
	ErrExists       = errors.New("destination already exists")
	ErrVerifyFailed = errors.New("post-operation verification failed")
)

// Info is the subset of stat(2) that playlink cares about.
type Info struct {
	Dev     uint64      `json:"dev"`
	Ino     uint64      `json:"ino"`
	Nlink   uint64      `json:"nlink"`
	Size    int64       `json:"size"`
	MtimeNs int64       `json:"mtime_ns"`
	Mode    fs.FileMode `json:"-"`
}

// SameInode reports whether a and b are the same file on disk.
func (i Info) SameInode(o Info) bool { return i.Dev == o.Dev && i.Ino == o.Ino }

// Unchanged reports whether size and mtime still match a previous snapshot.
func (i Info) Unchanged(size, mtimeNs int64) bool { return i.Size == size && i.MtimeNs == mtimeNs }

// Lstat stats p without following a trailing symlink.
func Lstat(p string) (Info, error) {
	fi, err := os.Lstat(p)
	if err != nil {
		return Info{}, err
	}
	return infoFrom(fi), nil
}

func infoFrom(fi fs.FileInfo) Info {
	in := Info{Size: fi.Size(), MtimeNs: fi.ModTime().UnixNano(), Mode: fi.Mode()}
	if st, ok := fi.Sys().(*syscall.Stat_t); ok {
		in.Dev = uint64(st.Dev)
		in.Ino = uint64(st.Ino)
		in.Nlink = uint64(st.Nlink)
	}
	return in
}

// RegularFile lstat's p and fails unless it is a plain regular file (not a
// symlink, device, directory, ...).
func RegularFile(p string) (Info, error) {
	in, err := Lstat(p)
	if err != nil {
		return in, err
	}
	if !in.Mode.IsRegular() {
		return in, fmt.Errorf("%s: %w", p, ErrNotRegular)
	}
	return in, nil
}

// Root is an absolute, symlink-free directory that confines every path
// playlink touches for a library.
type Root struct {
	Path string
}

// NewRoot validates dir and resolves symlinks so containment checks are exact.
func NewRoot(dir string) (Root, error) {
	if !filepath.IsAbs(dir) {
		return Root{}, fmt.Errorf("%q is not an absolute path", dir)
	}
	real, err := filepath.EvalSymlinks(filepath.Clean(dir))
	if err != nil {
		return Root{}, err
	}
	fi, err := os.Stat(real)
	if err != nil {
		return Root{}, err
	}
	if !fi.IsDir() {
		return Root{}, fmt.Errorf("%q is not a directory", dir)
	}
	return Root{Path: real}, nil
}

// Within reports whether p equals base or lies beneath it. Both must be clean.
func Within(base, p string) bool {
	if p == base {
		return true
	}
	if base == "/" {
		return strings.HasPrefix(p, "/")
	}
	return strings.HasPrefix(p, base+string(filepath.Separator))
}

// CleanRel validates a library-relative path coming from the database or an
// API request.
func CleanRel(rel string) (string, error) {
	if rel == "" || filepath.IsAbs(rel) || strings.ContainsRune(rel, 0) {
		return "", fmt.Errorf("invalid relative path %q", rel)
	}
	c := filepath.Clean(rel)
	if c == "." || c == ".." || strings.HasPrefix(c, "../") {
		return "", fmt.Errorf("%q: %w", rel, ErrOutsideRoot)
	}
	return c, nil
}

// Resolve maps a relative path to an absolute one inside the root. The
// deepest existing ancestor is resolved through symlinks and must still be
// inside the root, so a symlinked directory can never redirect a write
// outside the library. The final component itself is not followed.
func (r Root) Resolve(rel string) (string, error) {
	c, err := CleanRel(rel)
	if err != nil {
		return "", err
	}
	full := filepath.Join(r.Path, c)
	anc := filepath.Dir(full)
	for {
		real, err := filepath.EvalSymlinks(anc)
		if err == nil {
			if !Within(r.Path, real) {
				return "", fmt.Errorf("%q: %w", rel, ErrOutsideRoot)
			}
			// Re-anchor the missing tail on the resolved ancestor.
			tail, _ := filepath.Rel(anc, full)
			return filepath.Join(real, tail), nil
		}
		if !errors.Is(err, fs.ErrNotExist) {
			return "", err
		}
		if anc == r.Path || anc == "/" {
			return "", err
		}
		anc = filepath.Dir(anc)
	}
}

// Rel converts an absolute path inside the root back to a relative one.
func (r Root) Rel(abs string) (string, error) {
	if !Within(r.Path, abs) || abs == r.Path {
		return "", ErrOutsideRoot
	}
	return filepath.Rel(r.Path, abs)
}

// SameContent compares two files byte for byte (the equivalent of cmp(1)).
// It refuses to report equality if either file changed while being read.
func SameContent(a, b string) (bool, error) {
	fa, err := os.Open(a)
	if err != nil {
		return false, err
	}
	defer fa.Close()
	fb, err := os.Open(b)
	if err != nil {
		return false, err
	}
	defer fb.Close()

	sa, err := fa.Stat()
	if err != nil {
		return false, err
	}
	sb, err := fb.Stat()
	if err != nil {
		return false, err
	}
	if !sa.Mode().IsRegular() || !sb.Mode().IsRegular() {
		return false, ErrNotRegular
	}
	if sa.Size() != sb.Size() {
		return false, nil
	}
	ia, ib := infoFrom(sa), infoFrom(sb)
	if ia.SameInode(ib) {
		return true, nil
	}

	const chunk = 1 << 20
	bufA := make([]byte, chunk)
	bufB := make([]byte, chunk)
	for {
		na, errA := io.ReadFull(fa, bufA)
		nb, errB := io.ReadFull(fb, bufB)
		if na != nb || !bytes.Equal(bufA[:na], bufB[:nb]) {
			return false, nil
		}
		endA := errA == io.EOF || errA == io.ErrUnexpectedEOF
		endB := errB == io.EOF || errB == io.ErrUnexpectedEOF
		if errA != nil && !endA {
			return false, errA
		}
		if errB != nil && !endB {
			return false, errB
		}
		if endA || endB {
			if endA != endB {
				return false, nil
			}
			break
		}
	}

	// Guard against a writer modifying either file mid-compare.
	for _, pair := range []struct {
		p    string
		prev Info
	}{{a, ia}, {b, ib}} {
		now, err := Lstat(pair.p)
		if err != nil {
			return false, err
		}
		if !now.SameInode(pair.prev) || !now.Unchanged(pair.prev.Size, pair.prev.MtimeNs) {
			return false, ErrChanged
		}
	}
	return true, nil
}

// SyncDir fsyncs a directory so renames and new links inside it are durable.
func SyncDir(dir string) error {
	d, err := os.Open(dir)
	if err != nil {
		return err
	}
	defer d.Close()
	if err := d.Sync(); err != nil && !errors.Is(err, syscall.EINVAL) {
		return err
	}
	return nil
}

func tmpName(dir string) (string, error) {
	var b [8]byte
	if _, err := rand.Read(b[:]); err != nil {
		return "", err
	}
	return filepath.Join(dir, TmpPrefix+hex.EncodeToString(b[:])), nil
}

// StageLink creates a temporary hardlink to src inside dstDir and verifies it
// points at the same inode. The caller must rename or remove the result.
func StageLink(src string, srcInfo Info, dstDir string) (string, error) {
	tmp, err := tmpName(dstDir)
	if err != nil {
		return "", err
	}
	if err := os.Link(src, tmp); err != nil {
		if errors.Is(err, syscall.EXDEV) {
			return "", ErrCrossDevice
		}
		return "", err
	}
	got, err := Lstat(tmp)
	if err != nil || !got.SameInode(srcInfo) {
		_ = os.Remove(tmp)
		if err == nil {
			err = fmt.Errorf("%w: staged link does not share the source inode", ErrVerifyFailed)
		}
		return "", err
	}
	return tmp, nil
}

// LinkNew hardlinks src to dst, which must not exist. Verified by inode.
func LinkNew(src string, srcInfo Info, dst string) error {
	if _, err := os.Lstat(dst); err == nil {
		return fmt.Errorf("%s: %w", dst, ErrExists)
	}
	if err := os.Link(src, dst); err != nil {
		if errors.Is(err, syscall.EXDEV) {
			return ErrCrossDevice
		}
		return err
	}
	got, err := Lstat(dst)
	if err != nil || !got.SameInode(srcInfo) {
		// dst was created by us a moment ago and is only an extra link.
		_ = os.Remove(dst)
		if err == nil {
			err = fmt.Errorf("%w: %s does not share the source inode", ErrVerifyFailed, dst)
		}
		return err
	}
	return SyncDir(filepath.Dir(dst))
}

// ReplaceWithLink atomically replaces dst with a hardlink to src. dst must be
// a regular file on the same device. The staged link is renamed over dst, so
// dst never disappears, even on a crash.
func ReplaceWithLink(src string, srcInfo Info, dst string) error {
	tmp, err := StageLink(src, srcInfo, filepath.Dir(dst))
	if err != nil {
		return err
	}
	if err := os.Rename(tmp, dst); err != nil {
		_ = os.Remove(tmp)
		return err
	}
	if err := SyncDir(filepath.Dir(dst)); err != nil {
		return err
	}
	got, err := Lstat(dst)
	if err != nil {
		return err
	}
	if !got.SameInode(srcInfo) {
		return fmt.Errorf("%w: %s does not share the source inode after rename", ErrVerifyFailed, dst)
	}
	return nil
}

// RemoveLink deletes p only if it is a hardlink of keep (same inode and at
// least one other link). This is the only way playlink deletes a file outside
// the quarantine, and it never destroys the last copy of any data.
func RemoveLink(p string, keep Info) error {
	in, err := RegularFile(p)
	if err != nil {
		return err
	}
	if !in.SameInode(keep) || in.Nlink < 2 {
		return fmt.Errorf("refusing to remove %s: it is not a hardlink of the file being kept", p)
	}
	if err := os.Remove(p); err != nil {
		return err
	}
	return SyncDir(filepath.Dir(p))
}

// CopyReplace replaces dst with an independent byte copy of src (used to undo
// a link between identical files). The copy is fsynced and verified before
// being renamed into place.
func CopyReplace(src, dst string, mode fs.FileMode) error {
	tmp, err := tmpName(filepath.Dir(dst))
	if err != nil {
		return err
	}
	in, err := os.Open(src)
	if err != nil {
		return err
	}
	defer in.Close()
	out, err := os.OpenFile(tmp, os.O_WRONLY|os.O_CREATE|os.O_EXCL, mode.Perm())
	if err != nil {
		return err
	}
	cleanup := func(e error) error { out.Close(); _ = os.Remove(tmp); return e }
	if _, err := io.Copy(out, in); err != nil {
		return cleanup(err)
	}
	if err := out.Sync(); err != nil {
		return cleanup(err)
	}
	if err := out.Close(); err != nil {
		_ = os.Remove(tmp)
		return err
	}
	same, err := SameContent(src, tmp)
	if err != nil || !same {
		_ = os.Remove(tmp)
		if err == nil {
			err = fmt.Errorf("%w: copy differs from source", ErrVerifyFailed)
		}
		return err
	}
	if err := os.Rename(tmp, dst); err != nil {
		_ = os.Remove(tmp)
		return err
	}
	return SyncDir(filepath.Dir(dst))
}

// MkdirAllLike creates dir and any missing parents with 0775 permissions.
func MkdirAllLike(dir string) error {
	return os.MkdirAll(dir, 0o775)
}