// Package archive is the files archive of the move-any-app design §8: a
// deterministic tar of an instance's files directory and its <host>.env,
// compressed with zstd, then sealed under the deployment's config keypair.
//
// The source box builds and seals it; the target box opens, verifies and
// extracts it. The target treats every archive as hostile. The config
// public key is widely held, so anyone who can write the staging bucket can
// seal an archive that opens: authentication proves who it was sealed for,
// not who sealed it. Every limit Build obeys, Verify and Extract enforce
// again, and Build obeys every limit Verify and Extract enforce, so an
// archive Build wrote always lands.
package archive

import (
	"archive/tar"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"hash"
	"io"
	"io/fs"
	"os"
	"path/filepath"
	"sort"

	"github.com/klauspost/compress/zstd"
)

// ManifestVersion is the Manifest.V this package writes and reads.
const ManifestVersion = 1

// Errors from Build (ErrSpecialFile, ErrOverCap, ErrTooManyEntries,
// ErrChanged and the long-path ErrBadPath) reach the operator through the
// source report's S1; their texts are rows MV-N77 to MV-N81 and MV-N86 of
// the copy record. The rest are the target's refusals.
var (
	ErrSpecialFile    = errors.New("archive: only regular files and directories move")
	ErrOverCap        = errors.New("archive: the files directory is over 256 MiB")
	ErrTooManyEntries = fmt.Errorf("archive: the files directory holds more than %d entries", MaxEntries)
	ErrChanged        = errors.New("archive: a file changed while it was being archived")
	ErrBadPath        = errors.New("archive: refused path")
	ErrTooLarge       = errors.New("archive: the decoded archive is over its size limit")
	ErrNotEmpty       = errors.New("archive: the destination files directory is not empty")
	// ErrPastTreeCap is Build's refusal of a path longer than TreePathLen
	// (hostfiles' cap) but within MaxPathLen; the source says MV-N502.
	ErrPastTreeCap = errors.New("archive: a path is past the files tree's length cap")
)

// PathError is a Build refusal that names a file: Err is the sentinel
// (errors.Is sees it), Path the file as Build names it (relative to the
// files directory, or the env file's base name). It lets a caller say
// which file in its own words without reading the error's text; Error() is
// the text the sentinel's rows record (MV-N77, MV-N80, MV-N81, MV-N86).
type PathError struct {
	Err  error
	Path string
	msg  string
}

func (e *PathError) Error() string {
	if e.msg != "" {
		return e.msg
	}
	return e.Err.Error() + ": " + e.Path
}
func (e *PathError) Unwrap() error { return e.Err }

// The refusals that name a file, each one whole recorded sentence (copy
// record MV-N77, MV-N80, MV-N81 and MV-N86) so the copy gate can read it.
// Err is the sentinel errors.Is sees; the text is the sentinel's, a colon
// and the path, as the rows say.
func specialFileError(path string) error {
	return &PathError{Err: ErrSpecialFile, Path: path, msg: fmt.Sprintf("archive: only regular files and directories move: %s", path)}
}

func hardLinkError(path string) error {
	return &PathError{Err: ErrSpecialFile, Path: path, msg: fmt.Sprintf("archive: only regular files and directories move: %s is a hard link", path)}
}

func changedError(path string) error {
	return &PathError{Err: ErrChanged, Path: path, msg: fmt.Sprintf("archive: a file changed while it was being archived: %s", path)}
}

// The row says 4096, so the sentence does; this fails to compile if
// MaxPathLen is ever changed without the row.
const _ = uint(MaxPathLen-4096) + uint(4096-MaxPathLen)

// treePathError is a path past TreePathLen but within MaxPathLen: the
// archive format could carry it, the target's files manifest could not.
// The operator's sentence is the caller's (MV-N502, from the sentinel);
// this text reaches the log only.
func treePathError(path string) error {
	return &PathError{Err: ErrPastTreeCap, Path: path, msg: fmt.Sprintf("archive: refused path: %s is longer than the files tree's %d-byte cap", path, TreePathLen)}
}

func longPathError(path string) error {
	return &PathError{Err: ErrBadPath, Path: path, msg: fmt.Sprintf("archive: refused path: %s is longer than 4096 bytes", path)}
}

// The limits as enforced. Variables only so tests can lower them; nothing
// else writes them.
var (
	entryLimit   = MaxEntries
	decodedLimit = MaxDecoded
)

// Entry is one regular file in the manifest: its archive path ("env" or
// "files/<path>"), size and sha256.
type Entry struct {
	Path   string `json:"path"`
	Size   int64  `json:"size"`
	SHA256 string `json:"sha256"`
}

// Manifest travels beside the sealed object (§8.2). SealedDataKey is the
// archive's data key sealed to the deployment's config keypair (a v2
// envelope bound to the app); it is not an object key (F29).
type Manifest struct {
	V             int     `json:"v"`
	Op            string  `json:"op"`
	Incarnation   string  `json:"incarnation"`
	Plain         string  `json:"plain"`
	Sealed        string  `json:"sealed"`
	Bytes         int64   `json:"bytes"`
	Entries       []Entry `json:"entries"`
	SealedDataKey string  `json:"sealed_data_key"`
	Chunk         int     `json:"chunk"`
}

// item is one listed entry: its archive name, where to open it, and its
// Lstat at listing time, which the open must match.
type item struct {
	name string      // "env", "files/<rel>"
	rel  string      // path inside the files root; "" for env
	root *os.Root    // the files root; nil for env
	path string      // envPath, for env
	info fs.FileInfo // as listed
}

// walk lists the files directory and envPath in archive order, refusing
// anything that is not a regular file or a directory and excluding the
// marker. The files directory is read through an os.Root, so nothing a
// tenant does inside it (a directory swapped for a symlink mid-walk) can
// make the walk or a later open leave it.
//
// skip, when non-nil, is asked each path relative to the files directory
// (slash-separated, as the archive names it) before anything else is read
// about it: a path it accepts is left out as the marker is, a directory
// with everything under it, and is neither typed nor counted, so a
// platform artifact of any kind (a socket, a FIFO) cannot fail the walk.
// envPath is never asked.
func walk(root *os.Root, envPath string, skip func(rel string) bool) ([]item, error) {
	var items []item
	add := func(it item) error {
		if len(items) >= entryLimit {
			return ErrTooManyEntries
		}
		items = append(items, it)
		return nil
	}
	if root != nil {
		err := fs.WalkDir(root.FS(), ".", func(p string, d fs.DirEntry, err error) error {
			if err != nil {
				return err
			}
			if p == "." {
				return nil
			}
			if p == MarkerName || (skip != nil && skip(p)) {
				// The marker is never archived, nor anything under it if
				// it is (wrongly) a directory; nor is a path the caller
				// skips.
				if d.IsDir() {
					return fs.SkipDir
				}
				return nil
			}
			// Through the root only (census ruling moves-overcap-cut-236,
			// binding): a read that fails refuses the walk, never a retry by
			// path.
			info, err := root.Lstat(p)
			if err != nil {
				return err
			}
			if !info.Mode().IsRegular() && !info.IsDir() {
				return specialFileError(p)
			}
			if len(p) > MaxPathLen {
				return longPathError(p)
			}
			if len(p) > TreePathLen {
				return treePathError(p)
			}
			return add(item{name: FilesPrefix + p, rel: p, root: root, info: info})
		})
		if err != nil {
			return nil, err
		}
	}
	if envPath != "" {
		info, err := os.Lstat(envPath)
		switch {
		case errors.Is(err, fs.ErrNotExist):
		case err != nil:
			return nil, err
		case !info.Mode().IsRegular():
			return nil, specialFileError(filepath.Base(envPath))
		default:
			if err := add(item{name: EnvEntry, path: envPath, info: info}); err != nil {
				return nil, err
			}
		}
	}
	sort.Slice(items, func(i, j int) bool { return items[i].name < items[j].name })
	return items, nil
}

// openItem opens a listed regular file. A variable so a test can stand in
// a file that grows after it was listed.
var openItem = openItemChecked

// openItemChecked opens it.rel inside the files root (or the env path) and
// requires the opened file to be the one listed: a regular file with the
// listed identity and one name. A file swapped after the walk (for a FIFO, another file,
// or a symlink the root would refuse to leave by) fails here. The open is
// non-blocking where the platform has the flag, so a swapped-in FIFO
// returns at once instead of waiting for a writer.
func openItemChecked(it item) (io.ReadCloser, error) {
	var f *os.File
	var err error
	if it.root != nil {
		f, err = it.root.OpenFile(it.rel, os.O_RDONLY|openNonblock, 0)
	} else {
		f, err = os.OpenFile(it.path, os.O_RDONLY|openNonblock, 0)
	}
	if err != nil {
		return nil, err
	}
	got, err := f.Stat()
	if err != nil {
		f.Close()
		return nil, err
	}
	if !got.Mode().IsRegular() || !os.SameFile(it.info, got) {
		f.Close()
		return nil, changedError(displayName(it))
	}
	// Phase 1 does not carry hard links. os.Root constrains names, not
	// inodes: a pre-existing link to a file outside the instance directory
	// is a regular file with the listed identity, and reading it would
	// carry the outside inode to the target. Counted on the opened file.
	if linkCount(got) > 1 {
		f.Close()
		return nil, hardLinkError(displayName(it))
	}
	return f, nil
}

func displayName(it item) string {
	if it.root == nil {
		return filepath.Base(it.path)
	}
	return it.rel
}

// hashWriter hashes and counts the tar stream on its way to the
// compressor. It enforces no limit: content is capped where sizes are
// admitted, and the stream's length follows from the entry limits.
type hashWriter struct {
	w io.Writer
	h hash.Hash
	n int64
}

func (c *hashWriter) Write(p []byte) (int, error) {
	c.h.Write(p)
	c.n += int64(len(p))
	return c.w.Write(p)
}

func digestString(h hash.Hash) string {
	return "sha256:" + hex.EncodeToString(h.Sum(nil))
}

// built is one pass over the tree.
type built struct {
	plain    string
	entries  []Entry
	content  int64
	tarBytes int64
	count    int // tar entries: files, directories and env
}

// openFilesRoot opens filesDir as an os.Root, requiring it to be a real
// directory (not a symlink to one) at both the Lstat and the open. A
// missing directory is an empty one: the archive then carries env only.
func openFilesRoot(filesDir string) (*os.Root, error) {
	if filesDir == "" {
		return nil, errors.New("archive: no files directory named")
	}
	info, err := os.Lstat(filesDir)
	if errors.Is(err, fs.ErrNotExist) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	if !info.IsDir() {
		// The files root itself is not a directory: untyped, so the caller
		// reports it as the files failing to archive, never as a <path>.
		return nil, fmt.Errorf("%w: %s", ErrSpecialFile, filesDir)
	}
	root, err := os.OpenRoot(filesDir)
	if err != nil {
		return nil, err
	}
	got, err := root.Stat(".")
	if err != nil {
		root.Close()
		return nil, err
	}
	if !os.SameFile(info, got) {
		root.Close()
		// The root itself, not a file in it: untyped, as above.
		return nil, fmt.Errorf("%w: %s", ErrChanged, filesDir)
	}
	return root, nil
}

// build writes the tar of filesDir and envPath to out and returns what
// Build and PlainDigest report.
func build(filesDir, envPath string, out io.Writer) (built, error) {
	return buildFiltered(filesDir, envPath, nil, out)
}

// buildFiltered is build over the paths skip leaves in (walk).
func buildFiltered(filesDir, envPath string, skip func(rel string) bool, out io.Writer) (built, error) {
	root, err := openFilesRoot(filesDir)
	if err != nil {
		return built{}, err
	}
	if root != nil {
		defer root.Close()
	}
	items, err := walk(root, envPath, skip)
	if err != nil {
		return built{}, err
	}
	var b built
	hw := &hashWriter{w: out, h: sha256.New()}
	tw := tar.NewWriter(hw)
	for _, it := range items {
		hdr := &tar.Header{Name: it.name, Mode: int64(it.info.Mode().Perm()), Format: tar.FormatPAX}
		if it.info.IsDir() {
			hdr.Typeflag = tar.TypeDir
			hdr.Name += "/"
		} else {
			hdr.Typeflag = tar.TypeReg
			hdr.Size = it.info.Size()
			// Admitted against the cap before the header commits the size.
			if hdr.Size > Cap-b.content {
				return built{}, ErrOverCap
			}
			b.content += hdr.Size
		}
		if err := tw.WriteHeader(hdr); err != nil {
			return built{}, err
		}
		b.count++
		if hdr.Typeflag != tar.TypeReg {
			continue
		}
		e, err := copyItem(tw, it, hdr.Size)
		if err != nil {
			return built{}, err
		}
		b.entries = append(b.entries, e)
	}
	if err := tw.Close(); err != nil {
		return built{}, err
	}
	b.plain, b.tarBytes = digestString(hw.h), hw.n
	return b, nil
}

// copyItem streams exactly the admitted size of one file into the tar.
// This is the cap while streaming: a file is never read past the size its
// header committed (and the cap admitted), so a file that grows after it
// was listed cannot carry the archive past Cap. It is refused as changed,
// as is one that shrank, rather than archived torn.
func copyItem(tw io.Writer, it item, size int64) (Entry, error) {
	rc, err := openItem(it)
	if err != nil {
		return Entry{}, err
	}
	defer rc.Close()
	fh := sha256.New()
	n, err := io.CopyN(io.MultiWriter(tw, fh), rc, size)
	if errors.Is(err, io.EOF) {
		return Entry{}, changedError(displayName(it))
	}
	if err != nil {
		return Entry{}, err
	}
	var one [1]byte
	m, err := io.ReadFull(rc, one[:])
	if m > 0 {
		return Entry{}, changedError(displayName(it))
	}
	if err != io.EOF {
		return Entry{}, err
	}
	return Entry{Path: it.name, Size: n, SHA256: digestString(fh)}, nil
}

// Build writes the archive (tar then zstd) to w and returns the plaintext
// digest (of the tar stream), the regular-file entries and the content
// byte count. Owners and times are zeroed, modes kept, so equal content
// gives equal plaintext (§8.1). The files directory's entries live under
// "files/" and <host>.env is "env", in separate namespaces, so an
// application file or directory named "env" is carried as "files/env" and
// can never collide with the overlay. A missing envPath (or "") means the
// archive has no env entry; a missing filesDir is an empty one.
func Build(filesDir, envPath string, w io.Writer) (plain string, entries []Entry, bytes int64, err error) {
	return BuildFiltered(filesDir, envPath, nil, w)
}

// BuildFiltered is Build over the files directory without the paths skip
// accepts. skip is asked each path relative to filesDir, slash-separated
// (a top-level entry is its bare name); a skipped directory is skipped
// whole, and a skipped entry of any type is never read, typed or counted.
// The env entry is never asked. A nil skip is Build. Nothing calls it with
// a filter today: the offline move's sealer archives a nested instance's
// files directory (<instance dir>/<host>, which holds nothing of the
// platform's but the marker) with Build, and refuses the seal over any
// entry of the app's beside it.
func BuildFiltered(filesDir, envPath string, skip func(rel string) bool, w io.Writer) (plain string, entries []Entry, bytes int64, err error) {
	zw, err := zstd.NewWriter(w, zstd.WithEncoderConcurrency(1), zstd.WithWindowSize(zstdWindow))
	if err != nil {
		return "", nil, 0, err
	}
	b, err := buildFiltered(filesDir, envPath, skip, zw)
	if err != nil {
		zw.Close()
		return "", nil, 0, err
	}
	if err := zw.Close(); err != nil {
		return "", nil, 0, err
	}
	return b.plain, b.entries, b.content, nil
}

// PlainDigest runs Build's walk and tar without compressing or writing
// anything, and returns the same plaintext digest Build would, the content
// byte count and the number of regular-file entries (env included). It is
// the baseline the source records at `stopped` before any archive exists
// (§8.3), and the comparison every recovery makes.
func PlainDigest(filesDir, envPath string) (plain string, bytes int64, entries int, err error) {
	b, err := build(filesDir, envPath, io.Discard)
	if err != nil {
		return "", 0, 0, err
	}
	return b.plain, b.content, len(b.entries), nil
}