crfs/stargz: add start of package

Basic API, format, tests.

Good enough checkpoint.

Updates golang/go#30829

Change-Id: Iaec5b205314d64fca5056f6b19a7bae52e5cef94
Reviewed-on: https://go-review.googlesource.com/c/build/+/167769
Reviewed-by: Brad Fitzpatrick <[email protected]>

crfs/stargz/stargz.go

@@ -0,0 +1,345 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The stargz package reads & writes tar.gz ("tarball") files in a
+// seekable, indexed format call "stargz". A stargz file is still a
+// valid tarball, but it's slightly bigger with new gzip streams for
+// each new file & throughout large files, and has an index in a magic
+// file at the end.
+package stargz
+
+import (
+	"archive/tar"
+	"bufio"
+	"bytes"
+	"compress/gzip"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"strconv"
+	"time"
+)
+
+// TOCTarName is the name of the JSON file in the tar archive in the
+// table of contents gzip stream.
+const TOCTarName = "stargz.index.json"
+
+// FooterSize is the number of bytes in the stargz footer.
+//
+// The footer is an empty gzip stream with no compression and an Extra
+// header of the form "%016xSTARGZ", where the 64 bit hex-encoded
+// number is the offset to the gzip stream of JSON TOC.
+//
+// 47 comes from:
+//
+//   10 byte gzip header +
+//   2 byte (LE16) length of extra, encoding 22 (16 hex digits + len("STARGZ")) == "\x16\x00" +
+//   22 bytes of extra (fmt.Sprintf("%016xSTARGZ", tocGzipOffset))
+//   5 byte flate header
+//   8 byte gzip footer (two little endian uint32s: digest, size)
+const FooterSize = 47
+
+// A Reader permits random access reads from a stargz file.
+type Reader struct {
+	sr  *io.SectionReader
+	TOC *TOC
+}
+
+// Open opens a stargz file for reading.
+func Open(sr *io.SectionReader) (*Reader, error) {
+	if sr.Size() < FooterSize {
+		return nil, fmt.Errorf("stargz size %d is smaller than the stargz footer size", sr.Size())
+	}
+	// TODO: read a bigger chunk (1MB?) at once here to hopefully
+	// get the TOC + footer in one go.
+	var footer [FooterSize]byte
+	if _, err := sr.ReadAt(footer[:], sr.Size()-FooterSize); err != nil {
+		return nil, fmt.Errorf("error reading footer: %v", err)
+	}
+	tocOff, ok := parseFooter(footer[:])
+	if !ok {
+		return nil, fmt.Errorf("error parsing footer")
+	}
+	tocTargz := make([]byte, sr.Size()-tocOff-FooterSize)
+	if _, err := sr.ReadAt(tocTargz, tocOff); err != nil {
+		return nil, fmt.Errorf("error reading %d byte TOC targz: %v", len(tocTargz), err)
+	}
+	zr, err := gzip.NewReader(bytes.NewReader(tocTargz))
+	if err != nil {
+		return nil, fmt.Errorf("malformed TOC gzip header: %v", err)
+	}
+	zr.Multistream(false)
+	tr := tar.NewReader(zr)
+	h, err := tr.Next()
+	if err != nil {
+		return nil, fmt.Errorf("failed to find tar header in TOC gzip stream: %v", err)
+	}
+	if h.Name != TOCTarName {
+		return nil, fmt.Errorf("TOC tar entry had name %q; expected %q", h.Name, TOCTarName)
+	}
+	toc := new(TOC)
+	if err := json.NewDecoder(tr).Decode(&toc); err != nil {
+		return nil, fmt.Errorf("error decoding TOC JSON: %v", err)
+	}
+	return &Reader{sr: sr, TOC: toc}, nil
+}
+
+// TOCEntry is an entry in the stargz file's TOC (Table of Contents).
+type TOCEntry struct {
+	Offset   int64  `json:"offset,omitempty"` // offset to gzip stream of tar entry (for regular files only)
+	Name     string `json:"name"`
+	Type     string `json:"type"` // "dir", "reg", TODO
+	Size     int64  `json:"size,omitempty"`
+	LinkName string `json:"linkName,omitempty"`  // for symlinks
+	Mode     int64  `json:"mode,omitempty"`      // Permission and mode bits
+	Uid      int    `json:"uid,omitempty"`       // User ID of owner
+	Gid      int    `json:"gid,omitempty"`       // Group ID of owner
+	Uname    string `json:"userName,omitempty"`  // User name of owner
+	Gname    string `json:"groupName,omitempty"` // Group name of owner
+	ModTime  string `json:"modtime,omitempty"`
+
+	// ChunkOffset is non-zero if this is a chunk of a large,
+	// regular file. If so, the Offset is where the gzip header of
+	// ChunkSize bytes at ChunkOffset in Name begin. If both
+	// ChunkOffset and ChunkSize are zero, the file contents are
+	// completely represented at the tar gzip stream starting at
+	// Offset.
+	ChunkOffset int64 `json:"chunkOffset,omitempty"`
+	ChunkSize   int64 `json:"chunkSize,omitempty"`
+}
+
+// TOC is the table of contents index of the files in the stargz file.
+type TOC struct {
+	Version int        `json:"version"`
+	Entries []TOCEntry `json:"entries"`
+}
+
+// A Writer writes stargz files.
+//
+// Use NewWriter to create a new Writer.
+type Writer struct {
+	bw  *bufio.Writer
+	cw  *countWriter
+	toc *TOC
+
+	closed bool
+	gz     *gzip.Writer
+}
+
+// NewWriter returns a new stargz writer writing to w.
+//
+// The writer must be closed to write its trailing table of contents.
+func NewWriter(w io.Writer) *Writer {
+	bw := bufio.NewWriter(w)
+	cw := &countWriter{w: bw}
+	return &Writer{
+		bw:  bw,
+		cw:  cw,
+		toc: &TOC{Version: 1},
+	}
+}
+
+// Close writes the stargz's table of contents and flushes all the
+// buffers, returning any error.
+func (w *Writer) Close() error {
+	if w.closed {
+		return nil
+	}
+	defer func() { w.closed = true }()
+
+	if err := w.closeGz(); err != nil {
+		return err
+	}
+
+	// Write the TOC index.
+	tocOff := w.cw.n
+	w.gz, _ = gzip.NewWriterLevel(w.cw, gzip.BestCompression)
+	tw := tar.NewWriter(w.gz)
+	tocJSON, err := json.MarshalIndent(w.toc, "", "\t")
+	if err != nil {
+		return err
+	}
+	if err := tw.WriteHeader(&tar.Header{
+		Typeflag: tar.TypeReg,
+		Name:     TOCTarName,
+		Size:     int64(len(tocJSON)),
+	}); err != nil {
+		return err
+	}
+	if _, err := tw.Write(tocJSON); err != nil {
+		return err
+	}
+
+	if err := tw.Close(); err != nil {
+		return err
+	}
+	if err := w.closeGz(); err != nil {
+		return err
+	}
+
+	// And a little footer with pointer to the TOC gzip stream.
+	if _, err := w.bw.Write(footerBytes(tocOff)); err != nil {
+		return err
+	}
+
+	if err := w.bw.Flush(); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (w *Writer) closeGz() error {
+	if w.closed {
+		return errors.New("write on closed Writer")
+	}
+	if w.gz != nil {
+		if err := w.gz.Close(); err != nil {
+			return err
+		}
+		w.gz = nil
+	}
+	return nil
+}
+
+// AppendTar reads the tar or tar.gz file from r and appends
+// each of its contents to w.
+//
+// The input r can optionally be gzip compressed but the output will
+// always be gzip compressed.
+func (w *Writer) AppendTar(r io.Reader) error {
+	br := bufio.NewReader(r)
+	var tr *tar.Reader
+	if isGzip(br) {
+		// NewReader can't fail if isGzip returned true.
+		zr, _ := gzip.NewReader(br)
+		tr = tar.NewReader(zr)
+	} else {
+		tr = tar.NewReader(br)
+	}
+	for {
+		h, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return fmt.Errorf("error reading from source tar: tar.Reader.Next: %v", err)
+		}
+		ent := TOCEntry{
+			Name:    h.Name,
+			Mode:    h.Mode,
+			Uid:     h.Uid,
+			Gid:     h.Gid,
+			Uname:   h.Uname,
+			Gname:   h.Gname,
+			ModTime: formatModtime(h.ModTime),
+		}
+		switch h.Typeflag {
+		case tar.TypeLink:
+			return fmt.Errorf("TODO: unsupported hardlink %q => %q", h.Name, h.Linkname)
+		case tar.TypeSymlink:
+			ent.Type = "symlink"
+			ent.LinkName = h.Linkname
+		case tar.TypeDir:
+			ent.Type = "dir"
+		case tar.TypeReg:
+			ent.Offset = w.cw.n
+			ent.Type = "reg"
+			ent.Size = h.Size
+
+			// Start a new gzip stream for regular files.
+			if err := w.closeGz(); err != nil {
+				return err
+			}
+		default:
+			return fmt.Errorf("unsupported input tar entry %q", h.Typeflag)
+		}
+		w.toc.Entries = append(w.toc.Entries, ent)
+		if w.gz == nil {
+			w.gz, err = gzip.NewWriterLevel(w.cw, gzip.BestCompression)
+			if err != nil {
+				return err
+			}
+		}
+		tw := tar.NewWriter(w.gz)
+		if err := tw.WriteHeader(h); err != nil {
+			return err
+		}
+		if _, err := io.Copy(tw, tr); err != nil {
+			return err
+		}
+		if err := tw.Flush(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// footerBytes the 47 byte footer.
+func footerBytes(tocOff int64) []byte {
+	buf := bytes.NewBuffer(make([]byte, 0, FooterSize))
+	gz, _ := gzip.NewWriterLevel(buf, gzip.NoCompression)
+	gz.Header.Extra = []byte(fmt.Sprintf("%016xSTARGZ", tocOff))
+	gz.Close()
+	if buf.Len() != FooterSize {
+		panic(fmt.Sprintf("footer buffer = %d, not %d", buf.Len(), FooterSize))
+	}
+	return buf.Bytes()
+}
+
+func parseFooter(p []byte) (tocOffset int64, ok bool) {
+	if len(p) != FooterSize {
+		return 0, false
+	}
+	zr, err := gzip.NewReader(bytes.NewReader(p))
+	if err != nil {
+		return 0, false
+	}
+	extra := zr.Header.Extra
+	if len(extra) != 16+len("STARGZ") {
+		return 0, false
+	}
+	if string(extra[16:]) != "STARGZ" {
+		return 0, false
+	}
+	tocOffset, err = strconv.ParseInt(string(extra[:16]), 16, 64)
+	return tocOffset, err == nil
+}
+
+func formatModtime(t time.Time) string {
+	if t.IsZero() || t.Unix() == 0 {
+		return ""
+	}
+	t = t.UTC()
+	if t.Equal(t.Round(time.Second)) {
+		return t.UTC().Format(time.RFC3339)
+	}
+	return t.UTC().Format(time.RFC3339Nano)
+}
+
+// countWriter counts how many bytes have been written to its wrapped
+// io.Writer.
+type countWriter struct {
+	w io.Writer
+	n int64
+}
+
+func (cw *countWriter) Write(p []byte) (n int, err error) {
+	n, err = cw.w.Write(p)
+	cw.n += int64(n)
+	return
+}
+
+// isGzip reports whether br is positioned right before an upcoming gzip stream.
+// It does not consume any bytes from br.
+func isGzip(br *bufio.Reader) bool {
+	const (
+		gzipID1     = 0x1f
+		gzipID2     = 0x8b
+		gzipDeflate = 8
+	)
+	peek, _ := br.Peek(3)
+	return len(peek) >= 3 && peek[0] == gzipID1 && peek[1] == gzipID2 && peek[2] == gzipDeflate
+}