Use advrefs in gituploadpackinfo (#92)

* add advrefs encoder and parser

* modify advrefs encoder to resemble json encoder

* turn advrefs parser into a decoder

* clean code

* improve documentation

* improve documentation

* clean code

* upgrade to new pktline.Add and add Flush const to easy integration

* gometalinter

* Use packp/advrefs for GitUploadPackInfo parsing

- GitUploadPackInfo now uses packp/advrefs instead of parsing the
  message by itself.

- Capabilities has been moved from clients/common to packp to avoid a
  circular import.

- Cleaning of advrefs_test code.

- Add support for prefix encoding and decoding in advrefs.

* clean advrefs test code

* clean advrefs test code

* clean advrefs test code

* gometalinter

* add pktline encoder

* change pktline.EncodeFlush to pktline.Flush

* make scanner tests use the encoder instead of Pktlines

* check errors on flush and clean constants

* ubstitute the PktLines type with a pktline.Encoder

* use pktline.Encoder in all go-git

* add example of pktline.Encodef()

* add package overview

* documentation

* support symbolic links other than HEAD

* simplify decoding of shallows

* packp: fix mcuadros comments

- all abbreviates removed (by visual inspection, some may remain)
- all empty maps are initialized using make
- simplify readRef with a switch
- make decodeShallow malformed error more verbose
- add pktline.Encoder.encodeLine
- remove infamous panic in checkPayloadLength by refactoring out
  the whole function

14 files changed, 2873 insertions, 387 deletions

diff --git a/formats/packp/advrefs/advrefs.go b/formats/packp/advrefs/advrefs.go
new file mode 100644
index 0000000..ab4bcf5
--- /dev/null
+++ b/formats/packp/advrefs/advrefs.go
@@ -0,0 +1,58 @@
+// Package advrefs implements encoding and decoding advertised-refs
+// messages from a git-upload-pack command.
+package advrefs
+
+import (
+	"gopkg.in/src-d/go-git.v4/core"
+	"gopkg.in/src-d/go-git.v4/formats/packp"
+)
+
+const (
+	hashSize = 40
+	head     = "HEAD"
+	noHead   = "capabilities^{}"
+)
+
+var (
+	sp         = []byte(" ")
+	null       = []byte("\x00")
+	eol        = []byte("\n")
+	peeled     = []byte("^{}")
+	shallow    = []byte("shallow ")
+	noHeadMark = []byte(" capabilities^{}\x00")
+)
+
+// AdvRefs values represent the information transmitted on an
+// advertised-refs message.  Values from this type are not zero-value
+// safe, use the New function instead.
+//
+// When using this messages over (smart) HTTP, you have to add a pktline
+// before the whole thing with the following payload:
+//
+// '# service=$servicename" LF
+//
+// Moreover, some (all) git HTTP smart servers will send a flush-pkt
+// just after the first pkt-line.
+//
+// To accomodate both situations, the Prefix field allow you to store
+// any data you want to send before the actual pktlines.  It will also
+// be filled up with whatever is found on the line.
+type AdvRefs struct {
+	Prefix       [][]byte // payloads of the prefix
+	Head         *core.Hash
+	Capabilities *packp.Capabilities
+	References   map[string]core.Hash
+	Peeled       map[string]core.Hash
+	Shallows     []core.Hash
+}
+
+// New returns a pointer to a new AdvRefs value, ready to be used.
+func New() *AdvRefs {
+	return &AdvRefs{
+		Prefix:       [][]byte{},
+		Capabilities: packp.NewCapabilities(),
+		References:   make(map[string]core.Hash),
+		Peeled:       make(map[string]core.Hash),
+		Shallows:     []core.Hash{},
+	}
+}
diff --git a/formats/packp/advrefs/advrefs_test.go b/formats/packp/advrefs/advrefs_test.go
new file mode 100644
index 0000000..6950ba5
--- /dev/null
+++ b/formats/packp/advrefs/advrefs_test.go
@@ -0,0 +1,315 @@
+package advrefs_test
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"strings"
+	"testing"
+
+	"gopkg.in/src-d/go-git.v4/core"
+	"gopkg.in/src-d/go-git.v4/formats/packp/advrefs"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+
+	. "gopkg.in/check.v1"
+)
+
+func Test(t *testing.T) { TestingT(t) }
+
+type SuiteDecodeEncode struct{}
+
+var _ = Suite(&SuiteDecodeEncode{})
+
+func (s *SuiteDecodeEncode) test(c *C, in []string, exp []string) {
+	var err error
+	var input io.Reader
+	{
+		var buf bytes.Buffer
+		p := pktline.NewEncoder(&buf)
+		err = p.EncodeString(in...)
+		c.Assert(err, IsNil)
+		input = &buf
+	}
+
+	var expected []byte
+	{
+		var buf bytes.Buffer
+		p := pktline.NewEncoder(&buf)
+		err = p.EncodeString(exp...)
+		c.Assert(err, IsNil)
+
+		expected = buf.Bytes()
+	}
+
+	var obtained []byte
+	{
+		ar := advrefs.New()
+		d := advrefs.NewDecoder(input)
+		err = d.Decode(ar)
+		c.Assert(err, IsNil)
+
+		var buf bytes.Buffer
+		e := advrefs.NewEncoder(&buf)
+		err := e.Encode(ar)
+		c.Assert(err, IsNil)
+
+		obtained = buf.Bytes()
+	}
+
+	c.Assert(obtained, DeepEquals, expected,
+		Commentf("input = %v\nobtained = %q\nexpected = %q\n",
+			in, string(obtained), string(expected)))
+}
+
+func (s *SuiteDecodeEncode) TestNoHead(c *C) {
+	input := []string{
+		"0000000000000000000000000000000000000000 capabilities^{}\x00",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"0000000000000000000000000000000000000000 capabilities^{}\x00\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func (s *SuiteDecodeEncode) TestNoHeadSmart(c *C) {
+	input := []string{
+		"# service=git-upload-pack\n",
+		"0000000000000000000000000000000000000000 capabilities^{}\x00",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"# service=git-upload-pack\n",
+		"0000000000000000000000000000000000000000 capabilities^{}\x00\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func (s *SuiteDecodeEncode) TestNoHeadSmartBug(c *C) {
+	input := []string{
+		"# service=git-upload-pack\n",
+		pktline.FlushString,
+		"0000000000000000000000000000000000000000 capabilities^{}\x00\n",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"# service=git-upload-pack\n",
+		pktline.FlushString,
+		"0000000000000000000000000000000000000000 capabilities^{}\x00\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func (s *SuiteDecodeEncode) TestRefs(c *C) {
+	input := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func (s *SuiteDecodeEncode) TestPeeled(c *C) {
+	input := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func (s *SuiteDecodeEncode) TestAll(c *C) {
+	input := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}",
+		"shallow 1111111111111111111111111111111111111111",
+		"shallow 2222222222222222222222222222222222222222\n",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}\n",
+		"shallow 1111111111111111111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func (s *SuiteDecodeEncode) TestAllSmart(c *C) {
+	input := []string{
+		"# service=git-upload-pack\n",
+		pktline.FlushString,
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}\n",
+		"shallow 1111111111111111111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222\n",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"# service=git-upload-pack\n",
+		pktline.FlushString,
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}\n",
+		"shallow 1111111111111111111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func (s *SuiteDecodeEncode) TestAllSmartBug(c *C) {
+	input := []string{
+		"# service=git-upload-pack\n",
+		pktline.FlushString,
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:/refs/heads/master ofs-delta multi_ack\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}\n",
+		"shallow 1111111111111111111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222\n",
+		pktline.FlushString,
+	}
+
+	expected := []string{
+		"# service=git-upload-pack\n",
+		pktline.FlushString,
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"7777777777777777777777777777777777777777 refs/tags/v2.6.12-tree\n",
+		"8888888888888888888888888888888888888888 refs/tags/v2.6.12-tree^{}\n",
+		"shallow 1111111111111111111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222\n",
+		pktline.FlushString,
+	}
+
+	s.test(c, input, expected)
+}
+
+func ExampleDecoder_Decode() {
+	// Here is a raw advertised-ref message.
+	raw := "" +
+		"0065a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n" +
+		"003fa6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n" +
+		"00441111111111111111111111111111111111111111 refs/tags/v2.6.11-tree\n" +
+		"00475555555555555555555555555555555555555555 refs/tags/v2.6.11-tree^{}\n" +
+		"0035shallow 5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c\n" +
+		"0000"
+
+	// Use the raw message as our input.
+	input := strings.NewReader(raw)
+
+	// Create a advref.Decoder reading from our input.
+	d := advrefs.NewDecoder(input)
+
+	// Decode the input into a newly allocated AdvRefs value.
+	ar := advrefs.New()
+	_ = d.Decode(ar) // error check ignored for brevity
+
+	// Do something interesting with the AdvRefs, e.g. print its contents.
+	fmt.Println("head =", ar.Head)
+	fmt.Println("capabilities =", ar.Capabilities.String())
+	fmt.Println("...")
+	fmt.Println("shallows =", ar.Shallows)
+	// Output: head = a6930aaee06755d1bdcfd943fbf614e4d92bb0c7
+	// capabilities = multi_ack ofs-delta symref=HEAD:/refs/heads/master
+	// ...
+	// shallows = [5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c]
+}
+
+func ExampleEncoder_Encode() {
+	// Create an AdvRefs with the contents you want...
+	ar := advrefs.New()
+
+	// ...add a hash for the HEAD...
+	head := core.NewHash("1111111111111111111111111111111111111111")
+	ar.Head = &head
+
+	// ...add some server capabilities...
+	ar.Capabilities.Add("symref", "HEAD:/refs/heads/master")
+	ar.Capabilities.Add("ofs-delta")
+	ar.Capabilities.Add("multi_ack")
+
+	// ...add a couple of references...
+	ar.References["refs/heads/master"] = core.NewHash("2222222222222222222222222222222222222222")
+	ar.References["refs/tags/v1"] = core.NewHash("3333333333333333333333333333333333333333")
+
+	// ...including a peeled ref...
+	ar.Peeled["refs/tags/v1"] = core.NewHash("4444444444444444444444444444444444444444")
+
+	// ...and finally add a shallow
+	ar.Shallows = append(ar.Shallows, core.NewHash("5555555555555555555555555555555555555555"))
+
+	// Encode the advrefs.Contents to a bytes.Buffer.
+	// You can encode into stdout too, but you will not be able
+	// see the '\x00' after "HEAD".
+	var buf bytes.Buffer
+	e := advrefs.NewEncoder(&buf)
+	_ = e.Encode(ar) // error checks ignored for brevity
+
+	// Print the contents of the buffer as a quoted string.
+	// Printing is as a non-quoted string will be prettier but you
+	// will miss the '\x00' after "HEAD".
+	fmt.Printf("%q", buf.String())
+	// Output:
+	// "00651111111111111111111111111111111111111111 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n003f2222222222222222222222222222222222222222 refs/heads/master\n003a3333333333333333333333333333333333333333 refs/tags/v1\n003d4444444444444444444444444444444444444444 refs/tags/v1^{}\n0035shallow 5555555555555555555555555555555555555555\n0000"
+}
diff --git a/formats/packp/advrefs/decoder.go b/formats/packp/advrefs/decoder.go
new file mode 100644
index 0000000..a0cf5e6
--- /dev/null
+++ b/formats/packp/advrefs/decoder.go
@@ -0,0 +1,288 @@
+package advrefs
+
+import (
+	"bytes"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+
+	"gopkg.in/src-d/go-git.v4/core"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+)
+
+// A Decoder reads and decodes AdvRef values from an input stream.
+type Decoder struct {
+	s     *pktline.Scanner // a pkt-line scanner from the input stream
+	line  []byte           // current pkt-line contents, use parser.nextLine() to make it advance
+	nLine int              // current pkt-line number for debugging, begins at 1
+	hash  core.Hash        // last hash read
+	err   error            // sticky error, use the parser.error() method to fill this out
+	data  *AdvRefs         // parsed data is stored here
+}
+
+// ErrEmpty is returned by Decode when there was no advertised-message at all
+var ErrEmpty = errors.New("empty advertised-ref message")
+
+// NewDecoder returns a new decoder that reads from r.
+//
+// Will not read more data from r than necessary.
+func NewDecoder(r io.Reader) *Decoder {
+	return &Decoder{
+		s: pktline.NewScanner(r),
+	}
+}
+
+// Decode reads the next advertised-refs message form its input and
+// stores it in the value pointed to by v.
+func (d *Decoder) Decode(v *AdvRefs) error {
+	d.data = v
+
+	for state := decodePrefix; state != nil; {
+		state = state(d)
+	}
+
+	return d.err
+}
+
+type decoderStateFn func(*Decoder) decoderStateFn
+
+// fills out the parser stiky error
+func (d *Decoder) error(format string, a ...interface{}) {
+	d.err = fmt.Errorf("pkt-line %d: %s", d.nLine,
+		fmt.Sprintf(format, a...))
+}
+
+// Reads a new pkt-line from the scanner, makes its payload available as
+// p.line and increments p.nLine.  A successful invocation returns true,
+// otherwise, false is returned and the sticky error is filled out
+// accordingly.  Trims eols at the end of the payloads.
+func (d *Decoder) nextLine() bool {
+	d.nLine++
+
+	if !d.s.Scan() {
+		if d.err = d.s.Err(); d.err != nil {
+			return false
+		}
+
+		if d.nLine == 1 {
+			d.err = ErrEmpty
+			return false
+		}
+
+		d.error("EOF")
+		return false
+	}
+
+	d.line = d.s.Bytes()
+	d.line = bytes.TrimSuffix(d.line, eol)
+
+	return true
+}
+
+// The HTTP smart prefix is often followed by a flush-pkt.
+func decodePrefix(d *Decoder) decoderStateFn {
+	if ok := d.nextLine(); !ok {
+		return nil
+	}
+
+	if isPrefix(d.line) {
+		tmp := make([]byte, len(d.line))
+		copy(tmp, d.line)
+		d.data.Prefix = append(d.data.Prefix, tmp)
+		if ok := d.nextLine(); !ok {
+			return nil
+		}
+	}
+
+	if isFlush(d.line) {
+		d.data.Prefix = append(d.data.Prefix, pktline.Flush)
+		if ok := d.nextLine(); !ok {
+			return nil
+		}
+	}
+
+	return decodeFirstHash
+}
+
+func isPrefix(payload []byte) bool {
+	return payload[0] == '#'
+}
+
+func isFlush(payload []byte) bool {
+	return len(payload) == 0
+}
+
+// If the first hash is zero, then a no-refs is comming. Otherwise, a
+// list-of-refs is comming, and the hash will be followed by the first
+// advertised ref.
+func decodeFirstHash(p *Decoder) decoderStateFn {
+	if len(p.line) < hashSize {
+		p.error("cannot read hash, pkt-line too short")
+		return nil
+	}
+
+	if _, err := hex.Decode(p.hash[:], p.line[:hashSize]); err != nil {
+		p.error("invalid hash text: %s", err)
+		return nil
+	}
+
+	p.line = p.line[hashSize:]
+
+	if p.hash.IsZero() {
+		return decodeSkipNoRefs
+	}
+
+	return decodeFirstRef
+}
+
+// Skips SP "capabilities^{}" NUL
+func decodeSkipNoRefs(p *Decoder) decoderStateFn {
+	if len(p.line) < len(noHeadMark) {
+		p.error("too short zero-id ref")
+		return nil
+	}
+
+	if !bytes.HasPrefix(p.line, noHeadMark) {
+		p.error("malformed zero-id ref")
+		return nil
+	}
+
+	p.line = p.line[len(noHeadMark):]
+
+	return decodeCaps
+}
+
+// decode the refname, expectes SP refname NULL
+func decodeFirstRef(l *Decoder) decoderStateFn {
+	if len(l.line) < 3 {
+		l.error("line too short after hash")
+		return nil
+	}
+
+	if !bytes.HasPrefix(l.line, sp) {
+		l.error("no space after hash")
+		return nil
+	}
+	l.line = l.line[1:]
+
+	chunks := bytes.SplitN(l.line, null, 2)
+	if len(chunks) < 2 {
+		l.error("NULL not found")
+		return nil
+	}
+	ref := chunks[0]
+	l.line = chunks[1]
+
+	if bytes.Equal(ref, []byte(head)) {
+		l.data.Head = &l.hash
+	} else {
+		l.data.References[string(ref)] = l.hash
+	}
+
+	return decodeCaps
+}
+
+func decodeCaps(p *Decoder) decoderStateFn {
+	if len(p.line) == 0 {
+		return decodeOtherRefs
+	}
+
+	for _, c := range bytes.Split(p.line, sp) {
+		name, values := readCapability(c)
+		p.data.Capabilities.Add(name, values...)
+	}
+
+	return decodeOtherRefs
+}
+
+// Capabilities are a single string or a name=value.
+// Even though we are only going to read at moust 1 value, we return
+// a slice of values, as Capability.Add receives that.
+func readCapability(data []byte) (name string, values []string) {
+	pair := bytes.SplitN(data, []byte{'='}, 2)
+	if len(pair) == 2 {
+		values = append(values, string(pair[1]))
+	}
+
+	return string(pair[0]), values
+}
+
+// The refs are either tips (obj-id SP refname) or a peeled (obj-id SP refname^{}).
+// If there are no refs, then there might be a shallow or flush-ptk.
+func decodeOtherRefs(p *Decoder) decoderStateFn {
+	if ok := p.nextLine(); !ok {
+		return nil
+	}
+
+	if bytes.HasPrefix(p.line, shallow) {
+		return decodeShallow
+	}
+
+	if len(p.line) == 0 {
+		return nil
+	}
+
+	saveTo := p.data.References
+	if bytes.HasSuffix(p.line, peeled) {
+		p.line = bytes.TrimSuffix(p.line, peeled)
+		saveTo = p.data.Peeled
+	}
+
+	ref, hash, err := readRef(p.line)
+	if err != nil {
+		p.error("%s", err)
+		return nil
+	}
+	saveTo[ref] = hash
+
+	return decodeOtherRefs
+}
+
+// Reads a ref-name
+func readRef(data []byte) (string, core.Hash, error) {
+	chunks := bytes.Split(data, sp)
+	switch {
+	case len(chunks) == 1:
+		return "", core.ZeroHash, fmt.Errorf("malformed ref data: no space was found")
+	case len(chunks) > 2:
+		return "", core.ZeroHash, fmt.Errorf("malformed ref data: more than one space found")
+	default:
+		return string(chunks[1]), core.NewHash(string(chunks[0])), nil
+	}
+}
+
+// Keeps reading shallows until a flush-pkt is found
+func decodeShallow(p *Decoder) decoderStateFn {
+	if !bytes.HasPrefix(p.line, shallow) {
+		p.error("malformed shallow prefix, found %q... instead", p.line[:len(shallow)])
+		return nil
+	}
+	p.line = bytes.TrimPrefix(p.line, shallow)
+
+	if len(p.line) != hashSize {
+		p.error(fmt.Sprintf(
+			"malformed shallow hash: wrong length, expected 40 bytes, read %d bytes",
+			len(p.line)))
+		return nil
+	}
+
+	text := p.line[:hashSize]
+	var h core.Hash
+	if _, err := hex.Decode(h[:], text); err != nil {
+		p.error("invalid hash text: %s", err)
+		return nil
+	}
+
+	p.data.Shallows = append(p.data.Shallows, h)
+
+	if ok := p.nextLine(); !ok {
+		return nil
+	}
+
+	if len(p.line) == 0 {
+		return nil // succesfull parse of the advertised-refs message
+	}
+
+	return decodeShallow
+}
diff --git a/formats/packp/advrefs/decoder_test.go b/formats/packp/advrefs/decoder_test.go
new file mode 100644
index 0000000..ee2f5ae
--- /dev/null
+++ b/formats/packp/advrefs/decoder_test.go
@@ -0,0 +1,500 @@
+package advrefs_test
+
+import (
+	"bytes"
+	"io"
+	"strings"
+
+	"gopkg.in/src-d/go-git.v4/core"
+	"gopkg.in/src-d/go-git.v4/formats/packp"
+	"gopkg.in/src-d/go-git.v4/formats/packp/advrefs"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+
+	. "gopkg.in/check.v1"
+)
+
+type SuiteDecoder struct{}
+
+var _ = Suite(&SuiteDecoder{})
+
+func (s *SuiteDecoder) TestEmpty(c *C) {
+	ar := advrefs.New()
+	var buf bytes.Buffer
+	d := advrefs.NewDecoder(&buf)
+
+	err := d.Decode(ar)
+	c.Assert(err, Equals, advrefs.ErrEmpty)
+}
+
+func (s *SuiteDecoder) TestShortForHash(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*too short")
+}
+
+func toPktLines(c *C, payloads []string) io.Reader {
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+	err := e.EncodeString(payloads...)
+	c.Assert(err, IsNil)
+
+	return &buf
+}
+
+func testDecoderErrorMatches(c *C, input io.Reader, pattern string) {
+	ar := advrefs.New()
+	d := advrefs.NewDecoder(input)
+
+	err := d.Decode(ar)
+	c.Assert(err, ErrorMatches, pattern)
+}
+
+func (s *SuiteDecoder) TestInvalidFirstHash(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796alberto2219af86ec6584e5 HEAD\x00multi_ack thin-pack\n",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*invalid hash.*")
+}
+
+func (s *SuiteDecoder) TestZeroId(c *C) {
+	payloads := []string{
+		"0000000000000000000000000000000000000000 capabilities^{}\x00multi_ack thin-pack\n",
+		pktline.FlushString,
+	}
+	ar := testDecodeOK(c, payloads)
+	c.Assert(ar.Head, IsNil)
+}
+
+func testDecodeOK(c *C, payloads []string) *advrefs.AdvRefs {
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+	err := e.EncodeString(payloads...)
+	c.Assert(err, IsNil)
+
+	ar := advrefs.New()
+	d := advrefs.NewDecoder(&buf)
+
+	err = d.Decode(ar)
+	c.Assert(err, IsNil)
+
+	return ar
+}
+
+func (s *SuiteDecoder) TestMalformedZeroId(c *C) {
+	payloads := []string{
+		"0000000000000000000000000000000000000000 wrong\x00multi_ack thin-pack\n",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*malformed zero-id.*")
+}
+
+func (s *SuiteDecoder) TestShortZeroId(c *C) {
+	payloads := []string{
+		"0000000000000000000000000000000000000000 capabi",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*too short zero-id.*")
+}
+
+func (s *SuiteDecoder) TestHead(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00",
+		pktline.FlushString,
+	}
+	ar := testDecodeOK(c, payloads)
+	c.Assert(*ar.Head, Equals,
+		core.NewHash("6ecf0ef2c2dffb796033e5a02219af86ec6584e5"))
+}
+
+func (s *SuiteDecoder) TestFirstIsNotHead(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 refs/heads/master\x00",
+		pktline.FlushString,
+	}
+	ar := testDecodeOK(c, payloads)
+	c.Assert(ar.Head, IsNil)
+	c.Assert(ar.References["refs/heads/master"], Equals,
+		core.NewHash("6ecf0ef2c2dffb796033e5a02219af86ec6584e5"))
+}
+
+func (s *SuiteDecoder) TestShortRef(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 H",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*too short.*")
+}
+
+func (s *SuiteDecoder) TestNoNULL(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEADofs-delta multi_ack",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*NULL not found.*")
+}
+
+func (s *SuiteDecoder) TestNoSpaceAfterHash(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5-HEAD\x00",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*no space after hash.*")
+}
+
+func (s *SuiteDecoder) TestNoCaps(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00",
+		pktline.FlushString,
+	}
+	ar := testDecodeOK(c, payloads)
+	c.Assert(ar.Capabilities.IsEmpty(), Equals, true)
+}
+
+func (s *SuiteDecoder) TestCaps(c *C) {
+	for _, test := range [...]struct {
+		input        []string
+		capabilities []packp.Capability
+	}{
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00",
+				pktline.FlushString,
+			},
+			capabilities: []packp.Capability{},
+		},
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00\n",
+				pktline.FlushString,
+			},
+			capabilities: []packp.Capability{},
+		},
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta",
+				pktline.FlushString,
+			},
+			capabilities: []packp.Capability{
+				{
+					Name:   "ofs-delta",
+					Values: []string(nil),
+				},
+			},
+		},
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta multi_ack",
+				pktline.FlushString,
+			},
+			capabilities: []packp.Capability{
+				{Name: "ofs-delta", Values: []string(nil)},
+				{Name: "multi_ack", Values: []string(nil)},
+			},
+		},
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta multi_ack\n",
+				pktline.FlushString,
+			},
+			capabilities: []packp.Capability{
+				{Name: "ofs-delta", Values: []string(nil)},
+				{Name: "multi_ack", Values: []string(nil)},
+			},
+		},
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:refs/heads/master agent=foo=bar\n",
+				pktline.FlushString,
+			},
+			capabilities: []packp.Capability{
+				{Name: "symref", Values: []string{"HEAD:refs/heads/master"}},
+				{Name: "agent", Values: []string{"foo=bar"}},
+			},
+		},
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00symref=HEAD:refs/heads/master agent=foo=bar agent=new-agent\n",
+				pktline.FlushString,
+			},
+			capabilities: []packp.Capability{
+				{Name: "symref", Values: []string{"HEAD:refs/heads/master"}},
+				{Name: "agent", Values: []string{"foo=bar", "new-agent"}},
+			},
+		},
+	} {
+		ar := testDecodeOK(c, test.input)
+		for _, fixCap := range test.capabilities {
+			c.Assert(ar.Capabilities.Supports(fixCap.Name), Equals, true,
+				Commentf("input = %q, capability = %q", test.input, fixCap.Name))
+			c.Assert(ar.Capabilities.Get(fixCap.Name).Values, DeepEquals, fixCap.Values,
+				Commentf("input = %q, capability = %q", test.input, fixCap.Name))
+		}
+	}
+}
+
+func (s *SuiteDecoder) TestWithPrefix(c *C) {
+	payloads := []string{
+		"# this is a prefix\n",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00foo\n",
+		pktline.FlushString,
+	}
+	ar := testDecodeOK(c, payloads)
+	c.Assert(len(ar.Prefix), Equals, 1)
+	c.Assert(ar.Prefix[0], DeepEquals, []byte("# this is a prefix"))
+}
+
+func (s *SuiteDecoder) TestWithPrefixAndFlush(c *C) {
+	payloads := []string{
+		"# this is a prefix\n",
+		pktline.FlushString,
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00foo\n",
+		pktline.FlushString,
+	}
+	ar := testDecodeOK(c, payloads)
+	c.Assert(len(ar.Prefix), Equals, 2)
+	c.Assert(ar.Prefix[0], DeepEquals, []byte("# this is a prefix"))
+	c.Assert(ar.Prefix[1], DeepEquals, []byte(pktline.FlushString))
+}
+
+func (s *SuiteDecoder) TestOtherRefs(c *C) {
+	for _, test := range [...]struct {
+		input      []string
+		references map[string]core.Hash
+		peeled     map[string]core.Hash
+	}{
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				pktline.FlushString,
+			},
+			references: make(map[string]core.Hash),
+			peeled:     make(map[string]core.Hash),
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"1111111111111111111111111111111111111111 ref/foo",
+				pktline.FlushString,
+			},
+			references: map[string]core.Hash{
+				"ref/foo": core.NewHash("1111111111111111111111111111111111111111"),
+			},
+			peeled: make(map[string]core.Hash),
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"1111111111111111111111111111111111111111 ref/foo\n",
+				pktline.FlushString,
+			},
+			references: map[string]core.Hash{
+				"ref/foo": core.NewHash("1111111111111111111111111111111111111111"),
+			},
+			peeled: make(map[string]core.Hash),
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"1111111111111111111111111111111111111111 ref/foo\n",
+				"2222222222222222222222222222222222222222 ref/bar",
+				pktline.FlushString,
+			},
+			references: map[string]core.Hash{
+				"ref/foo": core.NewHash("1111111111111111111111111111111111111111"),
+				"ref/bar": core.NewHash("2222222222222222222222222222222222222222"),
+			},
+			peeled: make(map[string]core.Hash),
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"1111111111111111111111111111111111111111 ref/foo^{}\n",
+				pktline.FlushString,
+			},
+			references: make(map[string]core.Hash),
+			peeled: map[string]core.Hash{
+				"ref/foo": core.NewHash("1111111111111111111111111111111111111111"),
+			},
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"1111111111111111111111111111111111111111 ref/foo\n",
+				"2222222222222222222222222222222222222222 ref/bar^{}",
+				pktline.FlushString,
+			},
+			references: map[string]core.Hash{
+				"ref/foo": core.NewHash("1111111111111111111111111111111111111111"),
+			},
+			peeled: map[string]core.Hash{
+				"ref/bar": core.NewHash("2222222222222222222222222222222222222222"),
+			},
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+				"51b8b4fb32271d39fbdd760397406177b2b0fd36 refs/pull/10/head\n",
+				"02b5a6031ba7a8cbfde5d65ff9e13ecdbc4a92ca refs/pull/100/head\n",
+				"c284c212704c43659bf5913656b8b28e32da1621 refs/pull/100/merge\n",
+				"3d6537dce68c8b7874333a1720958bd8db3ae8ca refs/pull/101/merge\n",
+				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11\n",
+				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11^{}\n",
+				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+				pktline.FlushString,
+			},
+			references: map[string]core.Hash{
+				"refs/heads/master":      core.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
+				"refs/pull/10/head":      core.NewHash("51b8b4fb32271d39fbdd760397406177b2b0fd36"),
+				"refs/pull/100/head":     core.NewHash("02b5a6031ba7a8cbfde5d65ff9e13ecdbc4a92ca"),
+				"refs/pull/100/merge":    core.NewHash("c284c212704c43659bf5913656b8b28e32da1621"),
+				"refs/pull/101/merge":    core.NewHash("3d6537dce68c8b7874333a1720958bd8db3ae8ca"),
+				"refs/tags/v2.6.11":      core.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
+				"refs/tags/v2.6.11-tree": core.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
+			},
+			peeled: map[string]core.Hash{
+				"refs/tags/v2.6.11":      core.NewHash("c39ae07f393806ccf406ef966e9a15afc43cc36a"),
+				"refs/tags/v2.6.11-tree": core.NewHash("c39ae07f393806ccf406ef966e9a15afc43cc36a"),
+			},
+		},
+	} {
+		ar := testDecodeOK(c, test.input)
+		comment := Commentf("input = %v\n", test.input)
+		c.Assert(ar.References, DeepEquals, test.references, comment)
+		c.Assert(ar.Peeled, DeepEquals, test.peeled, comment)
+	}
+}
+
+func (s *SuiteDecoder) TestMalformedOtherRefsNoSpace(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack thin-pack\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8crefs/tags/v2.6.11\n",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*malformed ref data.*")
+}
+
+func (s *SuiteDecoder) TestMalformedOtherRefsMultipleSpaces(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack thin-pack\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags v2.6.11\n",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*malformed ref data.*")
+}
+
+func (s *SuiteDecoder) TestShallow(c *C) {
+	for _, test := range [...]struct {
+		input    []string
+		shallows []core.Hash
+	}{
+		{
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+				pktline.FlushString,
+			},
+			shallows: []core.Hash{},
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+				"shallow 1111111111111111111111111111111111111111\n",
+				pktline.FlushString,
+			},
+			shallows: []core.Hash{core.NewHash("1111111111111111111111111111111111111111")},
+		}, {
+			input: []string{
+				"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+				"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+				"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+				"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+				"shallow 1111111111111111111111111111111111111111\n",
+				"shallow 2222222222222222222222222222222222222222\n",
+				pktline.FlushString,
+			},
+			shallows: []core.Hash{
+				core.NewHash("1111111111111111111111111111111111111111"),
+				core.NewHash("2222222222222222222222222222222222222222"),
+			},
+		},
+	} {
+		ar := testDecodeOK(c, test.input)
+		comment := Commentf("input = %v\n", test.input)
+		c.Assert(ar.Shallows, DeepEquals, test.shallows, comment)
+	}
+}
+
+func (s *SuiteDecoder) TestInvalidShallowHash(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"shallow 11111111alcortes111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222\n",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*invalid hash text.*")
+}
+
+func (s *SuiteDecoder) TestGarbageAfterShallow(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"shallow 1111111111111111111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222\n",
+		"b5be40b90dbaa6bd337f3b77de361bfc0723468b refs/tags/v4.4",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*malformed shallow prefix.*")
+}
+
+func (s *SuiteDecoder) TestMalformedShallowHash(c *C) {
+	payloads := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n",
+		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
+		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
+		"c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n",
+		"shallow 1111111111111111111111111111111111111111\n",
+		"shallow 2222222222222222222222222222222222222222 malformed\n",
+		pktline.FlushString,
+	}
+	r := toPktLines(c, payloads)
+	testDecoderErrorMatches(c, r, ".*malformed shallow hash.*")
+}
+
+func (s *SuiteDecoder) TestEOFRefs(c *C) {
+	input := strings.NewReader("" +
+		"005b6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n" +
+		"003fa6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n" +
+		"00355dc01c595e6c6ec9ccda4f6ffbf614e4d92bb0c7 refs/foo\n",
+	)
+	testDecoderErrorMatches(c, input, ".*invalid pkt-len.*")
+}
+
+func (s *SuiteDecoder) TestEOFShallows(c *C) {
+	input := strings.NewReader("" +
+		"005b6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00ofs-delta symref=HEAD:/refs/heads/master\n" +
+		"003fa6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n" +
+		"00445dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n" +
+		"0047c39ae07f393806ccf406ef966e9a15afc43cc36a refs/tags/v2.6.11-tree^{}\n" +
+		"0035shallow 1111111111111111111111111111111111111111\n" +
+		"0034shallow 222222222222222222222222")
+	testDecoderErrorMatches(c, input, ".*unexpected EOF.*")
+}
diff --git a/formats/packp/advrefs/encoder.go b/formats/packp/advrefs/encoder.go
new file mode 100644
index 0000000..9874884
--- /dev/null
+++ b/formats/packp/advrefs/encoder.go
@@ -0,0 +1,155 @@
+package advrefs
+
+import (
+	"bytes"
+	"io"
+	"sort"
+
+	"gopkg.in/src-d/go-git.v4/core"
+	"gopkg.in/src-d/go-git.v4/formats/packp"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+)
+
+// An Encoder writes AdvRefs values to an output stream.
+type Encoder struct {
+	data *AdvRefs         // data to encode
+	pe   *pktline.Encoder // where to write the encoded data
+	err  error            // sticky error
+}
+
+// NewEncoder returns a new encoder that writes to w.
+func NewEncoder(w io.Writer) *Encoder {
+	return &Encoder{
+		pe: pktline.NewEncoder(w),
+	}
+}
+
+// Encode writes the AdvRefs encoding of v to the stream.
+//
+// All the payloads will end with a newline character.  Capabilities,
+// references and shallows are writen in alphabetical order, except for
+// peeled references that always follow their corresponding references.
+func (e *Encoder) Encode(v *AdvRefs) error {
+	e.data = v
+
+	for state := encodePrefix; state != nil; {
+		state = state(e)
+	}
+
+	return e.err
+}
+
+type encoderStateFn func(*Encoder) encoderStateFn
+
+func encodePrefix(e *Encoder) encoderStateFn {
+	for _, p := range e.data.Prefix {
+		if bytes.Equal(p, pktline.Flush) {
+			if e.err = e.pe.Flush(); e.err != nil {
+				return nil
+			}
+			continue
+		}
+		if e.err = e.pe.Encodef("%s\n", string(p)); e.err != nil {
+			return nil
+		}
+	}
+
+	return encodeFirstLine
+}
+
+// Adds the first pkt-line payload: head hash, head ref and capabilities.
+// Also handle the special case when no HEAD ref is found.
+func encodeFirstLine(e *Encoder) encoderStateFn {
+	head := formatHead(e.data.Head)
+	separator := formatSeparator(e.data.Head)
+	capabilities := formatCaps(e.data.Capabilities)
+
+	if e.err = e.pe.Encodef("%s %s\x00%s\n", head, separator, capabilities); e.err != nil {
+		return nil
+	}
+
+	return encodeRefs
+}
+
+func formatHead(h *core.Hash) string {
+	if h == nil {
+		return core.ZeroHash.String()
+	}
+
+	return h.String()
+}
+
+func formatSeparator(h *core.Hash) string {
+	if h == nil {
+		return noHead
+	}
+
+	return head
+}
+
+func formatCaps(c *packp.Capabilities) string {
+	if c == nil {
+		return ""
+	}
+
+	c.Sort()
+
+	return c.String()
+}
+
+// Adds the (sorted) refs: hash SP refname EOL
+// and their peeled refs if any.
+func encodeRefs(e *Encoder) encoderStateFn {
+	refs := sortRefs(e.data.References)
+	for _, r := range refs {
+		hash, _ := e.data.References[r]
+		if e.err = e.pe.Encodef("%s %s\n", hash.String(), r); e.err != nil {
+			return nil
+		}
+
+		if hash, ok := e.data.Peeled[r]; ok {
+			if e.err = e.pe.Encodef("%s %s^{}\n", hash.String(), r); e.err != nil {
+				return nil
+			}
+		}
+	}
+
+	return encodeShallow
+}
+
+func sortRefs(m map[string]core.Hash) []string {
+	ret := make([]string, 0, len(m))
+	for k := range m {
+		ret = append(ret, k)
+	}
+	sort.Strings(ret)
+
+	return ret
+}
+
+// Adds the (sorted) shallows: "shallow" SP hash EOL
+func encodeShallow(e *Encoder) encoderStateFn {
+	sorted := sortShallows(e.data.Shallows)
+	for _, hash := range sorted {
+		if e.err = e.pe.Encodef("shallow %s\n", hash); e.err != nil {
+			return nil
+		}
+	}
+
+	return encodeFlush
+}
+
+func sortShallows(c []core.Hash) []string {
+	ret := []string{}
+	for _, h := range c {
+		ret = append(ret, h.String())
+	}
+	sort.Strings(ret)
+
+	return ret
+}
+
+func encodeFlush(e *Encoder) encoderStateFn {
+	e.err = e.pe.Flush()
+	return nil
+}
diff --git a/formats/packp/advrefs/encoder_test.go b/formats/packp/advrefs/encoder_test.go
new file mode 100644
index 0000000..8fb475b
--- /dev/null
+++ b/formats/packp/advrefs/encoder_test.go
@@ -0,0 +1,249 @@
+package advrefs_test
+
+import (
+	"bytes"
+	"strings"
+
+	"gopkg.in/src-d/go-git.v4/core"
+	"gopkg.in/src-d/go-git.v4/formats/packp"
+	"gopkg.in/src-d/go-git.v4/formats/packp/advrefs"
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+
+	. "gopkg.in/check.v1"
+)
+
+type SuiteEncoder struct{}
+
+var _ = Suite(&SuiteEncoder{})
+
+// returns a byte slice with the pkt-lines for the given payloads.
+func pktlines(c *C, payloads ...[]byte) []byte {
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+	err := e.Encode(payloads...)
+	c.Assert(err, IsNil, Commentf("building pktlines for %v\n", payloads))
+
+	return buf.Bytes()
+}
+
+func testEncode(c *C, input *advrefs.AdvRefs, expected []byte) {
+	var buf bytes.Buffer
+	e := advrefs.NewEncoder(&buf)
+	err := e.Encode(input)
+	c.Assert(err, IsNil)
+	obtained := buf.Bytes()
+
+	comment := Commentf("\nobtained = %s\nexpected = %s\n", string(obtained), string(expected))
+
+	c.Assert(obtained, DeepEquals, expected, comment)
+}
+
+func (s *SuiteEncoder) TestZeroValue(c *C) {
+	ar := &advrefs.AdvRefs{}
+
+	expected := pktlines(c,
+		[]byte("0000000000000000000000000000000000000000 capabilities^{}\x00\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestHead(c *C) {
+	hash := core.NewHash("6ecf0ef2c2dffb796033e5a02219af86ec6584e5")
+	ar := &advrefs.AdvRefs{
+		Head: &hash,
+	}
+
+	expected := pktlines(c,
+		[]byte("6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestCapsNoHead(c *C) {
+	capabilities := packp.NewCapabilities()
+	capabilities.Add("symref", "HEAD:/refs/heads/master")
+	capabilities.Add("ofs-delta")
+	capabilities.Add("multi_ack")
+	ar := &advrefs.AdvRefs{
+		Capabilities: capabilities,
+	}
+
+	expected := pktlines(c,
+		[]byte("0000000000000000000000000000000000000000 capabilities^{}\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestCapsWithHead(c *C) {
+	hash := core.NewHash("6ecf0ef2c2dffb796033e5a02219af86ec6584e5")
+	capabilities := packp.NewCapabilities()
+	capabilities.Add("symref", "HEAD:/refs/heads/master")
+	capabilities.Add("ofs-delta")
+	capabilities.Add("multi_ack")
+	ar := &advrefs.AdvRefs{
+		Head:         &hash,
+		Capabilities: capabilities,
+	}
+
+	expected := pktlines(c,
+		[]byte("6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestRefs(c *C) {
+	references := map[string]core.Hash{
+		"refs/heads/master":      core.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
+		"refs/tags/v2.6.12-tree": core.NewHash("1111111111111111111111111111111111111111"),
+		"refs/tags/v2.7.13-tree": core.NewHash("3333333333333333333333333333333333333333"),
+		"refs/tags/v2.6.13-tree": core.NewHash("2222222222222222222222222222222222222222"),
+		"refs/tags/v2.6.11-tree": core.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
+	}
+	ar := &advrefs.AdvRefs{
+		References: references,
+	}
+
+	expected := pktlines(c,
+		[]byte("0000000000000000000000000000000000000000 capabilities^{}\x00\n"),
+		[]byte("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n"),
+		[]byte("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n"),
+		[]byte("1111111111111111111111111111111111111111 refs/tags/v2.6.12-tree\n"),
+		[]byte("2222222222222222222222222222222222222222 refs/tags/v2.6.13-tree\n"),
+		[]byte("3333333333333333333333333333333333333333 refs/tags/v2.7.13-tree\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestPeeled(c *C) {
+	references := map[string]core.Hash{
+		"refs/heads/master":      core.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
+		"refs/tags/v2.6.12-tree": core.NewHash("1111111111111111111111111111111111111111"),
+		"refs/tags/v2.7.13-tree": core.NewHash("3333333333333333333333333333333333333333"),
+		"refs/tags/v2.6.13-tree": core.NewHash("2222222222222222222222222222222222222222"),
+		"refs/tags/v2.6.11-tree": core.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
+	}
+	peeled := map[string]core.Hash{
+		"refs/tags/v2.7.13-tree": core.NewHash("4444444444444444444444444444444444444444"),
+		"refs/tags/v2.6.12-tree": core.NewHash("5555555555555555555555555555555555555555"),
+	}
+	ar := &advrefs.AdvRefs{
+		References: references,
+		Peeled:     peeled,
+	}
+
+	expected := pktlines(c,
+		[]byte("0000000000000000000000000000000000000000 capabilities^{}\x00\n"),
+		[]byte("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n"),
+		[]byte("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n"),
+		[]byte("1111111111111111111111111111111111111111 refs/tags/v2.6.12-tree\n"),
+		[]byte("5555555555555555555555555555555555555555 refs/tags/v2.6.12-tree^{}\n"),
+		[]byte("2222222222222222222222222222222222222222 refs/tags/v2.6.13-tree\n"),
+		[]byte("3333333333333333333333333333333333333333 refs/tags/v2.7.13-tree\n"),
+		[]byte("4444444444444444444444444444444444444444 refs/tags/v2.7.13-tree^{}\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestShallow(c *C) {
+	shallows := []core.Hash{
+		core.NewHash("1111111111111111111111111111111111111111"),
+		core.NewHash("4444444444444444444444444444444444444444"),
+		core.NewHash("3333333333333333333333333333333333333333"),
+		core.NewHash("2222222222222222222222222222222222222222"),
+	}
+	ar := &advrefs.AdvRefs{
+		Shallows: shallows,
+	}
+
+	expected := pktlines(c,
+		[]byte("0000000000000000000000000000000000000000 capabilities^{}\x00\n"),
+		[]byte("shallow 1111111111111111111111111111111111111111\n"),
+		[]byte("shallow 2222222222222222222222222222222222222222\n"),
+		[]byte("shallow 3333333333333333333333333333333333333333\n"),
+		[]byte("shallow 4444444444444444444444444444444444444444\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestAll(c *C) {
+	hash := core.NewHash("6ecf0ef2c2dffb796033e5a02219af86ec6584e5")
+
+	capabilities := packp.NewCapabilities()
+	capabilities.Add("symref", "HEAD:/refs/heads/master")
+	capabilities.Add("ofs-delta")
+	capabilities.Add("multi_ack")
+
+	references := map[string]core.Hash{
+		"refs/heads/master":      core.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
+		"refs/tags/v2.6.12-tree": core.NewHash("1111111111111111111111111111111111111111"),
+		"refs/tags/v2.7.13-tree": core.NewHash("3333333333333333333333333333333333333333"),
+		"refs/tags/v2.6.13-tree": core.NewHash("2222222222222222222222222222222222222222"),
+		"refs/tags/v2.6.11-tree": core.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
+	}
+
+	peeled := map[string]core.Hash{
+		"refs/tags/v2.7.13-tree": core.NewHash("4444444444444444444444444444444444444444"),
+		"refs/tags/v2.6.12-tree": core.NewHash("5555555555555555555555555555555555555555"),
+	}
+
+	shallows := []core.Hash{
+		core.NewHash("1111111111111111111111111111111111111111"),
+		core.NewHash("4444444444444444444444444444444444444444"),
+		core.NewHash("3333333333333333333333333333333333333333"),
+		core.NewHash("2222222222222222222222222222222222222222"),
+	}
+
+	ar := &advrefs.AdvRefs{
+		Head:         &hash,
+		Capabilities: capabilities,
+		References:   references,
+		Peeled:       peeled,
+		Shallows:     shallows,
+	}
+
+	expected := pktlines(c,
+		[]byte("6ecf0ef2c2dffb796033e5a02219af86ec6584e5 HEAD\x00multi_ack ofs-delta symref=HEAD:/refs/heads/master\n"),
+		[]byte("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n"),
+		[]byte("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n"),
+		[]byte("1111111111111111111111111111111111111111 refs/tags/v2.6.12-tree\n"),
+		[]byte("5555555555555555555555555555555555555555 refs/tags/v2.6.12-tree^{}\n"),
+		[]byte("2222222222222222222222222222222222222222 refs/tags/v2.6.13-tree\n"),
+		[]byte("3333333333333333333333333333333333333333 refs/tags/v2.7.13-tree\n"),
+		[]byte("4444444444444444444444444444444444444444 refs/tags/v2.7.13-tree^{}\n"),
+		[]byte("shallow 1111111111111111111111111111111111111111\n"),
+		[]byte("shallow 2222222222222222222222222222222222222222\n"),
+		[]byte("shallow 3333333333333333333333333333333333333333\n"),
+		[]byte("shallow 4444444444444444444444444444444444444444\n"),
+		pktline.Flush,
+	)
+
+	testEncode(c, ar, expected)
+}
+
+func (s *SuiteEncoder) TestErrorTooLong(c *C) {
+	references := map[string]core.Hash{
+		strings.Repeat("a", pktline.MaxPayloadSize): core.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
+	}
+	ar := &advrefs.AdvRefs{
+		References: references,
+	}
+
+	var buf bytes.Buffer
+	e := advrefs.NewEncoder(&buf)
+	err := e.Encode(ar)
+	c.Assert(err, ErrorMatches, ".*payload is too long.*")
+}
diff --git a/formats/packp/capabilities.go b/formats/packp/capabilities.go
new file mode 100644
index 0000000..d77c2fa
--- /dev/null
+++ b/formats/packp/capabilities.go
@@ -0,0 +1,136 @@
+package packp
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// Capabilities contains all the server capabilities
+// https://github.com/git/git/blob/master/Documentation/technical/protocol-capabilities.txt
+type Capabilities struct {
+	m map[string]*Capability
+	o []string
+}
+
+// Capability represents a server capability
+type Capability struct {
+	Name   string
+	Values []string
+}
+
+// NewCapabilities returns a new Capabilities struct
+func NewCapabilities() *Capabilities {
+	return &Capabilities{
+		m: make(map[string]*Capability),
+	}
+}
+
+func (c *Capabilities) IsEmpty() bool {
+	return len(c.o) == 0
+}
+
+// Decode decodes a string
+func (c *Capabilities) Decode(raw string) {
+	params := strings.Split(raw, " ")
+	for _, p := range params {
+		s := strings.SplitN(p, "=", 2)
+
+		var value string
+		if len(s) == 2 {
+			value = s[1]
+		}
+
+		c.Add(s[0], value)
+	}
+}
+
+// Get returns the values for a capability
+func (c *Capabilities) Get(capability string) *Capability {
+	return c.m[capability]
+}
+
+// Set sets a capability removing the values
+func (c *Capabilities) Set(capability string, values ...string) {
+	if _, ok := c.m[capability]; ok {
+		delete(c.m, capability)
+	}
+
+	c.Add(capability, values...)
+}
+
+// Add adds a capability, values are optional
+func (c *Capabilities) Add(capability string, values ...string) {
+	if !c.Supports(capability) {
+		c.m[capability] = &Capability{Name: capability}
+		c.o = append(c.o, capability)
+	}
+
+	if len(values) == 0 {
+		return
+	}
+
+	c.m[capability].Values = append(c.m[capability].Values, values...)
+}
+
+// Supports returns true if capability is present
+func (c *Capabilities) Supports(capability string) bool {
+	_, ok := c.m[capability]
+	return ok
+}
+
+// SymbolicReference returns the reference for a given symbolic reference
+func (c *Capabilities) SymbolicReference(sym string) string {
+	if !c.Supports("symref") {
+		return ""
+	}
+
+	for _, symref := range c.Get("symref").Values {
+		parts := strings.Split(symref, ":")
+		if len(parts) != 2 {
+			continue
+		}
+
+		if parts[0] == sym {
+			return parts[1]
+		}
+	}
+
+	return ""
+}
+
+// Sorts capabilities in increasing order of their name
+func (c *Capabilities) Sort() {
+	sort.Strings(c.o)
+}
+
+func (c *Capabilities) String() string {
+	if len(c.o) == 0 {
+		return ""
+	}
+
+	var o string
+	for _, key := range c.o {
+		cap := c.m[key]
+
+		added := false
+		for _, value := range cap.Values {
+			if value == "" {
+				continue
+			}
+
+			added = true
+			o += fmt.Sprintf("%s=%s ", key, value)
+		}
+
+		if len(cap.Values) == 0 || !added {
+			o += key + " "
+		}
+	}
+
+	if len(o) == 0 {
+		return o
+	}
+
+	return o[:len(o)-1]
+}
diff --git a/formats/packp/capabilities_test.go b/formats/packp/capabilities_test.go
new file mode 100644
index 0000000..e42a0c7
--- /dev/null
+++ b/formats/packp/capabilities_test.go
@@ -0,0 +1,46 @@
+package packp
+
+import (
+	"testing"
+
+	. "gopkg.in/check.v1"
+)
+
+func Test(t *testing.T) { TestingT(t) }
+
+type SuiteCapabilities struct{}
+
+var _ = Suite(&SuiteCapabilities{})
+
+func (s *SuiteCapabilities) TestDecode(c *C) {
+	cap := NewCapabilities()
+	cap.Decode("symref=foo symref=qux thin-pack")
+
+	c.Assert(cap.m, HasLen, 2)
+	c.Assert(cap.Get("symref").Values, DeepEquals, []string{"foo", "qux"})
+	c.Assert(cap.Get("thin-pack").Values, DeepEquals, []string{""})
+}
+
+func (s *SuiteCapabilities) TestSet(c *C) {
+	cap := NewCapabilities()
+	cap.Add("symref", "foo", "qux")
+	cap.Set("symref", "bar")
+
+	c.Assert(cap.m, HasLen, 1)
+	c.Assert(cap.Get("symref").Values, DeepEquals, []string{"bar"})
+}
+
+func (s *SuiteCapabilities) TestSetEmpty(c *C) {
+	cap := NewCapabilities()
+	cap.Set("foo", "bar")
+
+	c.Assert(cap.Get("foo").Values, HasLen, 1)
+}
+
+func (s *SuiteCapabilities) TestAdd(c *C) {
+	cap := NewCapabilities()
+	cap.Add("symref", "foo", "qux")
+	cap.Add("thin-pack")
+
+	c.Assert(cap.String(), Equals, "symref=foo symref=qux thin-pack")
+}
diff --git a/formats/packp/doc.go b/formats/packp/doc.go
new file mode 100644
index 0000000..4950d1d
--- /dev/null
+++ b/formats/packp/doc.go
@@ -0,0 +1,724 @@
+package packp
+
+/*
+
+A nice way to trace the real data transmitted and received by git, use:
+
+GIT_TRACE_PACKET=true git ls-remote http://github.com/src-d/go-git
+GIT_TRACE_PACKET=true git clone http://github.com/src-d/go-git
+
+Here follows a copy of the current protocol specification at the time of
+this writing.
+
+(Please notice that most http git servers will add a flush-pkt after the
+first pkt-line when using HTTP smart.)
+
+
+Documentation Common to Pack and Http Protocols
+===============================================
+
+ABNF Notation
+-------------
+
+ABNF notation as described by RFC 5234 is used within the protocol documents,
+except the following replacement core rules are used:
+----
+  HEXDIG    =  DIGIT / "a" / "b" / "c" / "d" / "e" / "f"
+----
+
+We also define the following common rules:
+----
+  NUL       =  %x00
+  zero-id   =  40*"0"
+  obj-id    =  40*(HEXDIGIT)
+
+  refname  =  "HEAD"
+  refname /=  "refs/" <see discussion below>
+----
+
+A refname is a hierarchical octet string beginning with "refs/" and
+not violating the 'git-check-ref-format' command's validation rules.
+More specifically, they:
+
+. They can include slash `/` for hierarchical (directory)
+  grouping, but no slash-separated component can begin with a
+  dot `.`.
+
+. They must contain at least one `/`. This enforces the presence of a
+  category like `heads/`, `tags/` etc. but the actual names are not
+  restricted.
+
+. They cannot have two consecutive dots `..` anywhere.
+
+. They cannot have ASCII control characters (i.e. bytes whose
+  values are lower than \040, or \177 `DEL`), space, tilde `~`,
+  caret `^`, colon `:`, question-mark `?`, asterisk `*`,
+  or open bracket `[` anywhere.
+
+. They cannot end with a slash `/` or a dot `.`.
+
+. They cannot end with the sequence `.lock`.
+
+. They cannot contain a sequence `@{`.
+
+. They cannot contain a `\\`.
+
+
+pkt-line Format
+---------------
+
+Much (but not all) of the payload is described around pkt-lines.
+
+A pkt-line is a variable length binary string.  The first four bytes
+of the line, the pkt-len, indicates the total length of the line,
+in hexadecimal.  The pkt-len includes the 4 bytes used to contain
+the length's hexadecimal representation.
+
+A pkt-line MAY contain binary data, so implementors MUST ensure
+pkt-line parsing/formatting routines are 8-bit clean.
+
+A non-binary line SHOULD BE terminated by an LF, which if present
+MUST be included in the total length. Receivers MUST treat pkt-lines
+with non-binary data the same whether or not they contain the trailing
+LF (stripping the LF if present, and not complaining when it is
+missing).
+
+The maximum length of a pkt-line's data component is 65516 bytes.
+Implementations MUST NOT send pkt-line whose length exceeds 65520
+(65516 bytes of payload + 4 bytes of length data).
+
+Implementations SHOULD NOT send an empty pkt-line ("0004").
+
+A pkt-line with a length field of 0 ("0000"), called a flush-pkt,
+is a special case and MUST be handled differently than an empty
+pkt-line ("0004").
+
+----
+  pkt-line     =  data-pkt / flush-pkt
+
+  data-pkt     =  pkt-len pkt-payload
+  pkt-len      =  4*(HEXDIG)
+  pkt-payload  =  (pkt-len - 4)*(OCTET)
+
+  flush-pkt    = "0000"
+----
+
+Examples (as C-style strings):
+
+----
+  pkt-line          actual value
+  ---------------------------------
+  "0006a\n"         "a\n"
+  "0005a"           "a"
+  "000bfoobar\n"    "foobar\n"
+  "0004"            ""
+----
+
+Packfile transfer protocols
+===========================
+
+Git supports transferring data in packfiles over the ssh://, git://, http:// and
+file:// transports.  There exist two sets of protocols, one for pushing
+data from a client to a server and another for fetching data from a
+server to a client.  The three transports (ssh, git, file) use the same
+protocol to transfer data. http is documented in http-protocol.txt.
+
+The processes invoked in the canonical Git implementation are 'upload-pack'
+on the server side and 'fetch-pack' on the client side for fetching data;
+then 'receive-pack' on the server and 'send-pack' on the client for pushing
+data.  The protocol functions to have a server tell a client what is
+currently on the server, then for the two to negotiate the smallest amount
+of data to send in order to fully update one or the other.
+
+pkt-line Format
+---------------
+
+The descriptions below build on the pkt-line format described in
+protocol-common.txt. When the grammar indicate `PKT-LINE(...)`, unless
+otherwise noted the usual pkt-line LF rules apply: the sender SHOULD
+include a LF, but the receiver MUST NOT complain if it is not present.
+
+Transports
+----------
+There are three transports over which the packfile protocol is
+initiated.  The Git transport is a simple, unauthenticated server that
+takes the command (almost always 'upload-pack', though Git
+servers can be configured to be globally writable, in which 'receive-
+pack' initiation is also allowed) with which the client wishes to
+communicate and executes it and connects it to the requesting
+process.
+
+In the SSH transport, the client just runs the 'upload-pack'
+or 'receive-pack' process on the server over the SSH protocol and then
+communicates with that invoked process over the SSH connection.
+
+The file:// transport runs the 'upload-pack' or 'receive-pack'
+process locally and communicates with it over a pipe.
+
+Git Transport
+-------------
+
+The Git transport starts off by sending the command and repository
+on the wire using the pkt-line format, followed by a NUL byte and a
+hostname parameter, terminated by a NUL byte.
+
+   0032git-upload-pack /project.git\0host=myserver.com\0
+
+--
+   git-proto-request = request-command SP pathname NUL [ host-parameter NUL ]
+   request-command   = "git-upload-pack" / "git-receive-pack" /
+		       "git-upload-archive"   ; case sensitive
+   pathname          = *( %x01-ff ) ; exclude NUL
+   host-parameter    = "host=" hostname [ ":" port ]
+--
+
+Only host-parameter is allowed in the git-proto-request. Clients
+MUST NOT attempt to send additional parameters. It is used for the
+git-daemon name based virtual hosting.  See --interpolated-path
+option to git daemon, with the %H/%CH format characters.
+
+Basically what the Git client is doing to connect to an 'upload-pack'
+process on the server side over the Git protocol is this:
+
+   $ echo -e -n \
+     "0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" |
+     nc -v example.com 9418
+
+If the server refuses the request for some reasons, it could abort
+gracefully with an error message.
+
+----
+  error-line     =  PKT-LINE("ERR" SP explanation-text)
+----
+
+
+SSH Transport
+-------------
+
+Initiating the upload-pack or receive-pack processes over SSH is
+executing the binary on the server via SSH remote execution.
+It is basically equivalent to running this:
+
+   $ ssh git.example.com "git-upload-pack '/project.git'"
+
+For a server to support Git pushing and pulling for a given user over
+SSH, that user needs to be able to execute one or both of those
+commands via the SSH shell that they are provided on login.  On some
+systems, that shell access is limited to only being able to run those
+two commands, or even just one of them.
+
+In an ssh:// format URI, it's absolute in the URI, so the '/' after
+the host name (or port number) is sent as an argument, which is then
+read by the remote git-upload-pack exactly as is, so it's effectively
+an absolute path in the remote filesystem.
+
+       git clone ssh://user@example.com/project.git
+		    |
+		    v
+    ssh user@example.com "git-upload-pack '/project.git'"
+
+In a "user@host:path" format URI, its relative to the user's home
+directory, because the Git client will run:
+
+     git clone user@example.com:project.git
+		    |
+		    v
+  ssh user@example.com "git-upload-pack 'project.git'"
+
+The exception is if a '~' is used, in which case
+we execute it without the leading '/'.
+
+      ssh://user@example.com/~alice/project.git,
+		     |
+		     v
+   ssh user@example.com "git-upload-pack '~alice/project.git'"
+
+A few things to remember here:
+
+- The "command name" is spelled with dash (e.g. git-upload-pack), but
+  this can be overridden by the client;
+
+- The repository path is always quoted with single quotes.
+
+Fetching Data From a Server
+---------------------------
+
+When one Git repository wants to get data that a second repository
+has, the first can 'fetch' from the second.  This operation determines
+what data the server has that the client does not then streams that
+data down to the client in packfile format.
+
+
+Reference Discovery
+-------------------
+
+When the client initially connects the server will immediately respond
+with a listing of each reference it has (all branches and tags) along
+with the object name that each reference currently points to.
+
+   $ echo -e -n "0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" |
+      nc -v example.com 9418
+   00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack thin-pack
+		side-band side-band-64k ofs-delta shallow no-progress include-tag
+   00441d3fcd5ced445d1abc402225c0b8a1299641f497 refs/heads/integration
+   003f7217a7c7e582c46cec22a130adf4b9d7d950fba0 refs/heads/master
+   003cb88d2441cac0977faf98efc80305012112238d9d refs/tags/v0.9
+   003c525128480b96c89e6418b1e40909bf6c5b2d580f refs/tags/v1.0
+   003fe92df48743b7bc7d26bcaabfddde0a1e20cae47c refs/tags/v1.0^{}
+   0000
+
+The returned response is a pkt-line stream describing each ref and
+its current value.  The stream MUST be sorted by name according to
+the C locale ordering.
+
+If HEAD is a valid ref, HEAD MUST appear as the first advertised
+ref.  If HEAD is not a valid ref, HEAD MUST NOT appear in the
+advertisement list at all, but other refs may still appear.
+
+The stream MUST include capability declarations behind a NUL on the
+first ref. The peeled value of a ref (that is "ref^{}") MUST be
+immediately after the ref itself, if presented. A conforming server
+MUST peel the ref if it's an annotated tag.
+
+----
+  advertised-refs  =  (no-refs / list-of-refs)
+		      *shallow
+		      flush-pkt
+
+  no-refs          =  PKT-LINE(zero-id SP "capabilities^{}"
+		      NUL capability-list)
+
+  list-of-refs     =  first-ref *other-ref
+  first-ref        =  PKT-LINE(obj-id SP refname
+		      NUL capability-list)
+
+  other-ref        =  PKT-LINE(other-tip / other-peeled)
+  other-tip        =  obj-id SP refname
+  other-peeled     =  obj-id SP refname "^{}"
+
+  shallow          =  PKT-LINE("shallow" SP obj-id)
+
+  capability-list  =  capability *(SP capability)
+  capability       =  1*(LC_ALPHA / DIGIT / "-" / "_")
+  LC_ALPHA         =  %x61-7A
+----
+
+Server and client MUST use lowercase for obj-id, both MUST treat obj-id
+as case-insensitive.
+
+See protocol-capabilities.txt for a list of allowed server capabilities
+and descriptions.
+
+Packfile Negotiation
+--------------------
+After reference and capabilities discovery, the client can decide to
+terminate the connection by sending a flush-pkt, telling the server it can
+now gracefully terminate, and disconnect, when it does not need any pack
+data. This can happen with the ls-remote command, and also can happen when
+the client already is up-to-date.
+
+Otherwise, it enters the negotiation phase, where the client and
+server determine what the minimal packfile necessary for transport is,
+by telling the server what objects it wants, its shallow objects
+(if any), and the maximum commit depth it wants (if any).  The client
+will also send a list of the capabilities it wants to be in effect,
+out of what the server said it could do with the first 'want' line.
+
+----
+  upload-request    =  want-list
+		       *shallow-line
+		       *1depth-request
+		       flush-pkt
+
+  want-list         =  first-want
+		       *additional-want
+
+  shallow-line      =  PKT-LINE("shallow" SP obj-id)
+
+  depth-request     =  PKT-LINE("deepen" SP depth) /
+		       PKT-LINE("deepen-since" SP timestamp) /
+		       PKT-LINE("deepen-not" SP ref)
+
+  first-want        =  PKT-LINE("want" SP obj-id SP capability-list)
+  additional-want   =  PKT-LINE("want" SP obj-id)
+
+  depth             =  1*DIGIT
+----
+
+Clients MUST send all the obj-ids it wants from the reference
+discovery phase as 'want' lines. Clients MUST send at least one
+'want' command in the request body. Clients MUST NOT mention an
+obj-id in a 'want' command which did not appear in the response
+obtained through ref discovery.
+
+The client MUST write all obj-ids which it only has shallow copies
+of (meaning that it does not have the parents of a commit) as
+'shallow' lines so that the server is aware of the limitations of
+the client's history.
+
+The client now sends the maximum commit history depth it wants for
+this transaction, which is the number of commits it wants from the
+tip of the history, if any, as a 'deepen' line.  A depth of 0 is the
+same as not making a depth request. The client does not want to receive
+any commits beyond this depth, nor does it want objects needed only to
+complete those commits. Commits whose parents are not received as a
+result are defined as shallow and marked as such in the server. This
+information is sent back to the client in the next step.
+
+Once all the 'want's and 'shallow's (and optional 'deepen') are
+transferred, clients MUST send a flush-pkt, to tell the server side
+that it is done sending the list.
+
+Otherwise, if the client sent a positive depth request, the server
+will determine which commits will and will not be shallow and
+send this information to the client. If the client did not request
+a positive depth, this step is skipped.
+
+----
+  shallow-update   =  *shallow-line
+		      *unshallow-line
+		      flush-pkt
+
+  shallow-line     =  PKT-LINE("shallow" SP obj-id)
+
+  unshallow-line   =  PKT-LINE("unshallow" SP obj-id)
+----
+
+If the client has requested a positive depth, the server will compute
+the set of commits which are no deeper than the desired depth. The set
+of commits start at the client's wants.
+
+The server writes 'shallow' lines for each
+commit whose parents will not be sent as a result. The server writes
+an 'unshallow' line for each commit which the client has indicated is
+shallow, but is no longer shallow at the currently requested depth
+(that is, its parents will now be sent). The server MUST NOT mark
+as unshallow anything which the client has not indicated was shallow.
+
+Now the client will send a list of the obj-ids it has using 'have'
+lines, so the server can make a packfile that only contains the objects
+that the client needs. In multi_ack mode, the canonical implementation
+will send up to 32 of these at a time, then will send a flush-pkt. The
+canonical implementation will skip ahead and send the next 32 immediately,
+so that there is always a block of 32 "in-flight on the wire" at a time.
+
+----
+  upload-haves      =  have-list
+		       compute-end
+
+  have-list         =  *have-line
+  have-line         =  PKT-LINE("have" SP obj-id)
+  compute-end       =  flush-pkt / PKT-LINE("done")
+----
+
+If the server reads 'have' lines, it then will respond by ACKing any
+of the obj-ids the client said it had that the server also has. The
+server will ACK obj-ids differently depending on which ack mode is
+chosen by the client.
+
+In multi_ack mode:
+
+  * the server will respond with 'ACK obj-id continue' for any common
+    commits.
+
+  * once the server has found an acceptable common base commit and is
+    ready to make a packfile, it will blindly ACK all 'have' obj-ids
+    back to the client.
+
+  * the server will then send a 'NAK' and then wait for another response
+    from the client - either a 'done' or another list of 'have' lines.
+
+In multi_ack_detailed mode:
+
+  * the server will differentiate the ACKs where it is signaling
+    that it is ready to send data with 'ACK obj-id ready' lines, and
+    signals the identified common commits with 'ACK obj-id common' lines.
+
+Without either multi_ack or multi_ack_detailed:
+
+ * upload-pack sends "ACK obj-id" on the first common object it finds.
+   After that it says nothing until the client gives it a "done".
+
+ * upload-pack sends "NAK" on a flush-pkt if no common object
+   has been found yet.  If one has been found, and thus an ACK
+   was already sent, it's silent on the flush-pkt.
+
+After the client has gotten enough ACK responses that it can determine
+that the server has enough information to send an efficient packfile
+(in the canonical implementation, this is determined when it has received
+enough ACKs that it can color everything left in the --date-order queue
+as common with the server, or the --date-order queue is empty), or the
+client determines that it wants to give up (in the canonical implementation,
+this is determined when the client sends 256 'have' lines without getting
+any of them ACKed by the server - meaning there is nothing in common and
+the server should just send all of its objects), then the client will send
+a 'done' command.  The 'done' command signals to the server that the client
+is ready to receive its packfile data.
+
+However, the 256 limit *only* turns on in the canonical client
+implementation if we have received at least one "ACK %s continue"
+during a prior round.  This helps to ensure that at least one common
+ancestor is found before we give up entirely.
+
+Once the 'done' line is read from the client, the server will either
+send a final 'ACK obj-id' or it will send a 'NAK'. 'obj-id' is the object
+name of the last commit determined to be common. The server only sends
+ACK after 'done' if there is at least one common base and multi_ack or
+multi_ack_detailed is enabled. The server always sends NAK after 'done'
+if there is no common base found.
+
+Then the server will start sending its packfile data.
+
+----
+  server-response = *ack_multi ack / nak
+  ack_multi       = PKT-LINE("ACK" SP obj-id ack_status)
+  ack_status      = "continue" / "common" / "ready"
+  ack             = PKT-LINE("ACK" SP obj-id)
+  nak             = PKT-LINE("NAK")
+----
+
+A simple clone may look like this (with no 'have' lines):
+
+----
+   C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
+     side-band-64k ofs-delta\n
+   C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
+   C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
+   C: 0032want 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
+   C: 0032want 74730d410fcb6603ace96f1dc55ea6196122532d\n
+   C: 0000
+   C: 0009done\n
+
+   S: 0008NAK\n
+   S: [PACKFILE]
+----
+
+An incremental update (fetch) response might look like this:
+
+----
+   C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
+     side-band-64k ofs-delta\n
+   C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
+   C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
+   C: 0000
+   C: 0032have 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
+   C: [30 more have lines]
+   C: 0032have 74730d410fcb6603ace96f1dc55ea6196122532d\n
+   C: 0000
+
+   S: 003aACK 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01 continue\n
+   S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d continue\n
+   S: 0008NAK\n
+
+   C: 0009done\n
+
+   S: 0031ACK 74730d410fcb6603ace96f1dc55ea6196122532d\n
+   S: [PACKFILE]
+----
+
+
+Packfile Data
+-------------
+
+Now that the client and server have finished negotiation about what
+the minimal amount of data that needs to be sent to the client is, the server
+will construct and send the required data in packfile format.
+
+See pack-format.txt for what the packfile itself actually looks like.
+
+If 'side-band' or 'side-band-64k' capabilities have been specified by
+the client, the server will send the packfile data multiplexed.
+
+Each packet starting with the packet-line length of the amount of data
+that follows, followed by a single byte specifying the sideband the
+following data is coming in on.
+
+In 'side-band' mode, it will send up to 999 data bytes plus 1 control
+code, for a total of up to 1000 bytes in a pkt-line.  In 'side-band-64k'
+mode it will send up to 65519 data bytes plus 1 control code, for a
+total of up to 65520 bytes in a pkt-line.
+
+The sideband byte will be a '1', '2' or a '3'. Sideband '1' will contain
+packfile data, sideband '2' will be used for progress information that the
+client will generally print to stderr and sideband '3' is used for error
+information.
+
+If no 'side-band' capability was specified, the server will stream the
+entire packfile without multiplexing.
+
+
+Pushing Data To a Server
+------------------------
+
+Pushing data to a server will invoke the 'receive-pack' process on the
+server, which will allow the client to tell it which references it should
+update and then send all the data the server will need for those new
+references to be complete.  Once all the data is received and validated,
+the server will then update its references to what the client specified.
+
+Authentication
+--------------
+
+The protocol itself contains no authentication mechanisms.  That is to be
+handled by the transport, such as SSH, before the 'receive-pack' process is
+invoked.  If 'receive-pack' is configured over the Git transport, those
+repositories will be writable by anyone who can access that port (9418) as
+that transport is unauthenticated.
+
+Reference Discovery
+-------------------
+
+The reference discovery phase is done nearly the same way as it is in the
+fetching protocol. Each reference obj-id and name on the server is sent
+in packet-line format to the client, followed by a flush-pkt.  The only
+real difference is that the capability listing is different - the only
+possible values are 'report-status', 'delete-refs', 'ofs-delta' and
+'push-options'.
+
+Reference Update Request and Packfile Transfer
+----------------------------------------------
+
+Once the client knows what references the server is at, it can send a
+list of reference update requests.  For each reference on the server
+that it wants to update, it sends a line listing the obj-id currently on
+the server, the obj-id the client would like to update it to and the name
+of the reference.
+
+This list is followed by a flush-pkt. Then the push options are transmitted
+one per packet followed by another flush-pkt. After that the packfile that
+should contain all the objects that the server will need to complete the new
+references will be sent.
+
+----
+  update-request    =  *shallow ( command-list | push-cert ) [packfile]
+
+  shallow           =  PKT-LINE("shallow" SP obj-id)
+
+  command-list      =  PKT-LINE(command NUL capability-list)
+		       *PKT-LINE(command)
+		       flush-pkt
+
+  command           =  create / delete / update
+  create            =  zero-id SP new-id  SP name
+  delete            =  old-id  SP zero-id SP name
+  update            =  old-id  SP new-id  SP name
+
+  old-id            =  obj-id
+  new-id            =  obj-id
+
+  push-cert         = PKT-LINE("push-cert" NUL capability-list LF)
+		      PKT-LINE("certificate version 0.1" LF)
+		      PKT-LINE("pusher" SP ident LF)
+		      PKT-LINE("pushee" SP url LF)
+		      PKT-LINE("nonce" SP nonce LF)
+		      PKT-LINE(LF)
+		      *PKT-LINE(command LF)
+		      *PKT-LINE(gpg-signature-lines LF)
+		      PKT-LINE("push-cert-end" LF)
+
+  packfile          = "PACK" 28*(OCTET)
+----
+
+If the receiving end does not support delete-refs, the sending end MUST
+NOT ask for delete command.
+
+If the receiving end does not support push-cert, the sending end
+MUST NOT send a push-cert command.  When a push-cert command is
+sent, command-list MUST NOT be sent; the commands recorded in the
+push certificate is used instead.
+
+The packfile MUST NOT be sent if the only command used is 'delete'.
+
+A packfile MUST be sent if either create or update command is used,
+even if the server already has all the necessary objects.  In this
+case the client MUST send an empty packfile.   The only time this
+is likely to happen is if the client is creating
+a new branch or a tag that points to an existing obj-id.
+
+The server will receive the packfile, unpack it, then validate each
+reference that is being updated that it hasn't changed while the request
+was being processed (the obj-id is still the same as the old-id), and
+it will run any update hooks to make sure that the update is acceptable.
+If all of that is fine, the server will then update the references.
+
+Push Certificate
+----------------
+
+A push certificate begins with a set of header lines.  After the
+header and an empty line, the protocol commands follow, one per
+line. Note that the trailing LF in push-cert PKT-LINEs is _not_
+optional; it must be present.
+
+Currently, the following header fields are defined:
+
+`pusher` ident::
+	Identify the GPG key in "Human Readable Name <email@address>"
+	format.
+
+`pushee` url::
+	The repository URL (anonymized, if the URL contains
+	authentication material) the user who ran `git push`
+	intended to push into.
+
+`nonce` nonce::
+	The 'nonce' string the receiving repository asked the
+	pushing user to include in the certificate, to prevent
+	replay attacks.
+
+The GPG signature lines are a detached signature for the contents
+recorded in the push certificate before the signature block begins.
+The detached signature is used to certify that the commands were
+given by the pusher, who must be the signer.
+
+Report Status
+-------------
+
+After receiving the pack data from the sender, the receiver sends a
+report if 'report-status' capability is in effect.
+It is a short listing of what happened in that update.  It will first
+list the status of the packfile unpacking as either 'unpack ok' or
+'unpack [error]'.  Then it will list the status for each of the references
+that it tried to update.  Each line is either 'ok [refname]' if the
+update was successful, or 'ng [refname] [error]' if the update was not.
+
+----
+  report-status     = unpack-status
+		      1*(command-status)
+		      flush-pkt
+
+  unpack-status     = PKT-LINE("unpack" SP unpack-result)
+  unpack-result     = "ok" / error-msg
+
+  command-status    = command-ok / command-fail
+  command-ok        = PKT-LINE("ok" SP refname)
+  command-fail      = PKT-LINE("ng" SP refname SP error-msg)
+
+  error-msg         = 1*(OCTECT) ; where not "ok"
+----
+
+Updates can be unsuccessful for a number of reasons.  The reference can have
+changed since the reference discovery phase was originally sent, meaning
+someone pushed in the meantime.  The reference being pushed could be a
+non-fast-forward reference and the update hooks or configuration could be
+set to not allow that, etc.  Also, some references can be updated while others
+can be rejected.
+
+An example client/server communication might look like this:
+
+----
+   S: 007c74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n
+   S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n
+   S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n
+   S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n
+   S: 0000
+
+   C: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n
+   C: 003e74730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n
+   C: 0000
+   C: [PACKDATA]
+
+   S: 000eunpack ok\n
+   S: 0018ok refs/heads/debug\n
+   S: 002ang refs/heads/master non-fast-forward\n
+----
+*/
diff --git a/formats/packp/pktline/encoder.go b/formats/packp/pktline/encoder.go
new file mode 100644
index 0000000..0a88a9b
--- /dev/null
+++ b/formats/packp/pktline/encoder.go
@@ -0,0 +1,123 @@
+// Package pktline implements reading payloads form pkt-lines and encoding pkt-lines from payloads.
+package pktline
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// An Encoder writes pkt-lines to an output stream.
+type Encoder struct {
+	w io.Writer
+}
+
+const (
+	// MaxPayloadSize is the maximum payload size of a pkt-line in bytes.
+	MaxPayloadSize = 65516
+)
+
+var (
+	// FlushPkt are the contents of a flush-pkt pkt-line.
+	FlushPkt = []byte{'0', '0', '0', '0'}
+	// Flush is the payload to use with the Encode method to encode a flush-pkt.
+	Flush = []byte{}
+	// FlushString is the payload to use with the EncodeString method to encode a flush-pkt.
+	FlushString = ""
+	// ErrPayloadTooLong is returned by the Encode methods when any of the
+	// provided payloads is bigger than MaxPayloadSize.
+	ErrPayloadTooLong = errors.New("payload is too long")
+)
+
+// NewEncoder returns a new encoder that writes to w.
+func NewEncoder(w io.Writer) *Encoder {
+	return &Encoder{
+		w: w,
+	}
+}
+
+// Flush encodes a flush-pkt to the output stream.
+func (e *Encoder) Flush() error {
+	_, err := e.w.Write(FlushPkt)
+	return err
+}
+
+// Encode encodes a pkt-line with the payload specified and write it to
+// the output stream.  If several payloads are specified, each of them
+// will get streamed in their own pkt-lines.
+func (e *Encoder) Encode(payloads ...[]byte) error {
+	for _, p := range payloads {
+		if err := e.encodeLine(p); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (e *Encoder) encodeLine(p []byte) error {
+	if len(p) > MaxPayloadSize {
+		return ErrPayloadTooLong
+	}
+
+	if bytes.Equal(p, Flush) {
+		if err := e.Flush(); err != nil {
+			return err
+		}
+		return nil
+	}
+
+	n := len(p) + 4
+	if _, err := e.w.Write(asciiHex16(n)); err != nil {
+		return err
+	}
+	if _, err := e.w.Write(p); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Returns the hexadecimal ascii representation of the 16 less
+// significant bits of n.  The length of the returned slice will always
+// be 4.  Example: if n is 1234 (0x4d2), the return value will be
+// []byte{'0', '4', 'd', '2'}.
+func asciiHex16(n int) []byte {
+	var ret [4]byte
+	ret[0] = byteToASCIIHex(byte(n & 0xf000 >> 12))
+	ret[1] = byteToASCIIHex(byte(n & 0x0f00 >> 8))
+	ret[2] = byteToASCIIHex(byte(n & 0x00f0 >> 4))
+	ret[3] = byteToASCIIHex(byte(n & 0x000f))
+
+	return ret[:]
+}
+
+// turns a byte into its hexadecimal ascii representation.  Example:
+// from 11 (0xb) to 'b'.
+func byteToASCIIHex(n byte) byte {
+	if n < 10 {
+		return '0' + n
+	}
+
+	return 'a' - 10 + n
+}
+
+// EncodeString works similarly as Encode but payloads are specified as strings.
+func (e *Encoder) EncodeString(payloads ...string) error {
+	for _, p := range payloads {
+		if err := e.Encode([]byte(p)); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Encodef encodes a single pkt-line with the payload formatted as
+// the format specifier and the rest of the arguments suggest.
+func (e *Encoder) Encodef(format string, a ...interface{}) error {
+	return e.EncodeString(
+		fmt.Sprintf(format, a...),
+	)
+}
diff --git a/formats/packp/pktline/encoder_test.go b/formats/packp/pktline/encoder_test.go
new file mode 100644
index 0000000..5990ea5
--- /dev/null
+++ b/formats/packp/pktline/encoder_test.go
@@ -0,0 +1,249 @@
+package pktline_test
+
+import (
+	"bytes"
+	"os"
+	"strings"
+	"testing"
+
+	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
+
+	. "gopkg.in/check.v1"
+)
+
+func Test(t *testing.T) { TestingT(t) }
+
+type SuiteEncoder struct{}
+
+var _ = Suite(&SuiteEncoder{})
+
+func (s *SuiteEncoder) TestFlush(c *C) {
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+
+	err := e.Flush()
+	c.Assert(err, IsNil)
+
+	obtained := buf.Bytes()
+	c.Assert(obtained, DeepEquals, pktline.FlushPkt)
+}
+
+func (s *SuiteEncoder) TestEncode(c *C) {
+	for i, test := range [...]struct {
+		input    [][]byte
+		expected []byte
+	}{
+		{
+			input: [][]byte{
+				[]byte("hello\n"),
+			},
+			expected: []byte("000ahello\n"),
+		}, {
+			input: [][]byte{
+				[]byte("hello\n"),
+				pktline.Flush,
+			},
+			expected: []byte("000ahello\n0000"),
+		}, {
+			input: [][]byte{
+				[]byte("hello\n"),
+				[]byte("world!\n"),
+				[]byte("foo"),
+			},
+			expected: []byte("000ahello\n000bworld!\n0007foo"),
+		}, {
+			input: [][]byte{
+				[]byte("hello\n"),
+				pktline.Flush,
+				[]byte("world!\n"),
+				[]byte("foo"),
+				pktline.Flush,
+			},
+			expected: []byte("000ahello\n0000000bworld!\n0007foo0000"),
+		}, {
+			input: [][]byte{
+				[]byte(strings.Repeat("a", pktline.MaxPayloadSize)),
+			},
+			expected: []byte(
+				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize)),
+		}, {
+			input: [][]byte{
+				[]byte(strings.Repeat("a", pktline.MaxPayloadSize)),
+				[]byte(strings.Repeat("b", pktline.MaxPayloadSize)),
+			},
+			expected: []byte(
+				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize) +
+					"fff0" + strings.Repeat("b", pktline.MaxPayloadSize)),
+		},
+	} {
+		comment := Commentf("input %d = %v\n", i, test.input)
+
+		var buf bytes.Buffer
+		e := pktline.NewEncoder(&buf)
+
+		err := e.Encode(test.input...)
+		c.Assert(err, IsNil, comment)
+
+		c.Assert(buf.Bytes(), DeepEquals, test.expected, comment)
+	}
+}
+
+func (s *SuiteEncoder) TestEncodeErrPayloadTooLong(c *C) {
+	for i, input := range [...][][]byte{
+		[][]byte{
+			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
+		},
+		[][]byte{
+			[]byte("hello world!"),
+			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
+		},
+		[][]byte{
+			[]byte("hello world!"),
+			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
+			[]byte("foo"),
+		},
+	} {
+		comment := Commentf("input %d = %v\n", i, input)
+
+		var buf bytes.Buffer
+		e := pktline.NewEncoder(&buf)
+
+		err := e.Encode(input...)
+		c.Assert(err, Equals, pktline.ErrPayloadTooLong, comment)
+	}
+}
+
+func (s *SuiteEncoder) TestEncodeStrings(c *C) {
+	for i, test := range [...]struct {
+		input    []string
+		expected []byte
+	}{
+		{
+			input: []string{
+				"hello\n",
+			},
+			expected: []byte("000ahello\n"),
+		}, {
+			input: []string{
+				"hello\n",
+				pktline.FlushString,
+			},
+			expected: []byte("000ahello\n0000"),
+		}, {
+			input: []string{
+				"hello\n",
+				"world!\n",
+				"foo",
+			},
+			expected: []byte("000ahello\n000bworld!\n0007foo"),
+		}, {
+			input: []string{
+				"hello\n",
+				pktline.FlushString,
+				"world!\n",
+				"foo",
+				pktline.FlushString,
+			},
+			expected: []byte("000ahello\n0000000bworld!\n0007foo0000"),
+		}, {
+			input: []string{
+				strings.Repeat("a", pktline.MaxPayloadSize),
+			},
+			expected: []byte(
+				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize)),
+		}, {
+			input: []string{
+				strings.Repeat("a", pktline.MaxPayloadSize),
+				strings.Repeat("b", pktline.MaxPayloadSize),
+			},
+			expected: []byte(
+				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize) +
+					"fff0" + strings.Repeat("b", pktline.MaxPayloadSize)),
+		},
+	} {
+		comment := Commentf("input %d = %v\n", i, test.input)
+
+		var buf bytes.Buffer
+		e := pktline.NewEncoder(&buf)
+
+		err := e.EncodeString(test.input...)
+		c.Assert(err, IsNil, comment)
+		c.Assert(buf.Bytes(), DeepEquals, test.expected, comment)
+	}
+}
+
+func (s *SuiteEncoder) TestEncodeStringErrPayloadTooLong(c *C) {
+	for i, input := range [...][]string{
+		[]string{
+			strings.Repeat("a", pktline.MaxPayloadSize+1),
+		},
+		[]string{
+			"hello world!",
+			strings.Repeat("a", pktline.MaxPayloadSize+1),
+		},
+		[]string{
+			"hello world!",
+			strings.Repeat("a", pktline.MaxPayloadSize+1),
+			"foo",
+		},
+	} {
+		comment := Commentf("input %d = %v\n", i, input)
+
+		var buf bytes.Buffer
+		e := pktline.NewEncoder(&buf)
+
+		err := e.EncodeString(input...)
+		c.Assert(err, Equals, pktline.ErrPayloadTooLong, comment)
+	}
+}
+
+func (s *SuiteEncoder) TestEncodef(c *C) {
+	format := " %s %d\n"
+	str := "foo"
+	d := 42
+
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+
+	err := e.Encodef(format, str, d)
+	c.Assert(err, IsNil)
+
+	expected := []byte("000c foo 42\n")
+	c.Assert(buf.Bytes(), DeepEquals, expected)
+}
+
+func ExampleEncoder() {
+	// Create an encoder that writes pktlines to stdout.
+	e := pktline.NewEncoder(os.Stdout)
+
+	// Encode some data as a new pkt-line.
+	_ = e.Encode([]byte("data\n")) // error checks removed for brevity
+
+	// Encode a flush-pkt.
+	_ = e.Flush()
+
+	// Encode a couple of byte slices and a flush in one go. Each of
+	// them will end up as payloads of their own pktlines.
+	_ = e.Encode(
+		[]byte("hello\n"),
+		[]byte("world!\n"),
+		pktline.Flush,
+	)
+
+	// You can also encode strings:
+	_ = e.EncodeString(
+		"foo\n",
+		"bar\n",
+		pktline.FlushString,
+	)
+
+	// You can also format and encode a payload:
+	_ = e.Encodef(" %s %d\n", "foo", 42)
+	// Output:
+	// 0009data
+	// 0000000ahello
+	// 000bworld!
+	// 00000008foo
+	// 0008bar
+	// 0000000c foo 42
+}
diff --git a/formats/packp/pktline/pktlines.go b/formats/packp/pktline/pktlines.go
deleted file mode 100644
index c19aa2e..0000000
--- a/formats/packp/pktline/pktlines.go
+++ /dev/null
@@ -1,140 +0,0 @@
-// Package pktline implements reading payloads form pkt-lines and creating pkt-lines from payloads.
-package pktline
-
-import (
-	"bytes"
-	"errors"
-	"io"
-	"strings"
-)
-
-const (
-	// MaxPayloadSize is the maximum payload size of a pkt-line in bytes.
-	MaxPayloadSize = 65516
-)
-
-var (
-	flush = []byte{'0', '0', '0', '0'}
-)
-
-// PktLines values represent a succession of pkt-lines.  Values from
-// this type are not zero-value safe, use the New function instead.
-type PktLines struct {
-	r io.Reader
-}
-
-var (
-	// ErrPayloadTooLong is returned by the Add methods when any of the
-	// provided payloads is bigger than MaxPayloadSize.
-	ErrPayloadTooLong = errors.New("payload is too long")
-	// ErrEmptyPayload is returned by the Add methods when an empty
-	// payload is provided.
-	ErrEmptyPayload = errors.New("cannot add empty payloads")
-)
-
-// New returns an empty PktLines (with no payloads) ready to be used.
-func New() *PktLines {
-	return &PktLines{
-		r: bytes.NewReader(nil),
-	}
-}
-
-// AddFlush adds a flush-pkt to p.
-func (p *PktLines) AddFlush() {
-	p.r = io.MultiReader(p.r, bytes.NewReader(flush))
-}
-
-// Add adds the slices in pp as the payloads of a
-// corresponding number of pktlines.
-func (p *PktLines) Add(pp ...[]byte) error {
-	tmp := []io.Reader{p.r}
-	for _, p := range pp {
-		if err := add(&tmp, p); err != nil {
-			return err
-		}
-	}
-	p.r = io.MultiReader(tmp...)
-
-	return nil
-}
-
-func add(dst *[]io.Reader, e []byte) error {
-	if err := checkPayloadLength(len(e)); err != nil {
-		return err
-	}
-
-	n := len(e) + 4
-	*dst = append(*dst, bytes.NewReader(asciiHex16(n)))
-	*dst = append(*dst, bytes.NewReader(e))
-
-	return nil
-}
-
-func checkPayloadLength(n int) error {
-	switch {
-	case n < 0:
-		panic("unexpected negative payload length")
-	case n == 0:
-		return ErrEmptyPayload
-	case n > MaxPayloadSize:
-		return ErrPayloadTooLong
-	default:
-		return nil
-	}
-}
-
-// Returns the hexadecimal ascii representation of the 16 less
-// significant bits of n.  The length of the returned slice will always
-// be 4.  Example: if n is 1234 (0x4d2), the return value will be
-// []byte{'0', '4', 'd', '2'}.
-func asciiHex16(n int) []byte {
-	var ret [4]byte
-	ret[0] = byteToASCIIHex(byte(n & 0xf000 >> 12))
-	ret[1] = byteToASCIIHex(byte(n & 0x0f00 >> 8))
-	ret[2] = byteToASCIIHex(byte(n & 0x00f0 >> 4))
-	ret[3] = byteToASCIIHex(byte(n & 0x000f))
-
-	return ret[:]
-}
-
-// turns a byte into its hexadecimal ascii representation.  Example:
-// from 11 (0xb) to 'b'.
-func byteToASCIIHex(n byte) byte {
-	if n < 10 {
-		return '0' + n
-	}
-
-	return 'a' - 10 + n
-}
-
-// AddString adds the strings in pp as payloads of a
-// corresponding number of pktlines.
-func (p *PktLines) AddString(pp ...string) error {
-	tmp := []io.Reader{p.r}
-	for _, p := range pp {
-		if err := addString(&tmp, p); err != nil {
-			return err
-		}
-	}
-
-	p.r = io.MultiReader(tmp...)
-
-	return nil
-}
-
-func addString(dst *[]io.Reader, s string) error {
-	if err := checkPayloadLength(len(s)); err != nil {
-		return err
-	}
-
-	n := len(s) + 4
-	*dst = append(*dst, bytes.NewReader(asciiHex16(n)))
-	*dst = append(*dst, strings.NewReader(s))
-
-	return nil
-}
-
-// Read reads the pktlines for the payloads added so far.
-func (p *PktLines) Read(b []byte) (n int, err error) {
-	return p.r.Read(b)
-}
diff --git a/formats/packp/pktline/pktlines_test.go b/formats/packp/pktline/pktlines_test.go
deleted file mode 100644
index e0ba16b..0000000
--- a/formats/packp/pktline/pktlines_test.go
+++ /dev/null
@@ -1,231 +0,0 @@
-package pktline_test
-
-import (
-	"io"
-	"io/ioutil"
-	"os"
-	"strings"
-	"testing"
-
-	"gopkg.in/src-d/go-git.v4/formats/packp/pktline"
-
-	. "gopkg.in/check.v1"
-)
-
-func Test(t *testing.T) { TestingT(t) }
-
-type SuitePktLine struct {
-}
-
-var _ = Suite(&SuitePktLine{})
-
-func (s *SuitePktLine) TestNewIsEmpty(c *C) {
-	p := pktline.New()
-
-	b, err := ioutil.ReadAll(p)
-	c.Assert(err, IsNil)
-	c.Assert(b, DeepEquals, []byte{})
-}
-
-func (s *SuitePktLine) TestAddFlush(c *C) {
-	p := pktline.New()
-	p.AddFlush()
-
-	b, err := ioutil.ReadAll(p)
-	c.Assert(err, IsNil)
-	c.Assert(string(b), DeepEquals, "0000")
-}
-
-func (s *SuitePktLine) TestAdd(c *C) {
-	for i, test := range [...]struct {
-		input    [][]byte
-		expected []byte
-	}{
-		{
-			input: [][]byte{
-				[]byte("hello\n"),
-			},
-			expected: []byte("000ahello\n"),
-		}, {
-			input: [][]byte{
-				[]byte("hello\n"),
-				[]byte("world!\n"),
-				[]byte("foo"),
-			},
-			expected: []byte("000ahello\n000bworld!\n0007foo"),
-		}, {
-			input: [][]byte{
-				[]byte(strings.Repeat("a", pktline.MaxPayloadSize)),
-			},
-			expected: []byte(
-				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize)),
-		}, {
-			input: [][]byte{
-				[]byte(strings.Repeat("a", pktline.MaxPayloadSize)),
-				[]byte(strings.Repeat("b", pktline.MaxPayloadSize)),
-			},
-			expected: []byte(
-				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize) +
-					"fff0" + strings.Repeat("b", pktline.MaxPayloadSize)),
-		},
-	} {
-		p := pktline.New()
-		err := p.Add(test.input...)
-		c.Assert(err, IsNil, Commentf("input %d = %v", i, test.input))
-
-		obtained, err := ioutil.ReadAll(p)
-		c.Assert(err, IsNil, Commentf("input %d = %v", i, test.input))
-
-		c.Assert(obtained, DeepEquals, test.expected,
-			Commentf("input %d = %v", i, test.input))
-	}
-}
-
-func (s *SuitePktLine) TestAddErrEmptyPayload(c *C) {
-	for _, input := range [...][][]byte{
-		[][]byte{
-			[]byte{},
-		},
-		[][]byte{
-			[]byte(nil),
-		},
-		[][]byte{
-			[]byte("hello world!"),
-			[]byte{},
-		},
-		[][]byte{
-			[]byte{},
-			[]byte("hello world!"),
-		},
-	} {
-		p := pktline.New()
-		err := p.Add(input...)
-		c.Assert(err, Equals, pktline.ErrEmptyPayload)
-	}
-}
-
-func (s *SuitePktLine) TestAddErrPayloadTooLong(c *C) {
-	for _, input := range [...][][]byte{
-		[][]byte{
-			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
-		},
-		[][]byte{
-			[]byte("hello world!"),
-			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
-		},
-		[][]byte{
-			[]byte("hello world!"),
-			[]byte(strings.Repeat("a", pktline.MaxPayloadSize+1)),
-			[]byte("foo"),
-		},
-	} {
-		p := pktline.New()
-		err := p.Add(input...)
-		c.Assert(err, Equals, pktline.ErrPayloadTooLong,
-			Commentf("%v\n", input))
-	}
-}
-
-func (s *SuitePktLine) TestAddString(c *C) {
-	for i, test := range [...]struct {
-		input    []string
-		expected []byte
-	}{
-		{
-			input: []string{
-				"hello\n",
-			},
-			expected: []byte("000ahello\n"),
-		}, {
-			input: []string{
-				"hello\n",
-				"world!\n",
-				"foo",
-			},
-			expected: []byte("000ahello\n000bworld!\n0007foo"),
-		}, {
-			input: []string{
-				strings.Repeat("a", pktline.MaxPayloadSize),
-			},
-			expected: []byte(
-				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize)),
-		}, {
-			input: []string{
-				strings.Repeat("a", pktline.MaxPayloadSize),
-				strings.Repeat("b", pktline.MaxPayloadSize),
-			},
-			expected: []byte(
-				"fff0" + strings.Repeat("a", pktline.MaxPayloadSize) +
-					"fff0" + strings.Repeat("b", pktline.MaxPayloadSize)),
-		},
-	} {
-		p := pktline.New()
-		err := p.AddString(test.input...)
-		c.Assert(err, IsNil, Commentf("input %d = %v", i, test.input))
-
-		obtained, err := ioutil.ReadAll(p)
-		c.Assert(err, IsNil, Commentf("input %d = %v", i, test.input))
-
-		c.Assert(obtained, DeepEquals, test.expected,
-			Commentf("input %d = %v", i, test.input))
-	}
-}
-
-func (s *SuitePktLine) TestAddStringErrEmptyPayload(c *C) {
-	for _, input := range [...][]string{
-		[]string{""},
-		[]string{"hello world!", ""},
-		[]string{"", "hello world!"},
-	} {
-		p := pktline.New()
-		err := p.AddString(input...)
-		c.Assert(err, Equals, pktline.ErrEmptyPayload)
-	}
-}
-
-func (s *SuitePktLine) TestAddStringErrPayloadTooLong(c *C) {
-	for _, input := range [...][]string{
-		[]string{
-			strings.Repeat("a", pktline.MaxPayloadSize+1),
-		},
-		[]string{
-			"hello world!",
-			strings.Repeat("a", pktline.MaxPayloadSize+1),
-		},
-		[]string{
-			"hello world!",
-			strings.Repeat("a", pktline.MaxPayloadSize+1),
-			"foo",
-		},
-	} {
-		p := pktline.New()
-		err := p.AddString(input...)
-		c.Assert(err, Equals, pktline.ErrPayloadTooLong,
-			Commentf("%v\n", input))
-	}
-}
-
-func ExamplePktLines() {
-	// Create an empty collection of pktlines.
-	p := pktline.New()
-
-	// Add two strings as payloads ("foo\n" and "bar\n"), they will
-	// end up as two consecutive pktlines.
-	p.AddString("foo\n", "bar\n") // error checks removed for brevity
-
-	// You can also add byte slices as payloads...
-	p.Add([]byte("hello\n"), []byte("world!\n"))
-
-	// Add a flush-pkt.
-	p.AddFlush()
-
-	// PktLines are Readers, so you can directly read the full sequence.
-	io.Copy(os.Stdout, p)
-
-	// Output:
-	// 0008foo
-	// 0008bar
-	// 000ahello
-	// 000bworld!
-	// 0000
-}
diff --git a/formats/packp/pktline/scanner_test.go b/formats/packp/pktline/scanner_test.go
index de0f8df..b5a3c7d 100644
--- a/formats/packp/pktline/scanner_test.go
+++ b/formats/packp/pktline/scanner_test.go
@@ -1,6 +1,7 @@
 package pktline_test
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"strings"
@@ -41,11 +42,14 @@ func (s *SuiteScanner) TestEmptyReader(c *C) {
 }
 
 func (s *SuiteScanner) TestFlush(c *C) {
-	p := pktline.New()
-	p.AddFlush()
-	sc := pktline.NewScanner(p)
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+	err := e.Flush()
+	c.Assert(err, IsNil)
 
+	sc := pktline.NewScanner(&buf)
 	c.Assert(sc.Scan(), Equals, true)
+
 	payload := sc.Bytes()
 	c.Assert(len(payload), Equals, 0)
 }
@@ -70,14 +74,16 @@ func (s *SuiteScanner) TestScanAndPayload(c *C) {
 		strings.Repeat("a", pktline.MaxPayloadSize),
 		strings.Repeat("a", pktline.MaxPayloadSize-1) + "\n",
 	} {
-		p := pktline.New()
-		err := p.AddString(test)
+		var buf bytes.Buffer
+		e := pktline.NewEncoder(&buf)
+		err := e.EncodeString(test)
 		c.Assert(err, IsNil,
 			Commentf("input len=%x, contents=%.10q\n", len(test), test))
-		sc := pktline.NewScanner(p)
 
+		sc := pktline.NewScanner(&buf)
 		c.Assert(sc.Scan(), Equals, true,
 			Commentf("test = %.20q...", test))
+
 		obtained := sc.Bytes()
 		c.Assert(obtained, DeepEquals, []byte(test),
 			Commentf("in = %.20q out = %.20q", test, string(obtained)))
@@ -107,16 +113,19 @@ func (s *SuiteScanner) TestSkip(c *C) {
 			expected: []byte("third"),
 		},
 	} {
-		p := pktline.New()
-		err := p.AddString(test.input...)
+		var buf bytes.Buffer
+		e := pktline.NewEncoder(&buf)
+		err := e.EncodeString(test.input...)
 		c.Assert(err, IsNil)
-		sc := pktline.NewScanner(p)
+
+		sc := pktline.NewScanner(&buf)
 		for i := 0; i < test.n; i++ {
 			c.Assert(sc.Scan(), Equals, true,
 				Commentf("scan error = %s", sc.Err()))
 		}
 		c.Assert(sc.Scan(), Equals, true,
 			Commentf("scan error = %s", sc.Err()))
+
 		obtained := sc.Bytes()
 		c.Assert(obtained, DeepEquals, test.expected,
 			Commentf("\nin = %.20q\nout = %.20q\nexp = %.20q",
@@ -125,10 +134,12 @@ func (s *SuiteScanner) TestSkip(c *C) {
 }
 
 func (s *SuiteScanner) TestEOF(c *C) {
-	p := pktline.New()
-	err := p.AddString("first", "second")
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+	err := e.EncodeString("first", "second")
 	c.Assert(err, IsNil)
-	sc := pktline.NewScanner(p)
+
+	sc := pktline.NewScanner(&buf)
 	for sc.Scan() {
 	}
 	c.Assert(sc.Err(), IsNil)
@@ -164,19 +175,22 @@ func (s *SuiteScanner) TestReadSomeSections(c *C) {
 // 0000
 // and so on
 func sectionsExample(c *C, nSections, nLines int) io.Reader {
-	p := pktline.New()
+	var buf bytes.Buffer
+	e := pktline.NewEncoder(&buf)
+
 	for section := 0; section < nSections; section++ {
 		ss := []string{}
 		for line := 0; line < nLines; line++ {
 			line := fmt.Sprintf(" %d.%d\n", section, line)
 			ss = append(ss, line)
 		}
-		err := p.AddString(ss...)
+		err := e.EncodeString(ss...)
+		c.Assert(err, IsNil)
+		err = e.Flush()
 		c.Assert(err, IsNil)
-		p.AddFlush()
 	}
 
-	return p
+	return &buf
 }
 
 func ExampleScanner() {