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

1 files changed, 155 insertions, 0 deletions

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
+}