protocol/packp: sideband muxer and demuxer (#143)

* format/pakp: sideband demuxer

* format/pakp: sideband muxer

* format/pakp: sideband demuxer and muxer

* protocol/pakp: sideband demuxer and muxer

* documentation and improvements

* improvements

* handle scan errors properly

1 files changed, 147 insertions, 0 deletions

diff --git a/plumbing/protocol/packp/sideband/demux.go b/plumbing/protocol/packp/sideband/demux.go
new file mode 100644
index 0000000..09fe57d
--- /dev/null
+++ b/plumbing/protocol/packp/sideband/demux.go
@@ -0,0 +1,147 @@
+package sideband
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+
+	"gopkg.in/src-d/go-git.v4/plumbing/format/pktline"
+)
+
+// ErrMaxPackedExceeded returned by Read, if the maximum packed size is exceeded
+var ErrMaxPackedExceeded = errors.New("max. packed size exceeded")
+
+// Progress allows to read the progress information
+type Progress interface {
+	io.Reader
+}
+
+// Demuxer demultiplex the progress reports and error info interleaved with the
+// packfile itself.
+//
+// A sideband has three diferent channels the main one call PackData contains
+// the packfile data, the ErrorMessage channel, that contains server errors and
+// the last one ProgressMessage channel containing information about the ongoing
+// tast happening in the server (optinal, can be suppressed sending NoProgress
+// or Quiet capabilities to the server)
+//
+// In order to demultiplex the data stream, method `Read` should be called to
+// retrieve the PackData channel, the incoming data from the ProgressMessage is
+// stored and can be read from `Progress` field, if any message is retrieved
+// from the ErrorMessage channel an error is returned and we can assume that the
+// conection has been closed.
+type Demuxer struct {
+	t Type
+	r io.Reader
+	s *pktline.Scanner
+
+	max     int
+	pending []byte
+
+	// Progress contains progress information
+	Progress Progress
+}
+
+// NewDemuxer returns a new Demuxer for the given t and read from r
+func NewDemuxer(t Type, r io.Reader) *Demuxer {
+	max := MaxPackedSize64k
+	if t == Sideband {
+		max = MaxPackedSize
+	}
+
+	return &Demuxer{
+		t:        t,
+		r:        r,
+		max:      max,
+		s:        pktline.NewScanner(r),
+		Progress: bytes.NewBuffer(nil),
+	}
+}
+
+// Read reads up to len(p) bytes from the PackData channel into p, an error can
+// be return if an error happends when reading or if a message is sent in the
+// ErrorMessage channel.
+//
+// If a ProgressMessage is read, it won't be copied to b. Instead of this, it is
+// stored and can be read through the reader Progress. If the n value returned
+// is zero, err will be nil unless an error reading happens.
+func (d *Demuxer) Read(b []byte) (n int, err error) {
+	var read, req int
+
+	req = len(b)
+	for read < req {
+		n, err := d.doRead(b[read:req])
+		read += n
+
+		if err != nil {
+			return read, err
+		}
+	}
+
+	return read, nil
+}
+
+func (d *Demuxer) doRead(b []byte) (int, error) {
+	read, err := d.nextPackData()
+	size := len(read)
+	wanted := len(b)
+
+	if size > wanted {
+		d.pending = read[wanted:]
+	}
+
+	if wanted > size {
+		wanted = size
+	}
+
+	size = copy(b, read[:wanted])
+	return size, err
+}
+
+func (d *Demuxer) nextPackData() ([]byte, error) {
+	content := d.getPending()
+	if len(content) != 0 {
+		return content, nil
+	}
+
+	if !d.s.Scan() {
+		if err := d.s.Err(); err != nil {
+			return nil, err
+		}
+
+		return nil, io.EOF
+	}
+
+	content = d.s.Bytes()
+
+	size := len(content)
+	if size == 0 {
+		return nil, nil
+	} else if size > d.max {
+		return nil, ErrMaxPackedExceeded
+	}
+
+	switch Channel(content[0]) {
+	case PackData:
+		return content[1:], nil
+	case ProgressMessage:
+		_, err := d.Progress.(io.Writer).Write(content[1:])
+		return nil, err
+	case ErrorMessage:
+		return nil, fmt.Errorf("unexpected error: %s", content[1:])
+	default:
+		return nil, fmt.Errorf("unknown channel %s", content)
+	}
+}
+
+func (d *Demuxer) getPending() (b []byte) {
+	if len(d.pending) == 0 {
+		return nil
+	}
+
+	content := d.pending
+	d.pending = nil
+
+	return content
+}