3 files changed, 128 insertions, 0 deletions
diff --git a/plumbing/format/pktline/error.go b/plumbing/format/pktline/error.go
new file mode 100644
index 0000000..2c0e5a7
--- /dev/null
+++ b/plumbing/format/pktline/error.go
@@ -0,0 +1,51 @@
+package pktline
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+)
+
+var (
+	// ErrInvalidErrorLine is returned by Decode when the packet line is not an
+	// error line.
+	ErrInvalidErrorLine = errors.New("expected an error-line")
+
+	errPrefix = []byte("ERR ")
+)
+
+// ErrorLine is a packet line that contains an error message.
+// Once this packet is sent by client or server, the data transfer process is
+// terminated.
+// See https://git-scm.com/docs/pack-protocol#_pkt_line_format
+type ErrorLine struct {
+	Text string
+}
+
+// Error implements the error interface.
+func (e *ErrorLine) Error() string {
+	return e.Text
+}
+
+// Encode encodes the ErrorLine into a packet line.
+func (e *ErrorLine) Encode(w io.Writer) error {
+	p := NewEncoder(w)
+	return p.Encodef("%s%s\n", string(errPrefix), e.Text)
+}
+
+// Decode decodes a packet line into an ErrorLine.
+func (e *ErrorLine) Decode(r io.Reader) error {
+	s := NewScanner(r)
+	if !s.Scan() {
+		return s.Err()
+	}
+
+	line := s.Bytes()
+	if !bytes.HasPrefix(line, errPrefix) {
+		return ErrInvalidErrorLine
+	}
+
+	e.Text = strings.TrimSpace(string(line[4:]))
+	return nil
+}
diff --git a/plumbing/format/pktline/error_test.go b/plumbing/format/pktline/error_test.go
new file mode 100644
index 0000000..3cffd20
--- /dev/null
+++ b/plumbing/format/pktline/error_test.go
@@ -0,0 +1,68 @@
+package pktline
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"testing"
+)
+
+func TestEncodeEmptyErrorLine(t *testing.T) {
+	e := &ErrorLine{}
+	err := e.Encode(io.Discard)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestEncodeErrorLine(t *testing.T) {
+	e := &ErrorLine{
+		Text: "something",
+	}
+	var buf bytes.Buffer
+	err := e.Encode(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if buf.String() != "0012ERR something\n" {
+		t.Fatalf("unexpected encoded error line: %q", buf.String())
+	}
+}
+
+func TestDecodeEmptyErrorLine(t *testing.T) {
+	var buf bytes.Buffer
+	e := &ErrorLine{}
+	err := e.Decode(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if e.Text != "" {
+		t.Fatalf("unexpected error line: %q", e.Text)
+	}
+}
+
+func TestDecodeErrorLine(t *testing.T) {
+	var buf bytes.Buffer
+	buf.WriteString("000eERR foobar")
+	var e *ErrorLine
+	err := e.Decode(&buf)
+	if !errors.As(err, &e) {
+		t.Fatalf("expected error line, got: %T: %v", err, err)
+	}
+	if e.Text != "foobar" {
+		t.Fatalf("unexpected error line: %q", e.Text)
+	}
+}
+
+func TestDecodeErrorLineLn(t *testing.T) {
+	var buf bytes.Buffer
+	buf.WriteString("000fERR foobar\n")
+	var e *ErrorLine
+	err := e.Decode(&buf)
+	if !errors.As(err, &e) {
+		t.Fatalf("expected error line, got: %T: %v", err, err)
+	}
+	if e.Text != "foobar" {
+		t.Fatalf("unexpected error line: %q", e.Text)
+	}
+}
diff --git a/plumbing/format/pktline/scanner.go b/plumbing/format/pktline/scanner.go
index 5e85ed0..fbb137d 100644
--- a/plumbing/format/pktline/scanner.go
+++ b/plumbing/format/pktline/scanner.go
@@ -1,8 +1,10 @@
 package pktline
 
 import (
+	"bytes"
 	"errors"
 	"io"
+	"strings"
 
 	"github.com/go-git/go-git/v5/utils/trace"
 )
@@ -69,6 +71,13 @@ func (s *Scanner) Scan() bool {
 	s.payload = s.payload[:l]
 	trace.Packet.Printf("packet: < %04x %s", l, s.payload)
 
+	if bytes.HasPrefix(s.payload, errPrefix) {
+		s.err = &ErrorLine{
+			Text: strings.TrimSpace(string(s.payload[4:])),
+		}
+		return false
+	}
+
 	return true
 }