format/pktline: fix readPayloadLen err handling

plumbing/format/pktline/scanner.go

@@ -80,10 +80,11 @@ func (s *Scanner) Bytes() []byte {
 // pkt-len and substracting the pkt-len size.
 func (s *Scanner) readPayloadLen() (int, error) {
 	if _, err := io.ReadFull(s.r, s.len[:]); err != nil {
-		if err == io.EOF {
-			return 0, err
+		if err == io.ErrUnexpectedEOF {
+			return 0, ErrInvalidPktLen
 		}
-		return 0, ErrInvalidPktLen
+
+		return 0, err
 	}
 
 	n, err := hexDecode(s.len)

plumbing/format/pktline/scanner_test.go

@@ -2,6 +2,7 @@ package pktline_test
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"strings"
@@ -145,6 +146,16 @@ func (s *SuiteScanner) TestEOF(c *C) {
 	c.Assert(sc.Err(), IsNil)
 }
 
+type mockReader struct{}
+
+func (r *mockReader) Read([]byte) (int, error) { return 0, errors.New("foo") }
+
+func (s *SuiteScanner) TestInternalReadError(c *C) {
+	sc := pktline.NewScanner(&mockReader{})
+	c.Assert(sc.Scan(), Equals, false)
+	c.Assert(sc.Err(), ErrorMatches, "foo")
+}
+
 // A section are several non flush-pkt lines followed by a flush-pkt, which
 // how the git protocol sends long messages.
 func (s *SuiteScanner) TestReadSomeSections(c *C) {