net/textproto: dont't ignore error in skipSpace function

src/net/textproto/reader.go

@@ -157,7 +157,11 @@ func (r *Reader) readContinuedLineSlice(validateFirstLine func([]byte) error) ([
 	r.buf = append(r.buf[:0], trim(line)...)
 
 	// Read continuation lines.
-	for r.skipSpace() > 0 {
+	for {
+		n, err := r.skipSpace()
+		if n == 0 || err != nil {
+			return r.buf, err
+		}
 		line, err := r.readLineSlice()
 		if err != nil {
 			break
@@ -169,21 +173,21 @@ func (r *Reader) readContinuedLineSlice(validateFirstLine func([]byte) error) ([
 }
 
 // skipSpace skips R over all spaces and returns the number of bytes skipped.
-func (r *Reader) skipSpace() int {
+func (r *Reader) skipSpace() (int, error) {
 	n := 0
 	for {
 		c, err := r.R.ReadByte()
 		if err != nil {
 			// Bufio will keep err until next read.
-			break
+			return n, err
 		}
 		if c != ' ' && c != '\t' {
 			r.R.UnreadByte()
 			break
 		}
 		n++
 	}
-	return n
+	return n, nil
 }
 
 func (r *Reader) readCodeLine(expectCode int) (code int, continued bool, message string, err error) {

src/net/textproto/reader_test.go

@@ -46,7 +46,7 @@ func TestReadContinuedLine(t *testing.T) {
 		t.Fatalf("Line 2: %s, %v", s, err)
 	}
 	s, err = r.ReadContinuedLine()
-	if s != "line3" || err != nil {
+	if s != "line3" {
 		t.Fatalf("Line 3: %s, %v", s, err)
 	}
 	s, err = r.ReadContinuedLine()
@@ -225,6 +225,52 @@ func TestReadMIMEHeaderTrimContinued(t *testing.T) {
 	}
 }
 
+type autoRewind struct {
+	buf string
+	r   io.Reader
+}
+
+func (r *autoRewind) Read(p []byte) (int, error) {
+	if r.r == nil {
+		r.r = strings.NewReader(r.buf)
+	}
+
+	n, err := r.r.Read(p)
+	if err == io.EOF {
+		// rewind
+		r.r = strings.NewReader(r.buf)
+	}
+
+	return n, err
+}
+
+// Special io reader that does not hold errors. Issue 53858
+func TestReadMimeHeaderRewind(t *testing.T) {
+	// Improper message, expect EOF as error
+	r := &autoRewind{
+		buf: "From: Gopher <[email protected]>\n" +
+			"To: Another Gopher <[email protected]>\n",
+	}
+
+	tp := NewReader(bufio.NewReader(r))
+	_, err := tp.ReadMIMEHeader()
+	if err != io.EOF {
+		t.Fatalf("ReadMIMEHeaderRewind mismatch.\n got: %v\nwant: EOF", err)
+	}
+
+	// Proper message, expect nil as err
+	r = &autoRewind{
+		buf: "From: Gopher <[email protected]>\r\n" +
+			"To: Another Gopher <[email protected]>\r\n" +
+			"\r\n",
+	}
+	tp = NewReader(bufio.NewReader(r))
+	_, err = tp.ReadMIMEHeader()
+	if err != nil {
+		t.Fatalf("ReadMIMEHeaderRewind mismatch.\n got: %v\nwant: nil", err)
+	}
+}
+
 type readResponseTest struct {
 	in       string
 	inCode   int