A resent message with repeating groups (that does not have a data dictionary on the session) will create an incorrect length and checksum

message.go

@@ -588,7 +588,7 @@ func formatCheckSum(value int) string {
 
 // Build constructs a []byte from a Message instance.
 func (m *Message) build() []byte {
-	m.cook()
+	m.cook(m.Body.length(), m.Body.total())
 
 	var b bytes.Buffer
 	m.Header.write(&b)
@@ -603,7 +603,7 @@ func (m *Message) build() []byte {
 // This func lets us pull the Message from the Store, parse it, update the Header, and then build it back into bytes using the original Body.
 // Note: The only standard non-Body group is NoHops.  If that is used in the Header, this workaround may fail.
 func (m *Message) buildWithBodyBytes(bodyBytes []byte) []byte {
-	m.cook()
+	m.cook(len(bodyBytes), bytesTotal(bodyBytes))
 
 	var b bytes.Buffer
 	m.Header.write(&b)
@@ -612,9 +612,9 @@ func (m *Message) buildWithBodyBytes(bodyBytes []byte) []byte {
 	return b.Bytes()
 }
 
-func (m *Message) cook() {
-	bodyLength := m.Header.length() + m.Body.length() + m.Trailer.length()
+func (m *Message) cook(bodyLen, bodyTotal int) {
+	bodyLength := m.Header.length() + bodyLen + m.Trailer.length()
 	m.Header.SetInt(tagBodyLength, bodyLength)
-	checkSum := (m.Header.total() + m.Body.total() + m.Trailer.total()) % 256
+	checkSum := (m.Header.total() + bodyTotal + m.Trailer.total()) % 256
 	m.Trailer.SetString(tagCheckSum, formatCheckSum(checkSum))
 }

message_test.go

@@ -418,6 +418,36 @@ func (s *MessageSuite) TestReBuildWithRepeatingGroupForResend() {
 	s.True(bytes.Equal(expectedBytes, resendBytes), "Unexpected bytes,\n expected: %s\n  but was: %s", expectedBytes, resendBytes)
 }
 
+func (s *MessageSuite) TestReBuildWithRepeatingGroupMultipleEntriesInGroupForResend() {
+	// Given the following message with a repeating group that has 2 entries
+	origHeader := "8=FIXT.1.1
9=184
35=8
34=3
49=ISLD
52=20240415-03:43:17.923
56=TW
"
+	origBody := "6=1.00
11=1
14=1.00
17=1
31=1.00
32=1.00
37=1
38=1.00
39=2
54=1
55=1
150=2
151=0.00
453=2
448=xyzzy
447=D
452=1
448=foobar
447=D
452=3
"
+	origTrailer := "10=152
"
+	rawMsg := bytes.NewBufferString(origHeader + origBody + origTrailer)
+
+	// When I reparse the message from the store during a resend request
+	s.Nil(ParseMessage(s.msg, rawMsg))
+
+	// And I update the headers for resend
+	s.msg.Header.SetField(tagOrigSendingTime, FIXString("20240415-03:43:17.923"))
+	s.msg.Header.SetField(tagSendingTime, FIXString("20240415-14:41:23.456"))
+	s.msg.Header.SetField(tagPossDupFlag, FIXBoolean(true))
+
+	// The bodyBytes will still be correct
+	origBodyBytes := []byte(origBody)
+	s.True(bytes.Equal(origBodyBytes, s.msg.bodyBytes), "Incorrect body bytes, \n expected: %s\n  but was: %s", origBodyBytes, s.msg.bodyBytes)
+
+	// So when I combine the updated header + the original bodyBytes + the as-is trailer
+	resendBytes := s.msg.buildWithBodyBytes(origBodyBytes)
+
+	// Then the reparsed, rebuilt message will retain the correct ordering of repeating group tags during resend
+	expectedResendHeader := "8=FIXT.1.1
9=215
35=8
34=3
43=Y
49=ISLD
52=20240415-14:41:23.456
56=TW
122=20240415-03:43:17.923
"
+	expectedResendBody := "6=1.00
11=1
14=1.00
17=1
31=1.00
32=1.00
37=1
38=1.00
39=2
54=1
55=1
150=2
151=0.00
453=2
448=xyzzy
447=D
452=1
448=foobar
447=D
452=3
"
+	expectedResendTrailer := "10=147
"
+	expectedResendBytes := []byte(expectedResendHeader + expectedResendBody + expectedResendTrailer)
+	s.True(bytes.Equal(expectedResendBytes, resendBytes), "Unexpected bytes,\n expected: %s\n  but was: %s", expectedResendBytes, resendBytes)
+}
+
 func (s *MessageSuite) TestReverseRoute() {
 	s.Nil(ParseMessage(s.msg, bytes.NewBufferString("8=FIX.4.2
9=171
35=D
34=2
49=TW
50=KK
52=20060102-15:04:05
56=ISLD
57=AP
144=BB
115=JCD
116=CS
128=MG
129=CB
142=JV
143=RY
145=BH
11=ID
21=3
38=100
40=w
54=1
55=INTC
60=20060102-15:04:05
10=123
")))
 

tag_value.go

@@ -86,14 +86,15 @@ func (tv TagValue) String() string {
 	return string(tv.bytes)
 }
 
-func (tv TagValue) total() int {
-	total := 0
-
-	for _, b := range []byte(tv.bytes) {
+func bytesTotal(bytes []byte) (total int) {
+	for _, b := range bytes {
 		total += int(b)
 	}
+	return
+}
 
-	return total
+func (tv TagValue) total() int {
+	return bytesTotal(tv.bytes)
 }
 
 func (tv TagValue) length() int {

tag_value_test.go

@@ -86,4 +86,5 @@ func TestTagValue_total(t *testing.T) {
 	var tv TagValue
 	require.Nil(t, tv.parse([]byte(stringField)))
 	assert.Equal(t, 643, tv.total(), "Total is the summation of the ascii byte values of the field string")
+	assert.Equal(t, 643, bytesTotal([]byte(stringField)))
 }