From 42443c0340f54d9441e887367473913729e25436 Mon Sep 17 00:00:00 2001
From: raphaelroshan <raphaelroshan@users.noreply.github.com>
Date: Tue, 14 Jul 2026 19:56:06 +0800
Subject: [PATCH] Fix duplicate 10= checksum on resend of message ending in a
 repeating group

When a stored message whose body ends in a repeating group is parsed with a
DataDictionary, parseGroup set trailerBytes to the (now-empty) rawBytes after
consuming the trailer field, so the caller's bodyBytes strip removed nothing
and left the 10= trailer embedded in bodyBytes. On resend, buildWithBodyBytes
wrote that contaminated bodyBytes and appended a fresh 10=, producing two
checksum tags and a wrong BodyLength, which the counterparty rejects.

parseGroup now restores the pre-extract buffer (which still holds the trailer)
when it detects the trailer field, so the strip removes it as in the non-group
path. Adds a dictionary-based resend test covering a body that ends in a group.
---
 message.go      |  5 +++++
 message_test.go | 20 ++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/message.go b/message.go
index 5bea1d375..69407cf5d 100644
--- a/message.go
+++ b/message.go
@@ -313,6 +313,7 @@ func parseGroup(mp *msgParser, tags []Tag) {
 	for {
 		mp.fieldIndex++
 		mp.parsedFieldBytes = &mp.msg.fields[mp.fieldIndex]
+		preExtractBytes := mp.rawBytes
 		mp.rawBytes, _ = extractField(mp.parsedFieldBytes, mp.rawBytes)
 		mp.trailerBytes = mp.rawBytes
 
@@ -337,6 +338,10 @@ func parseGroup(mp *msgParser, tags []Tag) {
 			mp.msg.Body.add(dm)
 			mp.msg.Trailer.add(mp.msg.fields[mp.fieldIndex : mp.fieldIndex+1])
 			mp.foundTrailer = true
+			// Restore the buffer that still holds the trailer so the caller strips
+			// it from bodyBytes; otherwise the trailer stays embedded and resend
+			// emits a duplicate 10= tag.
+			mp.trailerBytes = preExtractBytes
 			break
 		} else {
 			// Found a body field outside the group.
diff --git a/message_test.go b/message_test.go
index 72f11dcd0..a51c5231e 100644
--- a/message_test.go
+++ b/message_test.go
@@ -448,6 +448,26 @@ func (s *MessageSuite) TestReBuildWithRepeatingGroupMultipleEntriesInGroupForRes
 	s.True(bytes.Equal(expectedResendBytes, resendBytes), "Unexpected bytes,\n expected: %s\n  but was: %s", expectedResendBytes, resendBytes)
 }
 
+func (s *MessageSuite) TestReBuildWithRepeatingGroupWithDictionaryForResend() {
+	dict, dictErr := datadictionary.Parse("spec/FIX44.xml")
+	s.Nil(dictErr)
+
+	// A message whose body ends in a repeating group (453), parsed WITH a
+	// dictionary so parseGroup handles the group.
+	rawMsg := bytes.NewBufferString(
+		"8=FIX.4.4\x019=165\x0135=D\x0134=2\x0149=01001\x0150=01001a\x0152=20231231-20:19:41\x0156=TEST\x01" +
+			"1=acct1\x0111=13976\x0121=1\x0138=1\x0140=2\x0144=12\x0154=1\x0155=SYMABC\x0159=0\x0160=20231231-20:19:41\x01453=1\x01448=4501\x01447=D\x01452=28\x01" +
+			"10=026\x01")
+	s.Nil(ParseMessageWithDataDictionary(s.msg, rawMsg, dict, dict))
+
+	// The trailer must not stay embedded in bodyBytes.
+	s.False(bytes.Contains(s.msg.bodyBytes, []byte("\x0110=")), "trailer leaked into bodyBytes: %s", s.msg.bodyBytes)
+
+	// A resend rebuilt from bodyBytes must carry exactly one 10= checksum.
+	resendBytes := s.msg.buildWithBodyBytes(s.msg.bodyBytes)
+	s.Equal(1, bytes.Count(resendBytes, []byte("\x0110=")), "expected exactly one 10= checksum, got: %s", 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
")))