CheckSum error if the declared number of groups does not match the actual number of groups

The scenario:

The initiator parses the following message with quickfix.MessageUtils.parse(), and sends it to the acceptor.

8=FIX.4.4|9=238|35=AE|31=1.35|32=1000|48=EURUSD|22=6|55=EURUSD|64=20131106|194=1.35|15011=1.33|15012=1.33|461=RCSXXX|487=2|571=1105200000001234|829=0|572=M1006050412587|552=1|54=1|37=SYS1:4545556002|11=SYS2:1006050412587|526=SSP:BDID001|453=2|448=a123456|447=D|452=3|802=2|523=5730607|803=10|523=5730606|803=15|1=123456-789|

It is not a valid FIX message, as we have 453<NoPartyIDs>=2, but there is only one group in the message. We would expect the acceptor to return a validation error, but instead the following happens:

1) An error message is logged:

2013-12-07 15:47:30,919 [SocketAcceptorIoProcessor-0.0] ERROR [] q.mina.acceptor.AcceptorIoHandler - Invalid message: Expected CheckSum=29, Received CheckSum=28 in 8=FIX.4.4?9=367?35=AE?34=82?49=SIDE1?52=20131207-14:47:30.618?56=SIDE2?22=6?31=1.35?32=1000?48=EURUSD?55=EURUSD?64=20131106?194=1.35?461=RCSXXX?487=2?571=1105200000001234?572=M1006050412587?829=0?15011=1.33?15012=1.33?552=1?54=1?37=SYS1:4545556002?11=SYS2:1006050412587?526=SSP:BDID001?453=2?448=a123456?447=D?452=3?802=2?523=5730607?803=10?523=5730606?803=15?1=123456-789?10=028?

2) The acceptor does not send any response and does not increase the message sequence count, either.

The suspect:

The acceptor validates the fix message with the checksum(String s) method of Message class. It calculates 29. But the checksum calculated on the initiator side is 28. The initiator calculates the checksum with a different method, which is called by the toString() method of Message class. This is the method which does the calculation (in FieldMap class) - my comments added:

int calculateTotal() {

int result = 0;
// JK: fields variable contains tags which are not group tags
for (final Field<?> field2 : fields.values()) {
final Field<?> field = field2;
if (field.getField() == CheckSum.FIELD || isGroupField(field.getField()))

{ continue; }

result += field.getTotal();
}

// JK: groups variable contains the group tags, but only the tag number (the key is Integer, not Field - why?)
final Iterator<Map.Entry<Integer, List<Group>>> iterator = groups.entrySet().iterator();
while (iterator.hasNext()) {
final Map.Entry<Integer, List<Group>> entry = iterator.next();
final List<Group> groupList = entry.getValue();
if (!groupList.isEmpty()) {
final IntField groupField = new IntField((entry.getKey()).intValue());
// JK: at this point we have 453=0, as this is a new field, no value yet
groupField.setValue(groupList.size());
// JK: now it is 453=1, because we have only one group
result += groupField.getTotal();
for (int i = 0; i < groupList.size(); i++)

{ final Group group = groupList.get(i); result += group.calculateTotal(); }

}
}

return result;
}

So the checksum will be the checksum of a different message actually, where 453=1.