Only lock fieldmap once during message parsing

```
goos: darwin
goarch: arm64
pkg: github.com/quickfixgo/quickfix
               │     old     │          new-parse-no-lock          │
               │   sec/op    │   sec/op     vs base                │
ParseMessage-8   695.2n ± 1%   591.0n ± 0%  -14.99% (p=0.000 n=20)
```

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

Author: sylr

field_map.go

@@ -115,6 +115,20 @@ func (m FieldMap) GetField(tag Tag, parser FieldValueReader) MessageRejectError
 	return nil
 }
 
+// GetField parses of a field with Tag tag. Returned reject may indicate the field is not present, or the field value is invalid.
+func (m FieldMap) getFieldNoLock(tag Tag, parser FieldValueReader) MessageRejectError {
+	f, ok := m.tagLookup[tag]
+	if !ok {
+		return ConditionallyRequiredFieldMissing(tag)
+	}
+
+	if err := parser.Read(f[0].value); err != nil {
+		return IncorrectDataFormatForValue(tag)
+	}
+
+	return nil
+}
+
 // GetBytes is a zero-copy GetField wrapper for []bytes fields.
 func (m FieldMap) GetBytes(tag Tag) ([]byte, MessageRejectError) {
 	m.rwLock.RLock()
@@ -128,6 +142,16 @@ func (m FieldMap) GetBytes(tag Tag) ([]byte, MessageRejectError) {
 	return f[0].value, nil
 }
 
+// getBytesNoLock is a lock free zero-copy GetField wrapper for []bytes fields.
+func (m FieldMap) getBytesNoLock(tag Tag) ([]byte, MessageRejectError) {
+	f, ok := m.tagLookup[tag]
+	if !ok {
+		return nil, ConditionallyRequiredFieldMissing(tag)
+	}
+
+	return f[0].value, nil
+}
+
 // GetBool is a GetField wrapper for bool fields.
 func (m FieldMap) GetBool(tag Tag) (bool, MessageRejectError) {
 	var val FIXBoolean
@@ -152,6 +176,21 @@ func (m FieldMap) GetInt(tag Tag) (int, MessageRejectError) {
 	return int(val), err
 }
 
+// GetInt is a lock free GetField wrapper for int fields.
+func (m FieldMap) getIntNoLock(tag Tag) (int, MessageRejectError) {
+	bytes, err := m.getBytesNoLock(tag)
+	if err != nil {
+		return 0, err
+	}
+
+	var val FIXInt
+	if val.Read(bytes) != nil {
+		err = IncorrectDataFormatForValue(tag)
+	}
+
+	return int(val), err
+}
+
 // GetTime is a GetField wrapper for utc timestamp fields.
 func (m FieldMap) GetTime(tag Tag) (t time.Time, err MessageRejectError) {
 	m.rwLock.RLock()
@@ -179,6 +218,15 @@ func (m FieldMap) GetString(tag Tag) (string, MessageRejectError) {
 	return string(val), nil
 }
 
+// GetString is a GetField wrapper for string fields.
+func (m FieldMap) getStringNoLock(tag Tag) (string, MessageRejectError) {
+	var val FIXString
+	if err := m.getFieldNoLock(tag, &val); err != nil {
+		return "", err
+	}
+	return string(val), nil
+}
+
 // GetGroup is a Get function specific to Group Fields.
 func (m FieldMap) GetGroup(parser FieldGroupReader) MessageRejectError {
 	m.rwLock.RLock()
@@ -246,6 +294,13 @@ func (m *FieldMap) Clear() {
 	}
 }
 
+func (m *FieldMap) clearNoLock() {
+	m.tags = m.tags[0:0]
+	for k := range m.tagLookup {
+		delete(m.tagLookup, k)
+	}
+}
+
 // CopyInto overwrites the given FieldMap with this one.
 func (m *FieldMap) CopyInto(to *FieldMap) {
 	m.rwLock.RLock()
@@ -263,9 +318,6 @@ func (m *FieldMap) CopyInto(to *FieldMap) {
 }
 
 func (m *FieldMap) add(f field) {
-	m.rwLock.Lock()
-	defer m.rwLock.Unlock()
-
 	t := fieldTag(f)
 	if _, ok := m.tagLookup[t]; !ok {
 		m.tags = append(m.tags, t)

message.go

@@ -181,10 +181,17 @@ func ParseMessageWithDataDictionary(
 
 // doParsing executes the message parsing process.
 func doParsing(mp *msgParser) (err error) {
+	mp.msg.Header.rwLock.Lock()
+	defer mp.msg.Header.rwLock.Unlock()
+	mp.msg.Body.rwLock.Lock()
+	defer mp.msg.Body.rwLock.Unlock()
+	mp.msg.Trailer.rwLock.Lock()
+	defer mp.msg.Trailer.rwLock.Unlock()
+
 	// Initialize for parsing.
-	mp.msg.Header.Clear()
-	mp.msg.Body.Clear()
-	mp.msg.Trailer.Clear()
+	mp.msg.Header.clearNoLock()
+	mp.msg.Body.clearNoLock()
+	mp.msg.Trailer.clearNoLock()
 
 	// Allocate expected message fields in one chunk.
 	fieldCount := 0
@@ -267,7 +274,7 @@ func doParsing(mp *msgParser) (err error) {
 		}
 
 		if mp.parsedFieldBytes.tag == tagXMLDataLen {
-			xmlDataLen, _ = mp.msg.Header.GetInt(tagXMLDataLen)
+			xmlDataLen, _ = mp.msg.Header.getIntNoLock(tagXMLDataLen)
 		}
 		mp.fieldIndex++
 	}
@@ -292,7 +299,7 @@ func doParsing(mp *msgParser) (err error) {
 		}
 	}
 
-	bodyLength, err := mp.msg.Header.GetInt(tagBodyLength)
+	bodyLength, err := mp.msg.Header.getIntNoLock(tagBodyLength)
 	if err != nil {
 		err = parseError{OrigError: err.Error()}
 	} else if length != bodyLength && !xmlDataMsg {
@@ -373,7 +380,7 @@ func parseGroup(mp *msgParser, tags []Tag) {
 // tags slice will contain multiple tags if the tag in question is found while processing a group already.
 func isNumInGroupField(msg *Message, tags []Tag, appDataDictionary *datadictionary.DataDictionary) bool {
 	if appDataDictionary != nil {
-		msgt, err := msg.MsgType()
+		msgt, err := msg.msgTypeNoLock()
 		if err != nil {
 			return false
 		}
@@ -406,7 +413,7 @@ func isNumInGroupField(msg *Message, tags []Tag, appDataDictionary *datadictiona
 // tags slice will contain multiple tags if the tag in question is found while processing a group already.
 func getGroupFields(msg *Message, tags []Tag, appDataDictionary *datadictionary.DataDictionary) (fields []*datadictionary.FieldDef) {
 	if appDataDictionary != nil {
-		msgt, err := msg.MsgType()
+		msgt, err := msg.msgTypeNoLock()
 		if err != nil {
 			return
 		}
@@ -476,6 +483,10 @@ func (m *Message) MsgType() (string, MessageRejectError) {
 	return m.Header.GetString(tagMsgType)
 }
 
+func (m *Message) msgTypeNoLock() (string, MessageRejectError) {
+	return m.Header.getStringNoLock(tagMsgType)
+}
+
 // IsMsgTypeOf returns true if the Header contains MsgType (tag 35) field and its value is the specified one.
 func (m *Message) IsMsgTypeOf(msgType string) bool {
 	if v, err := m.MsgType(); err == nil {