Add initial support for connection attributes.

This sets attribute _client_name with the value "Go MySQL Driver"

Author: dveeden

const.go

@@ -24,6 +24,8 @@ const (
 	iERR         byte = 0xff
 )
 
+// https://dev.mysql.com/doc/internals/en/capability-flags.html#packet-Protocol::CapabilityFlags
+
 type clientFlag uint32
 
 const (
@@ -45,6 +47,13 @@ const (
 	clientSecureConn
 	clientMultiStatements
 	clientMultiResults
+        clientPsMultiResults
+        clientPluginAuth
+        clientConnectAttrs
+        clientPluginAuthLenencClientData
+        clientExpiredPassword
+        clientSessionTrack
+        clientDeprecateEOF
 )
 
 const (

packets.go

@@ -214,6 +214,7 @@ func (mc *mysqlConn) writeAuthPacket(cipher []byte) error {
 		clientLongPassword |
 		clientTransactions |
 		clientLocalFiles |
+                clientConnectAttrs |
 		mc.flags&clientLongFlag
 
 	if mc.cfg.clientFoundRows {
@@ -228,7 +229,12 @@ func (mc *mysqlConn) writeAuthPacket(cipher []byte) error {
 	// User Password
 	scrambleBuff := scramblePassword(cipher, []byte(mc.cfg.passwd))
 
+        attrname := []byte("_client_name")
+        attrvalue := []byte("Go MySQL Driver")
+        attrlen := len(attrname) + len(attrvalue) + 2
+
 	pktLen := 4 + 4 + 1 + 23 + len(mc.cfg.user) + 1 + 1 + len(scrambleBuff)
+        pktLen += len(attrname) + len(attrvalue) + 3
 
 	// To specify a db name
 	if n := len(mc.cfg.dbname); n > 0 {
@@ -295,6 +301,17 @@ func (mc *mysqlConn) writeAuthPacket(cipher []byte) error {
 		pos += copy(data[pos:], mc.cfg.dbname)
 		data[pos] = 0x00
 	}
+        pos++
+
+        // Connection attributes
+        data[pos] = byte(attrlen)
+        pos++
+
+        data[pos] = byte(len(attrname))
+        pos += 1 + copy(data[pos+1:], attrname)
+
+        data[pos] = byte(len(attrvalue))
+        pos += 1 + copy(data[pos+1:], attrvalue)
 
 	// Send Auth packet
 	return mc.writePacket(data)