Fix handling zero byte string for session state changed (vitessio#11071…

…) (vitessio#996)

On MySQL 8.0 it's possible we get back a zero length state for session
state changed information. In this case, we treat this as no data being
present and continue.

This was exposed in vitessio#11026 but
I'm extracted this as a separate fix until that lands since this can
trigger spurious errors when it shouldn't.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

go/mysql/conn.go

@@ -1470,10 +1470,15 @@ func (c *Conn) parseOKPacket(in []byte) (*PacketOK, error) {
 	if c.Capabilities&uint32(CapabilityClientSessionTrack) == CapabilityClientSessionTrack {
 		// session tracking
 		if statusFlags&ServerSessionStateChanged == ServerSessionStateChanged {
-			_, ok := data.readLenEncInt()
+			length, ok := data.readLenEncInt()
 			if !ok {
 				return fail("invalid OK packet session state change length: %v", data)
 			}
+			// In case we have a zero length string, there's no additional information so
+			// we can return the packet.
+			if length == 0 {
+				return packetOK, nil
+			}
 			sscType, ok := data.readByte()
 			if !ok {
 				return fail("invalid OK packet session state change type: %v", sscType)