fix some write error handling (go-sql-driver#1595)

interpolateParams() returned ErrInvalidConn without closing the connection.
Since database/sql doesn't understand ErrInvalidConn, there is a risk
that database/sql reuse this connection and ErrInvalidConn is returned
repeatedly.

connection.go

@@ -230,8 +230,10 @@ func (mc *mysqlConn) interpolateParams(query string, args []driver.Value) (strin
  buf, err := mc.buf.takeCompleteBuffer()
  if err != nil {
  // can not take the buffer. Something must be wrong with the connection
- mc.log(err)
- return "", ErrInvalidConn
+ mc.cleanup()
+ // interpolateParams would be called before sending any query.
+ // So its safe to retry.
+ return "", driver.ErrBadConn
  }
  buf = buf[:0]
  argPos := 0

packets.go

@@ -117,6 +117,8 @@ func (mc *mysqlConn) writePacket(data []byte) error {
  // Write packet
  if mc.writeTimeout > 0 {
  if err := mc.netConn.SetWriteDeadline(time.Now().Add(mc.writeTimeout)); err != nil {
+ mc.cleanup()
+ mc.log(err)
  return err
  }
  }