Using RegisterReaderHandler to insert rows in mysql

I'm using go-sql-driver to connect to mysql from go, and I'd love to use a reader to insert a big number of rows (34M+).

I'm having a hard time using an io.Reader to load the data as shown on the RegisterReaderHandler documentation.

I have a table defined like this

CREATE TABLE `test` (
  `Id` int(11) NOT NULL,
  `One` varchar(45) DEFAULT NULL,
  `Two` varchar(45) DEFAULT NULL,
  `Three` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`Id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

And I'm trying to insert a row by doing

func (r MyReader) Read(p []byte) (n int, err error){
    buf := new(bytes.Buffer)
    wr := csv.NewWriter(buf)

    // just generate a csv line to see if this works
    // I've verified that the size of the result is lower than p's capacity
    wr.Write([]string{"1", "Something", "NULL", "Something"})
    wr.Flush()

    p = buf.Bytes()    
    return len(p), io.EOF
}

func main() {
    mysql.RegisterReaderHandler("data", func() io.Reader {
        // pass my reader
        return MyReader{}
    })
    
    db, err := sql.Open("mysql", "user:pass@/db")
    checkErr(err)
    
    _, err = db.Exec("LOAD DATA LOCAL INFILE 'Reader::data' INTO TABLE test FIELDS TERMINATED BY ','")
    checkErr(err)
}

I see that my Read operation is called, and after running this a new element is created. However, it has Id 0 and all the other elements are NULL, so it's not processing the data correctly.

FWIW, I run the LOAD DATA LOCAL command in mysql (with the same contents the reader generates) and the row is successfully inserted.

What am I missing?

Obviously, 3 minutes after posting the question I figured out the answer.

I was replacing the output buffer in MyReader. So changing the line

p = buf.Bytes()

With

out = buf.Bytes()
copy(p, out)

did the trick