如何使用Go修复泄漏的MySQL conns？

Question

I have data on my local computer in 2 MySQL databases (dbConnOuter and dbConnInner) that I want to process and collate into a 3rd database (dbConnTarget).

The code runs for about 17000 cycles, then stops with these error messages:

[mysql] 2018/08/06 18:20:57 packets.go:72: unexpected EOF
[mysql] 2018/08/06 18:20:57 packets.go:405: busy buffer

As far as I can tell I'm properly closing the connections where I'm reading from and I'm using Exec for writing, that I believe handles its own resources. I've also tried prepared statements, but it didn't help, the result was the same.

Below is the relevant part of my code, and it does similar database operations prior to this without any issues.

As this is one of my first experiments with Go, I can't yet see where I might be wasting my resources.

import (
    "database/sql"

    _ "github.com/go-sql-driver/mysql"
)

var dbConnOuter *sql.DB
var dbConnInner *sql.DB
var dbConnTarget *sql.DB

func main() {
    dbConnOuter = connectToDb(dbUserDataOne)
    dbConnInner = connectToDb(dbUserDataTwo)
    dbConnTarget = connectToDb(dbUserDataThree)
    // execute various db processing functions
    doStuff()
}

func connectToDb(dbUser dbUser) *sql.DB {
    dbConn, err := sql.Open("mysql", fmt.Sprintf("%v:%v@tcp(127.0.0.1:3306)/%v", dbUser.username, dbUser.password, dbUser.dbname))
    if err != nil {
        panic(err)
    }
    dbConn.SetMaxOpenConns(500)
    return dbConn
}

// omitted similar db processing functions that work just fine

func doStuff() {
    outerRes, err := dbConnOuter.Query("SELECT some outer data")
    if err != nil {
        panic(err)
    }
    defer outerRes.Close()

    for outerRes.Next() {
        outerRes.Scan(&data1)
        innerRes, err := dbConnInner.Query("SELECT some inner data using", data1)
        if err != nil {
            panic(err)
        }
        innerRes.Scan(&data2, &data3)
        innerRes.Close()

        dbConnTarget.Exec("REPLACE INTO whatever", data1, data2, data3)
    }
}