Reproduce https://github.com/go-sql-driver/mysql/issues/450

reproduce.go

package main

import (
	"database/sql"
	_ "github.com/go-sql-driver/mysql"
	"log"
	"strconv"
	"sync"
)

/*
-- MySQL dump 10.13  Distrib 5.7.9, for osx10.9 (x86_64)
--
-- Host: 127.0.0.1    Database: test
-- ------------------------------------------------------
-- Server version	5.5.5-10.1.14-MariaDB-1~jessie

--
-- Table structure for table `test`
--

DROP TABLE IF EXISTS `test`;
CREATE TABLE `test` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `val` varchar(128) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
*/

func main() {
	const numWorkers = 5

	db, err := sql.Open("mysql", "root@tcp(127.0.0.1:3306)/test")
	if err != nil {
		panic(err)
	}
	err = db.Ping()
	if err != nil {
		panic(err)
	}
	db.SetMaxOpenConns(numWorkers)
	db.SetMaxIdleConns(numWorkers / 2)

	jobs := make(chan int, numWorkers*2)
	var wg sync.WaitGroup
	for i := 0; i < numWorkers; i++ {
		go work(db, &wg, jobs)
		wg.Add(1)
	}
	for i := 0; i < 1000; i++ {
		jobs <- i
	}
	close(jobs)
	wg.Wait()
}

func work(db *sql.DB, wg *sync.WaitGroup, vals <-chan int) {
	defer wg.Done()
	for {
		select {

		case val, ok := <-vals:
			if !ok {
				return
			}
			insert(db, val)
		}

	}
}

func insert(db *sql.DB, val int) {
	const sql = `
INSERT INTO test SET val = ?
	`
	stmt, err := db.Prepare(sql)
	if err != nil {
		panic(err)
	}
	defer func() {
		err := stmt.Close()
		if err != nil {
			panic(err)
		}
	}()
	res, err := stmt.Exec(strconv.Itoa(val))
	if err != nil {
		panic(err)
	}
	id, err := res.LastInsertId()
	if err != nil {
		panic(err)
	}
	log.Println(id)
}