getting connection errors when using worker pool

[n-ozerov]

Issue description

Getting connection errors when trying to execute sql statements within specific amount of workers

Example code

package main

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

var (
	stmt *sql.Stmt
	wg   sync.WaitGroup
)

const WORKERS_COUNT = 140
const WORKERS_MODE = true // false to make Goroutine for each exec

func main() {

	db, err := sql.Open("mysql", "root:pass@/test")
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	db.SetMaxOpenConns(140)

	stmt, _ = db.Prepare("INSERT INTO test (zn, idtest) VALUES (?,?)")
	defer stmt.Close()
	dlt, _ := db.Prepare("DELETE FROM test WHERE (idtest > 0)")
	defer dlt.Close()
	dlt.Exec()

	jobs := make(chan int)

	for w := 1; w <= WORKERS_COUNT; w++ {
		go worker(jobs)
	}

	now := time.Now()
	for i := 1; i <= 100000; i++ {
		wg.Add(1)
		if WORKERS_MODE {
			jobs <- i // this does not
		} else {
			go DoSomeJob(i) // this works fine
		}

	}
	wg.Wait()
	fmt.Println(time.Now().Sub(now))
	// time.Sleep(5*time.Second)
}

func DoSomeJob(i int) {

	_, err := stmt.Exec("test", i)

	if err != nil {
		fmt.Println("Error: ", err.Error())
	}

	wg.Done()
}

func worker(jobs chan int) {
	for {
		select {
		case task := <-jobs:
			DoSomeJob(task)
		}
	}
}

MySQL Table:

CREATE TABLE `test` (
  `idtest` int(11) NOT NULL AUTO_INCREMENT,
  `zn` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`idtest`)
) ENGINE=InnoDB AUTO_INCREMENT=100001 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

Error log

getting:
1. dial tcp 127.0.0.1:3306: connectex: Only one usage of each socket address (protocol/network address/port) is normally permitted. -- after about 70k rows when workers_count is less than MaxOpenConns (e.g. 100) 

2. driver: bad connection -- after about 30k row when workers_count is greater than MaxOpenConns

3. works without any errors then workers_count is much greater than MaxOpenConns (e.g. 30000) or less than 80 or using goroutine for each exec

Configuration

*Driver version (or git SHA): d0a5481

Go version: 1.12.4

*Server version: MySQL 8.0.16

*Server OS: Windows 10

[n-ozerov]

seems like it was an issue of database/sql: golang/go#31480