repro for non-atomic create database if not exists

main.go

package main

import (
	"database/sql"
	"flag"
	"fmt"
	"log"
	"os"
	"sync"

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

func envOr(key, fallback string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return fallback
}

func main() {
	var (
		addr = flag.String("addr", envOr("DOLT_ADDR", "127.0.0.1:3306"), "host:port of MySQL/Dolt server")
		user = flag.String("user", envOr("DOLT_USER", "root"), "username")
		pass = flag.String("pass", os.Getenv("DOLT_PASS"), "password")
		dsn  = flag.String("dsn", os.Getenv("DOLT_DSN"), "full MySQL DSN (overrides other connection flags)")
	)
	flag.Parse()

	var cfg *mysql.Config
	if *dsn != "" {
		var err error
		cfg, err = mysql.ParseDSN(*dsn)
		if err != nil {
			log.Fatalf("parse dsn: %v", err)
		}
	} else {
		cfg = mysql.NewConfig()
		cfg.User = *user
		cfg.Passwd = *pass
		cfg.Net = "tcp"
		cfg.Addr = *addr
	}

	concurrency := 10
	dbName := "testconcurrentdb"

	// Clean up from any previous run.
	db, err := sql.Open("mysql", cfg.FormatDSN())
	if err != nil {
		log.Fatalf("open: %v", err)
	}
	defer db.Close()

	_, err = db.Exec(fmt.Sprintf("DROP DATABASE IF EXISTS %s", dbName))
	if err != nil {
		log.Fatalf("cleanup: %v", err)
	}

	var wg sync.WaitGroup
	wg.Add(concurrency)
	errs := make([]error, concurrency)

	for i := 0; i < concurrency; i++ {
		go func(id int) {
			defer wg.Done()
			conn, err := sql.Open("mysql", cfg.FormatDSN())
			if err != nil {
				errs[id] = fmt.Errorf("goroutine %d open: %w", id, err)
				return
			}
			defer conn.Close()

			_, err = conn.Exec(fmt.Sprintf("CREATE DATABASE IF NOT EXISTS %s", dbName))
			if err != nil {
				errs[id] = fmt.Errorf("goroutine %d: %w", id, err)
			}
		}(i)
	}

	wg.Wait()

	failed := 0
	for _, err := range errs {
		if err != nil {
			log.Printf("ERROR: %v", err)
			failed++
		}
	}

	if failed > 0 {
		log.Printf("%d/%d goroutines failed", failed, concurrency)
		os.Exit(1)
	}

	log.Printf("all %d goroutines succeeded", concurrency)

	// Clean up.
	_, _ = db.Exec(fmt.Sprintf("DROP DATABASE IF EXISTS %s", dbName))
}