MaxOpenConns is not working

[youzhixiaomutou]

Issue description

When I operate MySQL concurrently, the number of actually created connections is greater than the set maximum number of connections. Is my usage wrong? Or is the connection leaked somewhere?

Example code

import (
	"context"
	"database/sql"
	"testing"
	"time"

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

// fuzz_conn_test.go
func FuzzConn(f *testing.F) {
	db, _ := sql.Open("mysql", "dsn")
	db.SetMaxOpenConns(2)
	db.SetMaxIdleConns(2)
	db.SetConnMaxIdleTime(10 * time.Second)
	db.SetConnMaxLifetime(10 * time.Second)

	f.Fuzz(func(t *testing.T, a int) {
		go func() {
			r, _ := db.Query("select sleep(10)")
			r.Close()
		}()
	})
}

Error log

Configuration

Driver version (or git SHA): v1.8.0

Go version: go version go1.21.8 windows/amd64

Server version: MySQL 5.7+

[methane]

not reproducible

[methane]

Maybe, you are using multiple process. Each process makes two connections.
https://go.dev/doc/security/fuzz/#custom-settings

[youzhixiaomutou]

Thank you very much for your reply.
But I use go test -fuzz FuzzConn -parallel 1 to run. It still creating more than maxOpenConnections.

I know this situation is not easy to reproduce without fuzz testing. But It actually appears in my prod environment.
Maybe give me some ideas（I hava confirmed that it only create one global sql.DB and use it）?

[methane]

If it is really db issue, it must be able to reproduce without fuzzing.

So you must use multiple processes. Try this.

-			r, _ := db.Query("select sleep(10)")
+			r, _ := db.Query(fmt.Sprintf("select sleep(10) /* %d */", os.Getpid()))