SQLITE_BUSY error on concurrent SELECT queries while in WAL mode
Scanning the result of multiple concurrent `SELECT` queries (eg. each in their own goroutine) results in the following error:
```
database is locked (5) (SQLITE_BUSY)
```
If I switch to the `mattn/go-sqlite3` driver OR execute the queries in a single goroutine everything seems to work fine.
Here is a minimal reproducible example:
```go
package main
import (
"database/sql"
"log"
"sync"
_ "github.com/mattn/go-sqlite3"
_ "modernc.org/sqlite"
)
func main() {
var wg sync.WaitGroup
// mattn/go-sqlite3
// ---
// db, openErr := sql.Open("sqlite3", "data.db?_journal_mode=WAL")
// modernc.org/sqlite
// ---
db, openErr := sql.Open("sqlite", "data.db?_pragma=journal_mode(WAL)")
if openErr != nil {
log.Fatal(openErr)
}
// run the queries
for i := 0; i < 1000; i++ {
wg.Add(1)
go func() {
defer wg.Done()
var id int
row := db.QueryRow("SELECT 1")
if err := row.Scan(&id); err != nil {
log.Fatal(err)
}
}()
}
// wait the goroutines to complete
wg.Wait()
log.Println("Completed successfully!")
}
```
_For context, this error was reported initially in https://github.com/pocketbase/pocketbase/issues/875._
_edit1: simplified the example and left only the `select` statement_
_edit2: fixed `wg.Add` placement_
issue