MySQLConnection closes unexpectedly in Swift Command Line Tool

[huibert7]

Describe the bug

MySQLConnection closes unexpectedly in a Swift Command Line Tool that I am writing.

To Reproduce

import Foundation
import MySQLNIO

@main
struct InsertValuesIntoMySQLDB {
    
    static func main() {
        do {
            try insertRecords()
        } catch {
            print("There was an error!")
            print(error.localizedDescription)
        }
    }
    
    static func insertRecords() throws {
        let addr = try SocketAddress.makeAddressResolvingHost(
            "database.domain.com",
            port: 3306
        )
        let eventGroup = MultiThreadedEventLoopGroup(numberOfThreads: 4)
        let eventLoop = eventGroup.next()
        let conn = try MySQLConnection.connect(to: addr, username: "admin", database: "quora", password: "xyzzy", tlsConfiguration: nil, on: eventLoop).wait()
        defer {
            do {
                try conn.close().wait()
            } catch {
                print("Error: \(error)")
            }
        }
        let rows = try conn.simpleQuery("SELECT COUNT(*) from questions").wait()
        print("rows: \(rows.count)")
//      try eventGroup.syncShutdownGracefully()
    }
}

Steps to reproduce the behavior:
When I run the code I get:
rows: 0 (should be > 0)
Error: alreadyClosed

The query clearly reaches the database server since a misspelled table name generates an error (MySQL error: Server error: Table 'quora.question' doesn't exist), but when the SQL is valid, I get no response.

If I uncomment the last line (try eventGroup.syncShutdownGracefully), then I get the following error:

ERROR: Cannot schedule tasks on an EventLoop that has already shut down. This will be upgraded to a forced crash in future SwiftNIO versions.

Environment

• OS version: macOS Sonoma 14.3

This is my first time trying to use mysql-nio as well as swift-no, therefore I could be totally wrong on how to use these packages, but I have run out of ideas. Thanks in advance for any kind of help you could provide.

[0xTim]

@huibert7 sorry for the delay. There are a few things that you should improve with your code to start debugging this.

First I strongly suggest you start using the async APIs (and using .get() where these aren't available. That will avoid any issues around waiting on event loops. Second, you're creating a new event loop group on every insert which is a bad idea. You should ideally use the singleton event loop group that's managed for you.

Those 2 suggestions might fix your issue, but feel free to post if it doesn't help or produces a different error

[huibert7]

Thanks for your help, Tim. I will try your suggestions.