Connections not closed with NIO

[mbarnach]

Context and Description

When using compiling Kitura with Swift NIO, the connections are not released.
This can be easily demonstrated with a simple application limiting the number of simultaneous connections to 2.

let serverOptions = ServerOptions(connectionLimit: 2)
Kitura.addHTTPServer(onPort: 8080, with: router, options: serverOptions)

Then calling the server more than 2 times will always return a 503.
It seems that this behaviour is affect both Linux and MacOS.

Environment Details

Tried on MacOS 10.15.3 with Swift 5.1.3 and Kitura 2.9.1
Same on Linux with Swift 5.1.5 and Kitura 2.9.1 (Docker swift:5.1.4)

This issue doesn't affect Kitura without NIO!

Could it be a reason for the issue #1499?

Steps to Reproduce

1. Limit the number of connections to something trackable (eg. 2)
2. Query any endpoints more than the previous limit
3. Observe the 503 response.

Expected vs. Actual Behaviour

• Expected to happen:
  - Connections available again after some time
• Actually happened:
  - Connections never get available

[Davidde94]

@mbarnach could you tell me which version of NIO was resolved?

[mbarnach]

@Davidde94 I'm still having the issue with the 2.20.2 (latest). I've tried some previous version also, with no luck.

[Davidde94]

@mbarnach thanks for that. I'm having difficulty reproducing with nc. I've created this extremely simple server:

import Kitura

let router = Router()

let serverOptions = ServerOptions(connectionLimit: 2)
Kitura.addHTTPServer(onPort: 8080, with: router, options: serverOptions)
Kitura.run()

I've then created 3 terminals and run nc localhost 8080 in each of them to create 3 connections. On the third terminal the connection fails as expected. If I then terminate one of the other nc processes, I'm able to create a new connection without issue.

Could you walk me through an exact reproduction?

[mbarnach]

Are you compiling with NIO on for Kitura? Like KITURA_NIO swift run

[Davidde94]

Yep!

david@Davids-Work-MacBook-Pro KituraConnectionTest % env KITURA_NIO=1 swift run
Fetching https://github.com/apple/swift-nio-extras.git
Fetching https://github.com/IBM-Swift/Kitura-NIO.git
Fetching https://github.com/IBM-Swift/TypeDecoder.git
Fetching https://github.com/IBM-Swift/KituraContracts.git
Fetching https://github.com/IBM-Swift/Kitura-TemplateEngine.git
Fetching https://github.com/IBM-Swift/BlueSocket.git
Fetching https://github.com/IBM-Swift/LoggerAPI.git
Fetching https://github.com/apple/swift-nio.git
Fetching https://github.com/apple/swift-nio-ssl.git
Fetching https://github.com/IBM-Swift/BlueSSLService.git
Fetching https://github.com/IBM-Swift/Kitura
Fetching https://github.com/apple/swift-log.git
...

[Davidde94]

How're you making the connections?

[mbarnach]

I'm usually using POSTMAN. Let me try to reproduce your setup to see if I get the same behaviour.

[Davidde94]

I'm able to reproduce with postman, I suspect it's something todo with Keep-Alive. Postman is trying to make a new connection on every request, but not terminating connections from previous requests. I'll try and confirm this.

[Davidde94]

This seems suspiciously similar: https://community.postman.com/t/terminate-connection/2351

[Davidde94]

Running multiple requests via curl also succeeds. libcurl reuses connections by default.

[mbarnach]

When settings Connection: close on Postman, as suggested by the Postman thread, it still doesn't work.
The fact that curl is re-using the connection means they are not close, right? So it confirms the issue?

[Davidde94]

Alright I think I have a reproduction. Let me have a look and get back to you.

[Davidde94]

Sorry for the delay, it's a busy time here at Apple. I had a look again today and found the source of the issue. It's a minor fix in Kitura-NIO. Details are here: Kitura/Kitura-NIO#241