[SR-14194] URLSession shared - memory leak

[swift-ci]

Previous ID	SR-14194
Radar	rdar://problem/74274732
Original Reporter	telcy (JIRA User)
Type	Bug
Status	Resolved
Resolution	Invalid

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: 39fc7d661ef45654ab81ab331c820df6

Issue Description:

I have a scheduled timer requesting a url every x seconds. For some reason URLSession.shared fills the memory slowly over time and never frees it.

import Foundation
DispatchQueue.global(qos: .default).async {
    let url = URL(string: "https://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Italian_States-Piacenza_1626_2_Doppie.jpg/2880px-Italian_States-Piacenza_1626_2_Doppie.jpg")!
    Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { timer in
        URLSession.shared.dataTask(with: url) { _, _, _ in
            
        }.resume()
    }
    RunLoop.current.run()
}
RunLoop.main.run()

[typesanitizer]

@swift-ci create

[swift-ci]

Comment by Jascha Burmeister (JIRA)

Another example. Memory is growing over time and only gets freed after completing the loop.

import Foundation


let url = URL(string: "https://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Italian_States-Piacenza_1626_2_Doppie.jpg/2880px-Italian_States-Piacenza_1626_2_Doppie.jpg")!


for i in 1...200 {
    autoreleasepool {
        print(i)
        URLSession.shared.dataTask(with: url) { _, _, _ in


        }.resume()
        sleep(1)
    }
}

RunLoop.main.run()

[swift-ci]

Comment by Jascha Burmeister (JIRA)

I decided to rewrite my application in Rust and surprisingly had the same memory issue. Same code runs fine on Linux. There seems to be a memory leak down the network request stack.

use reqwest;
use std::{thread::sleep, time::Duration};

fn main() {
    // memory grows around 0.1 MB every 10 seconds
    loop {
        let _ = reqwest::blocking::get("https://jsonplaceholder.typicode.com/posts");
        sleep(Duration::from_millis(10));
    }
}

[typesanitizer]

Here's the message I received:

The session needs to be invalidated.

You can check out: https://developer.apple.com/documentation/foundation/urlsession

Important

The session object keeps a strong reference to the delegate until your app exits or explicitly invalidates the session. If you don’t invalidate the session, your app leaks memory until the app terminates.

[typesanitizer]

From what I understand based on the discussion in this reddit thread, the above doesn't quite answer how to fix the leak, since there is no delegate in the example. https://www.reddit.com/r/swift/comments/ms5orj/possibly_memory_leak_in_macos_network_request/

However, since this issue is related to the networking stack and not the open source toolchain (Swift, LLDB, SourceKit etc.), and a separate Feedback has been filed (FB9076036), I'm going to leave this as Resolved instead of reopening it.