Why does .bytes() hang indefinitely when switching from WiFi to Personal Hospot (or Cellular network)?

[bes]

Given this code:

use reqwest::ClientBuilder;
use std::time::Duration;

#[tokio::main()]
async fn main() {
    println!("Download started");
    let client = ClientBuilder::default()
        // Doesn't seem to help
        .tcp_keepalive(Some(Duration::from_secs(1)))
        // Doesn't seem to help
        .connect_timeout(Duration::from_secs(1))
        .build()
        .unwrap();
    let response = client.get("http://distribution.bbb3d.renderfarming.net/video/mp4/bbb_sunflower_native_60fps_stereo_abl.mp4").send().await.unwrap();

    // Hangs here as soon as switching WiFi network, and
    // then resumes when switching back.
    // Confirm using a network monitor, e.b. bmon
    let bytes = response.bytes().await.unwrap();
    println!("Download finished, {}", bytes.len());
}

Running the following way:

macOS

• Connect to home WiFi
• cargo run
• Switch to "Mobile hotspot" functionality in macOS

iOS

• Connect to home WiFi
• Run the above code in an iOS app (hard mode)
• Switch WiFi "off" -> Cellular connection

Then both macOS and iOS will hang indefinitely until I switch back to the original WiFi connection, at which point they will resume.

In my case, this is unexpected and unwanted. How can I make the response fail as soon as possible when this happens, so that I can retry with the new connection type?

[bes]

I now believe this is unavoidable and is caused by the network stack on macOS / iOS.

I have started another discussion for exploring if this can be mitigated through reqwest: #1776