block_in_place panic on runtime block_on

[driftluo]

version: tokio 0.2.1
feature: ["time", "io-util", "tcp", "dns", "rt-threaded", "blocking"]

fn main() {
    let mut rt = tokio::runtime::Runtime::new().unwrap();
    rt.block_on(async move {
        tokio::task::block_in_place(|| println!("test"))
    })
}

output:

thread 'main' panicked at 'can only call blocking when on Tokio runtime', src/libcore/option.rs:1190:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

The cause of this problem is that the ON_BLOCK of the current thread is not set.

Other workers On_BLOCK is set on here:

tokio/tokio/src/runtime/builder.rs

Line 425 in 632ee50

workers.spawn(&blocking_spawner);

But Runtime current thread not set on block_on function, is it a feature or a bug?

[carllerche]

The panic message needs to be improved... and block_in_place still needs to be generalized.

As a work around, you can do:

    let mut rt = tokio::runtime::Runtime::new().unwrap();
    rt.block_on(async move {
        tokio::spawn(async {
            tokio::task::block_in_place(|| println!("test"))
        }).await
    })

[jebrosen]

Is this a bug or intended behavior (and will it be different between the multithreaded and current_thread runtimes?)

Aside from any 'static or Send requirements, are there any important differences between spawn_blocking or spawn(block_in_place)?

[carllerche]

the main difference is "block_in_place" moves any runtime infrastructure co-located on the thread to a different thread.

It should be possible to implement this for everything except LocalSet.

[leoyvens]

A nested spawn_blocking will also panic, which is annoying, but the workaround fixes it.

[carllerche]

@leotvyens that is unexpected. Would you mind filing a follow up issue?

[dbcfd]

There's sometimes I can get the workaround to work, and other times I can't. The most common time this happens is in #[tokio::test] where I am testing an async function that is using block_in_place and then attempt tokio::spawn(async move { inside my test function. If my dev dependencies does not include blocking and rt-threaded it will fail, but even with adding those, I may still get this error.

[dbcfd]

I also get the failure when moving the tokio::spawn(async move { inside my function I am testing.

[dbcfd]

Had to change my #[tokio::test] to #[tokio::test(threaded_scheduler)] when the other approaches didn't work.

[jgrund]

Can this be considered fixed with #2410 ?

[carllerche]

I believe so 👍