tokio-macros 1.4.0 blocks usage of tokio::task::block_in_place even in multi_thread flavored tests

[tadman]

Version

tm_block_in_place v0.1.0
├── tokio v1.12.0
│   ├── bytes v1.1.0
│   ├── libc v0.2.103
│   ├── memchr v2.4.1
│   ├── mio v0.7.13
│   │   ├── libc v0.2.103
│   │   └── log v0.4.14
│   │       └── cfg-if v1.0.0
│   ├── num_cpus v1.13.0
│   │   └── libc v0.2.103
│   ├── once_cell v1.8.0
│   ├── parking_lot v0.11.2
│   │   ├── instant v0.1.11
│   │   │   └── cfg-if v1.0.0
│   │   ├── lock_api v0.4.5
│   │   │   └── scopeguard v1.1.0
│   │   └── parking_lot_core v0.8.5
│   │       ├── cfg-if v1.0.0
│   │       ├── instant v0.1.11 (*)
│   │       ├── libc v0.2.103
│   │       └── smallvec v1.7.0
│   ├── pin-project-lite v0.2.7
│   ├── signal-hook-registry v1.4.0
│   │   └── libc v0.2.103
│   └── tokio-macros v1.3.0 (proc-macro)
│       ├── proc-macro2 v1.0.29
│       │   └── unicode-xid v0.2.2
│       ├── quote v1.0.9
│       │   └── proc-macro2 v1.0.29 (*)
│       └── syn v1.0.77
│           ├── proc-macro2 v1.0.29 (*)
│           ├── quote v1.0.9 (*)
│           └── unicode-xid v0.2.2
│   [build-dependencies]
│   └── autocfg v1.0.1
└── tokio-macros v1.4.0 (proc-macro) (*)

Platform

Reproduced on both macOS (11.15.2) and Linux:

Darwin hammer 20.6.0 Darwin Kernel Version 20.6.0: Wed Jun 23 00:26:27 PDT 2021; root:xnu-7195.141.2~5/RELEASE_ARM64_T8101 arm64
Linux xa 5.4.0-81-generic #91-Ubuntu SMP Thu Jul 15 19:09:17 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Description

Under 1.3.0 and prior versions of tokio-macros the tests execute correctly. WIth 1.4.0 the following result occurs:

thread 'tests::dont_panic' panicked at 'can call blocking only when running on the multi-threaded runtime', .../.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.12.0/src/runtime/thread_pool/worker.rs:286:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Even though this is explicitly within a "multi_thread" runtime.

This is caused by any use of tokio::task::block_in_place().

Example test that fails:

#[cfg(test)]
mod tests {
    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
    async fn dont_panic() {
        tokio::task::block_in_place(move || { });
    }
}

This executes correctly in 1.3.0, but fails in 1.4.0. There should be multiple threads available as per the test definition, but this does not appear to be the case.

[Noah-Kennedy]

I suspect #4027 may be to blame.

[tadman]

It's worth noting that this doesn't just happen in tests, it seems to impact any use of the #[tokio] macros, but it seems super easy to reproduce within tests. I'm also seeing it within a main function like:

#[tokio::main(worker_threads = 2)]
async fn main() {
    tokio::task::block_in_place(move || { });
}

I found this while trying to figure out why tokio-diesel began to malfunction.

The project pivoted to block_in_place for a number of reasons expressed here.

[Noah-Kennedy]

I ran a bisect, #4027 introduced this issue.

[Darksonn]

Thanks, I am yanking tokio-macros 1.4.0.