"This is a bug" panic after unwind of invalid Runtime::block_on() call

[wabain]

Version
v1.21.2

Platform
Linux 5.15, Rust Playground

Description

I hit an error where some code had created a current-thread tokio runtime and called Runtime::block_on while already inside an async tokio context. As expected, this triggered a panic with a message explaining that blocking API calls are not allowed inside an async context. However, unless the runtime whose block_on call is dropped during the unwinding, a second panic is triggered when the runtime is dropped. The message for the second panic states it's an internal bug.

I tried this code (playground):

fn main() {
    // initially tested on tokio 1.21.2

    let outer_rt = tokio::runtime::Builder::new_multi_thread().build().unwrap();

    let inner_rt = tokio::runtime::Builder::new_current_thread()
        .build()
        .unwrap();

    outer_rt.block_on(async {
        _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            inner_rt.block_on(async {});
        }));
    });
}

And got these messages:

thread 'main' panicked at 'Cannot start a runtime from within a runtime. This happens because a function (like `block_on`) attempted to block the current thread while the thread is being used to drive asynchronous tasks.', /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:516:26
thread 'main' panicked at 'Oh no! We never placed the Core back, this is a bug!', /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:213:21

With backtraces

thread 'main' panicked at 'Cannot start a runtime from within a runtime. This happens because a function (like `block_on`) attempted to block the current thread while the thread is being used to drive asynchronous tasks.', /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:516:26
stack backtrace:
   0: std::panicking::begin_panic
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/panicking.rs:616:12
   1: tokio::runtime::enter::enter
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/enter.rs:40:9
   2: tokio::runtime::scheduler::current_thread::CoreGuard::block_on::{{closure}}
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:516:26
   3: tokio::runtime::scheduler::current_thread::CoreGuard::enter::{{closure}}
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:595:57
   4: tokio::macros::scoped_tls::ScopedKey<T>::set
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/macros/scoped_tls.rs:61:9
   5: tokio::runtime::scheduler::current_thread::CoreGuard::enter
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:595:27
   6: tokio::runtime::scheduler::current_thread::CoreGuard::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:515:19
   7: tokio::runtime::scheduler::current_thread::CurrentThread::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:161:24
   8: tokio::runtime::Runtime::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/mod.rs:490:46
   9: playground::main::{{closure}}::{{closure}}
             at ./[src/main.rs:12](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021#):13
  10: core::ops::function::FnOnce::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ops/function.rs:248:5
  11: <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/panic/unwind_safe.rs:271:9
  12: std::panicking::try::do_call
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/panicking.rs:492:40
  13: __rust_try
  14: std::panicking::try
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/panicking.rs:456:19
  15: std::panic::catch_unwind
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/panic.rs:137:14
  16: playground::main::{{closure}}
             at ./[src/main.rs:11](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021#):13
  17: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/future/mod.rs:91:19
  18: tokio::park::thread::CachedParkThread::block_on::{{closure}}
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/park/thread.rs:267:54
  19: tokio::coop::with_budget::{{closure}}
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/coop.rs:102:9
  20: std::thread::local::LocalKey<T>::try_with
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/thread/local.rs:445:16
  21: std::thread::local::LocalKey<T>::with
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/thread/local.rs:421:9
  22: tokio::coop::with_budget
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/coop.rs:95:5
  23: tokio::coop::budget
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/coop.rs:72:5
  24: tokio::park::thread::CachedParkThread::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/park/thread.rs:267:31
  25: tokio::runtime::enter::Enter::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/enter.rs:152:13
  26: tokio::runtime::scheduler::multi_thread::MultiThread::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/multi_thread/mod.rs:79:9
  27: tokio::runtime::Runtime::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/mod.rs:492:44
  28: playground::main
             at ./[src/main.rs:10](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021#):5
  29: core::ops::function::FnOnce::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
thread 'main' panicked at 'Oh no! We never placed the Core back, this is a bug!', /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:213:21
stack backtrace:
   0: std::panicking::begin_panic
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/panicking.rs:616:12
   1: <tokio::runtime::scheduler::current_thread::CurrentThread as core::ops::drop::Drop>::drop
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.21.2/src/runtime/scheduler/current_thread.rs:213:21
   2: core::ptr::drop_in_place<tokio::runtime::scheduler::current_thread::CurrentThread>
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ptr/mod.rs:487:1
   3: core::ptr::drop_in_place<tokio::runtime::Kind>
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ptr/mod.rs:487:1
   4: core::ptr::drop_in_place<tokio::runtime::Runtime>
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ptr/mod.rs:487:1
   5: playground::main
             at ./[src/main.rs:15](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021#):1
   6: core::ops::function::FnOnce::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I haven't found a way to reproduce this without API misuse, but ideally I would expect that dropping tokio data structures doesn't trigger a panic, even if panics occurred earlier in processing. Note that the original scenario where I hit this involved multiple threads rather than catch_unwind. In that version there wasn't a reference to the tokio runtime being passed across an explicit AssertUnwindSafe boundary in the user code.

[Darksonn]

Thank you for your report. This is indeed a bug.

[Darksonn]

It seems like this has already been fixed on master in #5163.