LocalSet drop impl panics when dropped with unfinished futures

[swlynch99]

Version

cargo tree | grep tokio
└── tokio v0.2.1

Platform

uname -a
Darwin tw-mbp-slynch 18.7.0 Darwin Kernel Version 18.7.0: Sat Oct 12 00:02:19 PDT 2019; root:xnu-4903.278.12~1/RELEASE_X86_64 x86_64

rustc --version
rustc 1.39.0 (4560ea788 2019-11-04

Description

The drop impl for tokio::task::LocalSet panics with

internal error: entered unreachable code: tasks should only be completed locally

when it is dropped while it still contains an incomplete future. Instead, I would expect this to just drop all the incomplete futures and continue normally.

The following code triggers this issue

use std::time::Duration;

use tokio::time::delay_for;
use tokio::runtime::Builder;
use tokio::task::LocalSet;

async fn run_forever() {
    loop {
        delay_for(Duration::from_secs(3600)).await;
    }
}

async fn run_fast() {
    delay_for(Duration::from_millis(10)).await;
}

fn main() -> std::io::Result<()> {
    let mut rt = Builder::new()
        .enable_time()
        .basic_scheduler()
        .build()?;

    let set = LocalSet::new();
    set.spawn_local(run_forever());
    set.block_on(&mut rt, run_fast());

    Ok(())
}

Cargo.toml

[package]
name = "mcve"
version = "0.1.0"
edition = "2018"

[dependencies.tokio]
version="0.2.1"
features=[ "time", "rt-core", "rt-util" ]

Stack Trace

thread 'main' panicked at 'internal error: entered unreachable code: tasks should only be completed locally', /Users/slynch/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.1/src/task/local.rs:349:9
stack backtrace:
   0:        0x10b53cf65 - std::sys_common::at_exit_imp::push::hbcb9accb9aed3616
   1:        0x10b554be0 - core::fmt::ArgumentV1::show_usize::h5612effbabb12159
   2:        0x10b53b2fb - std::io::Write::write_fmt::h7bd7b6fe8b47c5fb
   3:        0x10b53eb7a - std::panicking::default_hook::{{closure}}::heee79636c241547c
   4:        0x10b53e885 - std::panicking::default_hook::hfcbd07059d15441e
   5:        0x10b53f247 - <std::panicking::begin_panic::PanicPayload<A> as core::panic::BoxMeUp>::get::h2b8b0a58863fcf2c
   6:        0x10b53edbd - std::panicking::continue_panic_fmt::h0e74ab2b215a1401
   7:        0x10b55804e - std::thread::local::fast::Key<T>::try_initialize::hcdc98eb893033f5a
   8:        0x10b526a2f - <tokio::task::local::Scheduler as tokio::task::Schedule>::release::h354af237f36f45f0
                               at /Users/slynch/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.1/src/task/local.rs:349
   9:        0x10b501141 - tokio::task::harness::Harness<T,S>::do_cancel::h34aab5a1c9fefa0e
                               at /Users/slynch/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.1/src/task/harness.rs:391
  10:        0x10b5008bf - tokio::task::harness::Harness<T,S>::cancel::h5dc98c6a2e9acf9f
                               at /Users/slynch/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.1/src/task/harness.rs:342
  11:        0x10b4fe012 - tokio::task::raw::cancel::h713e03278d359f08
                               at /Users/slynch/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.1/src/task/raw.rs:196
  12:        0x10b51a4de - tokio::task::list::OwnedList<T>::shutdown::h819bd7f227889e20
                               at /Users/slynch/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.1/src/task/list.rs:68
  13:        0x10b527666 - <tokio::task::local::Scheduler as core::ops::drop::Drop>::drop::h38a36f038ca65249
                               at /Users/slynch/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.1/src/task/local.rs:490
  14:        0x10b505d55 - core::ptr::real_drop_in_place::hce300fa65c00fc82
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/ptr/mod.rs:175
  15:        0x10b5057ae - core::ptr::real_drop_in_place::h91d23830a1679dd7
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/ptr/mod.rs:175
  16:        0x10b4fc34e - core::ptr::drop_in_place::h9fbdc1f16152cc97
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/ptr/mod.rs:165
  17:        0x10b4fc34e - <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop::h2a5f174f42f1e30a
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/liballoc/rc.rs:1098
  18:        0x10b504f75 - core::ptr::real_drop_in_place::h083d13dc9290de0b
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/ptr/mod.rs:175
  19:        0x10b506075 - core::ptr::real_drop_in_place::heb6700c1e84481fd
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/ptr/mod.rs:175
  20:        0x10b5075e2 - mcve::main::hf863556eb750bbdf
                               at src/main.rs:31
  21:        0x10b4faf76 - std::rt::lang_start::{{closure}}::hf2f3af7d2e40962c
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libstd/rt.rs:64
  22:        0x10b53ec98 - std::panicking::try::do_call::ha1a4587e2a4eb439
  23:        0x10b54068f - panic_unwind::dwarf::eh::read_encoded_pointer::h8a85bd7bb8b1ba5e
  24:        0x10b53f5ee - <std::panicking::begin_panic::PanicPayload<A> as core::panic::BoxMeUp>::get::h2b8b0a58863fcf2c
  25:        0x10b4faf52 - std::rt::lang_start::h1808cc0130c38463
                               at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libstd/rt.rs:64
  26:        0x10b5076c2 - mcve::main::hf863556eb750bbdf

[hawkw]

Thanks for the report! I'll take a look at fixing that.

[hawkw]

PR #1843 should fix this. Thanks again for reporting!