unreachable triggered by Runtime::block_on and block_on_all when future panics!

[FSMaxB]

Version

tokio 0.1.21 (also latest master as of now)

Platform

Linux hostname 5.1.9-arch1-1-ARCH #1 SMP PREEMPT Tue Jun 11 16:18:09 UTC 2019 x86_64 GNU/Linux

Description

When running a future that panics on a runtime with block_on or block_on_all, it will trigger an unreachable macro.

It is documented behavior, that those functions panic if the provided future panics, but it shouldn't panic with unreachable since that code is clearly not unreachable.

The bug is here:

tokio/tokio/src/runtime/threadpool/mod.rs

Lines 253 to 263 in d4adeee

	pub fn block_on<F, R, E>(&self, future: F) -> Result<R, E>
	where
	F: Send + 'static + Future<Item = R, Error = E>,
	R: Send + 'static,
	E: Send + 'static,
	{
	let mut entered = enter().expect("nested block_on");
	let (tx, rx) = futures::sync::oneshot::channel();
	self.spawn(future.then(move |r| tx.send(r).map_err(|_| unreachable!())));
	entered.block_on(rx).unwrap()
	}

tokio/tokio/src/runtime/threadpool/mod.rs

Lines 279 to 295 in d4adeee

	pub fn block_on_all<F, R, E>(self, future: F) -> Result<R, E>
	where
	F: Send + 'static + Future<Item = R, Error = E>,
	R: Send + 'static,
	E: Send + 'static,
	{
	let mut entered = enter().expect("nested block_on_all");
	let (tx, rx) = futures::sync::oneshot::channel();
	self.spawn(future.then(move |r| tx.send(r).map_err(|_| unreachable!())));
	let block = rx
	.map_err(|_| unreachable!())
	.and_then(move |r| {
	self.shutdown_on_idle()
	.map(move |()| r)
	});
	entered.block_on(block).unwrap()
	}

The unreachable macro should be replaced with something like panic!("The provided future paniced") or something like that.

Code to reproduce:

use futures::prelude::*;

fn main() {
    let runtime = tokio::runtime::Runtime::new().unwrap();
    let future = futures::future::ok::<_,String>(())
        .map(|_| panic!());
    runtime.block_on_all(future).unwrap();
}

Or the same with block_on instead of block_on_all.

The unreachable get's triggered because tx.send errored because the receiving end of the oneshot channel was dropped because of the panic btw.

[carllerche]

Makes sense to me. Would you be able to provide a PR to update the panic message? Thanks.

[FSMaxB]

#1166

[tobz]

This was fixed in #1166. Thanks again for reporting/fixing it. :)