1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140

// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

use std::{
	sync::{
		atomic::{AtomicBool, Ordering},
		Arc,
	},
	time::Duration,
};

/// An error signifying that a task has been cancelled due to a timeout.
#[derive(Debug)]
pub struct Timeout;

impl std::error::Error for Timeout {}
impl std::fmt::Display for Timeout {
	fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
		fmt.write_str("task has been running too long")
	}
}

/// A handle which can be used to check whether the task has been cancelled due to a timeout.
#[repr(transparent)]
pub struct IsTimedOut(Arc<AtomicBool>);

impl IsTimedOut {
	#[must_use]
	pub fn check_if_timed_out(&self) -> std::result::Result<(), Timeout> {
		if self.0.load(Ordering::Relaxed) {
			Err(Timeout)
		} else {
			Ok(())
		}
	}
}

/// An error for a task which either panicked, or has been cancelled due to a timeout.
#[derive(Debug)]
pub enum SpawnWithTimeoutError {
	JoinError(tokio::task::JoinError),
	Timeout,
}

impl std::error::Error for SpawnWithTimeoutError {}
impl std::fmt::Display for SpawnWithTimeoutError {
	fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
		match self {
			SpawnWithTimeoutError::JoinError(error) => error.fmt(fmt),
			SpawnWithTimeoutError::Timeout => Timeout.fmt(fmt),
		}
	}
}

struct CancelOnDrop(Arc<AtomicBool>);
impl Drop for CancelOnDrop {
	fn drop(&mut self) {
		self.0.store(true, Ordering::Relaxed)
	}
}

/// Spawns a new blocking task with a given `timeout`.
///
/// The `callback` should continuously call [`IsTimedOut::check_if_timed_out`],
/// which will return an error once the task runs for longer than `timeout`.
///
/// If `timeout` is `None` then this works just as a regular `spawn_blocking`.
pub async fn spawn_blocking_with_timeout<R>(
	timeout: Option<Duration>,
	callback: impl FnOnce(IsTimedOut) -> std::result::Result<R, Timeout> + Send + 'static,
) -> Result<R, SpawnWithTimeoutError>
where
	R: Send + 'static,
{
	let is_timed_out_arc = Arc::new(AtomicBool::new(false));
	let is_timed_out = IsTimedOut(is_timed_out_arc.clone());
	let _cancel_on_drop = CancelOnDrop(is_timed_out_arc);
	let task = tokio::task::spawn_blocking(move || callback(is_timed_out));

	let result = if let Some(timeout) = timeout {
		tokio::select! {
			// Shouldn't really matter, but make sure the task is polled before the timeout,
			// in case the task finishes after the timeout and the timeout is really short.
			biased;

			task_result = task => task_result,
			_ = tokio::time::sleep(timeout) => Ok(Err(Timeout))
		}
	} else {
		task.await
	};

	match result {
		Ok(Ok(result)) => Ok(result),
		Ok(Err(Timeout)) => Err(SpawnWithTimeoutError::Timeout),
		Err(error) => Err(SpawnWithTimeoutError::JoinError(error)),
	}
}

#[cfg(test)]
mod tests {
	use super::*;

	#[tokio::test]
	async fn spawn_blocking_with_timeout_works() {
		let task: Result<(), SpawnWithTimeoutError> =
			spawn_blocking_with_timeout(Some(Duration::from_millis(100)), |is_timed_out| {
				std::thread::sleep(Duration::from_millis(200));
				is_timed_out.check_if_timed_out()?;
				unreachable!();
			})
			.await;

		assert_matches::assert_matches!(task, Err(SpawnWithTimeoutError::Timeout));

		let task = spawn_blocking_with_timeout(Some(Duration::from_millis(100)), |is_timed_out| {
			std::thread::sleep(Duration::from_millis(20));
			is_timed_out.check_if_timed_out()?;
			Ok(())
		})
		.await;

		assert_matches::assert_matches!(task, Ok(()));
	}
}