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
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171

use futures::future::BoxFuture;
use std::collections::HashMap;
use std::collections::HashSet;
use std::fmt::Debug;
use std::hash::Hash;
use std::sync::Arc;
use tokio::sync::oneshot;
use tokio::sync::{RwLock, RwLockWriteGuard};
use tokio::{select, task};
use tracing::debug;
use uuid::Uuid;

#[derive(Clone)]
pub struct TaskTracker<K, R>
where
    K: Clone + Eq + Hash + Sync + Send + Debug + 'static,
    R: Sync + Send + std::fmt::Debug + 'static,
{
    inner: Arc<RwLock<Inner<K, R>>>,
}

#[derive(Debug)]
pub enum TaskResult<R>
where
    R: Sync + Send + std::fmt::Debug + 'static,
{
    Done(R),
    Cancelled,
    JoinError(task::JoinError),
}

pub struct InnerTask<R>
where
    R: Sync + Send + std::fmt::Debug + 'static,
{
    id: Uuid,
    stop_tx: oneshot::Sender<()>,
    join_handle: task::JoinHandle<TaskResult<R>>,
}

impl<R> InnerTask<R>
where
    R: Sync + Send + std::fmt::Debug + 'static,
{
    async fn cancel_and_wait(self) -> TaskResult<R> {
        debug!(task_id = %self.id, "waiting for task to finish");
        self.stop_tx.send(()).unwrap();
        match self.join_handle.await {
            Ok(task_result) => task_result,
            Err(join_error) => TaskResult::JoinError(join_error),
        }
    }

    async fn wait(self) -> TaskResult<R> {
        match self.join_handle.await {
            Ok(task_result) => task_result,
            Err(join_error) => TaskResult::JoinError(join_error),
        }
    }
}

pub type Inner<K, R> = HashMap<K, InnerTask<R>>;

impl<K, R> TaskTracker<K, R>
where
    K: Clone + Eq + Hash + Sync + Send + Debug + 'static,
    R: Sync + Send + std::fmt::Debug + 'static,
{
    pub fn new() -> Self {
        Self {
            inner: Default::default(),
        }
    }

    pub async fn add(&self, key: K, task: BoxFuture<'static, R>) -> Option<TaskResult<R>> {
        let prev_task = {
            let mut inner = self.inner.write().await;
            self.add_inner(&mut inner, key.clone(), task)
        };
        if let Some(prev_task) = prev_task {
            Some(prev_task.cancel_and_wait().await)
        } else {
            None
        }
    }

    fn add_inner<'a>(
        &'a self,
        inner: &mut RwLockWriteGuard<'a, Inner<K, R>>,
        key: K,
        task: BoxFuture<'static, R>,
    ) -> Option<InnerTask<R>> {
        let task_id = Uuid::new_v4();
        let prev_task = Self::remove_inner(inner, &key);
        let (stop_tx, stop_rx) = oneshot::channel::<()>();
        debug!(?key, %task_id, "adding new task");
        inner.insert(
            key.clone(),
            InnerTask {
                id: task_id,
                stop_tx,
                join_handle: task::spawn(async move {
                    debug!(?key, %task_id, "task started");
                    let result = select! {
                        task_result = task => {
                            debug!(?key, %task_id, ?task_result, "task finished");
                            TaskResult::Done(task_result)
                        },
                        _ = stop_rx => {
                            debug!(?key, %task_id, "task cancelled");
                            TaskResult::Cancelled
                        },
                    };
                    result
                }),
            },
        );
        prev_task
    }

    pub async fn remove(&self, key: &K) -> Option<TaskResult<R>> {
        let prev_task = {
            let mut inner = self.inner.write().await;
            Self::remove_inner(&mut inner, key)
        };
        if let Some(prev_task) = prev_task {
            Some(prev_task.cancel_and_wait().await)
        } else {
            None
        }
    }

    fn remove_inner(
        inner: &mut RwLockWriteGuard<'_, Inner<K, R>>,
        key: &K,
    ) -> Option<InnerTask<R>> {
        inner.remove(key).map(|inner_task| {
            debug!(?key, %inner_task.id, "removed previous task");
            inner_task
        })
    }

    pub async fn apply<S, F>(&self, keys: S, create_task: F)
    where
        S: Iterator<Item = K>,
        F: Fn(&K) -> BoxFuture<'static, R>,
    {
        let mut inner = self.inner.write().await;
        let old_keys: HashSet<_> = inner.keys().cloned().collect();
        let new_keys: HashSet<_> = keys.collect();

        for to_remove in old_keys.difference(&new_keys) {
            Self::remove_inner(&mut inner, to_remove);
        }

        for to_add in new_keys.difference(&old_keys) {
            self.add_inner(&mut inner, to_add.to_owned(), create_task(to_add));
        }
    }

    pub async fn wait_for_tasks(&self) -> HashMap<K, TaskResult<R>> {
        let mut results = HashMap::new();
        debug!("waiting for all tasks to finish");
        let mut inner = self.inner.write().await;
        for (key, inner_task) in inner.drain() {
            let result = inner_task.wait().await;
            results.insert(key, result);
        }
        results
    }
}