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
172
173
174
175
176
177
178
|
use crate::SpawnError;
/// A value that spawns futures of a specific type.
///
/// The trait is generic over `T`: the type of future that can be spawened. This
/// is useful for implementing an executor that is only able to spawn a specific
/// type of future.
///
/// The [`spawn`] function is used to submit the future to the executor. Once
/// submitted, the executor takes ownership of the future and becomes
/// responsible for driving the future to completion.
///
/// This trait is useful as a bound for applications and libraries in order to
/// be generic over futures that are `Send` vs. `!Send`.
///
/// # Examples
///
/// Consider a function that provides an API for draining a `Stream` in the
/// background. To do this, a task must be spawned to perform the draining. As
/// such, the function takes a stream and an executor on which the background
/// task is spawned.
///
/// [`spawn`]: TypedExecutor::spawn
/// ```
/// use tokio::executor::TypedExecutor;
/// use tokio::sync::oneshot;
///
/// use futures_core::{ready, Stream};
/// use std::future::Future;
/// use std::pin::Pin;
/// use std::task::{Context, Poll};
///
/// async fn drain<T, E>(stream: T, executor: &mut E)
/// where
/// T: Stream + Unpin,
/// E: TypedExecutor<Drain<T>>
/// {
/// let (tx, rx) = oneshot::channel();
///
/// executor.spawn(Drain {
/// stream,
/// tx: Some(tx),
/// }).unwrap();
///
/// rx.await.unwrap()
/// }
///
/// // The background task
/// pub struct Drain<T> {
/// stream: T,
/// tx: Option<oneshot::Sender<()>>,
/// }
///
/// impl<T: Stream + Unpin> Future for Drain<T> {
/// type Output = ();
///
/// fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
/// loop {
/// let item = ready!(
/// Pin::new(&mut self.stream).poll_next(cx)
/// );
///
/// if item.is_none() { break; }
/// }
///
/// let _ = self.tx.take().unwrap().send(()).map_err(|_| ());
/// Poll::Ready(())
/// }
/// }
/// ```
///
/// By doing this, the `drain` fn can accept a stream that is `!Send` as long as
/// the supplied executor is able to spawn `!Send` types.
pub trait TypedExecutor<T> {
/// Spawns a future to run on this executor.
///
/// `future` is passed to the executor, which will begin running it. The
/// executor takes ownership of the future and becomes responsible for
/// driving the future to completion.
///
/// # Panics
///
/// Implementations are encouraged to avoid panics. However, panics are
/// permitted and the caller should check the implementation specific
/// documentation for more details on possible panics.
///
/// # Examples
///
/// ```rust
/// use tokio_executor::TypedExecutor;
///
/// use std::future::Future;
/// use std::pin::Pin;
/// use std::task::{Context, Poll};
///
/// fn example<T>(my_executor: &mut T)
/// where
/// T: TypedExecutor<MyFuture>,
/// {
/// my_executor.spawn(MyFuture).unwrap();
/// }
///
/// struct MyFuture;
///
/// impl Future for MyFuture {
/// type Output = ();
///
/// fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<()> {
/// println!("running on the executor");
/// Poll::Ready(())
/// }
/// }
/// ```
fn spawn(&mut self, future: T) -> Result<(), SpawnError>;
/// Provides a best effort **hint** to whether or not `spawn` will succeed.
///
/// This function may return both false positives **and** false negatives.
/// If `status` returns `Ok`, then a call to `spawn` will *probably*
/// succeed, but may fail. If `status` returns `Err`, a call to `spawn` will
/// *probably* fail, but may succeed.
///
/// This allows a caller to avoid creating the task if the call to `spawn`
/// has a high likelihood of failing.
///
/// # Panics
///
/// This function must not panic. Implementers must ensure that panics do
/// not happen.
///
/// # Examples
///
/// ```rust
/// use tokio_executor::TypedExecutor;
///
/// use std::future::Future;
/// use std::pin::Pin;
/// use std::task::{Context, Poll};
///
/// fn example<T>(my_executor: &mut T)
/// where
/// T: TypedExecutor<MyFuture>,
/// {
/// if my_executor.status().is_ok() {
/// my_executor.spawn(MyFuture).unwrap();
/// } else {
/// println!("the executor is not in a good state");
/// }
/// }
///
/// struct MyFuture;
///
/// impl Future for MyFuture {
/// type Output = ();
///
/// fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<()> {
/// println!("running on the executor");
/// Poll::Ready(())
/// }
/// }
/// ```
fn status(&self) -> Result<(), SpawnError> {
Ok(())
}
}
impl<E, T> TypedExecutor<T> for Box<E>
where
E: TypedExecutor<T>,
{
fn spawn(&mut self, future: T) -> Result<(), SpawnError> {
(**self).spawn(future)
}
fn status(&self) -> Result<(), SpawnError> {
(**self).status()
}
}
|
