futures_executor/
enter.rs

1use std::cell::Cell;
2use std::fmt;
3
4std::thread_local!(static ENTERED: Cell<bool> = const { Cell::new(false) });
5
6/// Represents an executor context.
7///
8/// For more details, see [`enter` documentation](enter()).
9pub struct Enter {
10    _priv: (),
11}
12
13/// An error returned by `enter` if an execution scope has already been
14/// entered.
15pub struct EnterError {
16    _priv: (),
17}
18
19impl fmt::Debug for EnterError {
20    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
21        f.debug_struct("EnterError").finish()
22    }
23}
24
25impl fmt::Display for EnterError {
26    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
27        write!(f, "an execution scope has already been entered")
28    }
29}
30
31impl std::error::Error for EnterError {}
32
33/// Marks the current thread as being within the dynamic extent of an
34/// executor.
35///
36/// Executor implementations should call this function before beginning to
37/// execute a task, and drop the returned [`Enter`] value after completing
38/// task execution:
39///
40/// ```
41/// use futures::executor::enter;
42///
43/// let enter = enter().expect("...");
44/// /* run task */
45/// drop(enter);
46/// ```
47///
48/// Doing so ensures that executors aren't
49/// accidentally invoked in a nested fashion.
50///
51/// # Error
52///
53/// Returns an error if the current thread is already marked, in which case the
54/// caller should panic with a tailored error message.
55pub fn enter() -> Result<Enter, EnterError> {
56    ENTERED.with(|c| {
57        if c.get() {
58            Err(EnterError { _priv: () })
59        } else {
60            c.set(true);
61
62            Ok(Enter { _priv: () })
63        }
64    })
65}
66
67impl fmt::Debug for Enter {
68    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
69        f.debug_struct("Enter").finish()
70    }
71}
72
73impl Drop for Enter {
74    fn drop(&mut self) {
75        ENTERED.with(|c| {
76            assert!(c.get());
77            c.set(false);
78        });
79    }
80}
futures_executor/enter.rs

futures_executor/
enter.rs
