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

use std::prelude::v1::*;
use std::cell::Cell;
use std::fmt;

thread_local!(static ENTERED: Cell<bool> = Cell::new(false));

/// Represents an executor context.
///
/// For more details, see [`enter` documentation](enter()).
pub struct Enter {
    _a: ()
}

/// An error returned by `enter` if an execution scope has already been
/// entered.
#[derive(Debug)]
pub struct EnterError {
    _a: (),
}

/// Marks the current thread as being within the dynamic extent of an
/// executor.
///
/// Executor implementations should call this function before beginning to
/// execute a tasks, and drop the returned [`Enter`](Enter) value after
/// completing task execution:
///
/// ```rust
/// # extern crate futures;
/// # use futures::executor::enter;
///
/// # fn main() {
/// let enter = enter().expect("...");
/// /* run task */
/// drop(enter);
/// # }
/// ```
///
/// Doing so ensures that executors aren't
/// accidentally invoked in a nested fashion.
///
/// # Error
///
/// Returns an error if the current thread is already marked, in which case the
/// caller should panic with a tailored error message.
pub fn enter() -> Result<Enter, EnterError> {
    ENTERED.with(|c| {
        if c.get() {
            Err(EnterError { _a: () })
        } else {
            c.set(true);

            Ok(Enter { _a: () })
        }
    })
}

impl fmt::Debug for Enter {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Enter").finish()
    }
}

impl Drop for Enter {
    fn drop(&mut self) {
        ENTERED.with(|c| {
            assert!(c.get());
            c.set(false);
        });
    }
}