1use std::cell::Cell;
2use std::fmt;
3
4thread_local!(static ENTERED: Cell<bool> = 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(name:"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`](Enter) value after
38/// completing 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: &Cell| {
57 if c.get() {
58 Err(EnterError { _priv: () })
59 } else {
60 c.set(val: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(name:"Enter").finish()
70 }
71}
72
73impl Drop for Enter {
74 fn drop(&mut self) {
75 ENTERED.with(|c: &Cell| {
76 assert!(c.get());
77 c.set(val:false);
78 });
79 }
80}
81