vendor/futures-executor/src/enter.rs

   1 use std::cell::Cell;
   2 use std::fmt;
   3
   4 thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
   5
   6 /// Represents an executor context.
   7 ///
   8 /// For more details, see [`enter` documentation](enter()).
   9 pub struct Enter {
  10     _priv: (),
  11 }
  12
  13 /// An error returned by `enter` if an execution scope has already been
  14 /// entered.
  15 pub struct EnterError {
  16     _priv: (),
  17 }
  18
  19 impl fmt::Debug for EnterError {
  20     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  21         f.debug_struct("EnterError").finish()
  22     }
  23 }
  24
  25 impl 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
  31 impl 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 tasks, 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.
  55 pub 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
  67 impl fmt::Debug for Enter {
  68     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  69         f.debug_struct("Enter").finish()
  70     }
  71 }
  72
  73 impl Drop for Enter {
  74     fn drop(&mut self) {
  75         ENTERED.with(|c| {
  76             assert!(c.get());
  77             c.set(false);
  78         });
  79     }
  80 }