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|
use std::cell::{Cell, RefCell};
use std::fmt;
use std::marker::PhantomData;
#[derive(Debug, Clone, Copy)]
pub(crate) enum EnterContext {
#[cfg_attr(not(feature = "rt"), allow(dead_code))]
Entered {
allow_blocking: bool,
},
NotEntered,
}
impl EnterContext {
pub(crate) fn is_entered(self) -> bool {
matches!(self, EnterContext::Entered { .. })
}
}
thread_local!(static ENTERED: Cell<EnterContext> = Cell::new(EnterContext::NotEntered));
/// Represents an executor context.
pub(crate) struct Enter {
_p: PhantomData<RefCell<()>>,
}
cfg_rt! {
use crate::park::thread::ParkError;
use std::time::Duration;
/// Marks the current thread as being within the dynamic extent of an
/// executor.
pub(crate) fn enter(allow_blocking: bool) -> Enter {
if let Some(enter) = try_enter(allow_blocking) {
return enter;
}
panic!(
"Cannot start a runtime from within a runtime. This happens \
because a function (like `block_on`) attempted to block the \
current thread while the thread is being used to drive \
asynchronous tasks."
);
}
/// Tries to enter a runtime context, returns `None` if already in a runtime
/// context.
pub(crate) fn try_enter(allow_blocking: bool) -> Option<Enter> {
ENTERED.with(|c| {
if c.get().is_entered() {
None
} else {
c.set(EnterContext::Entered { allow_blocking });
Some(Enter { _p: PhantomData })
}
})
}
}
// Forces the current "entered" state to be cleared while the closure
// is executed.
//
// # Warning
//
// This is hidden for a reason. Do not use without fully understanding
// executors. Misusing can easily cause your program to deadlock.
cfg_rt_multi_thread! {
pub(crate) fn exit<F: FnOnce() -> R, R>(f: F) -> R {
// Reset in case the closure panics
struct Reset(EnterContext);
impl Drop for Reset {
fn drop(&mut self) {
ENTERED.with(|c| {
assert!(!c.get().is_entered(), "closure claimed permanent executor");
c.set(self.0);
});
}
}
let was = ENTERED.with(|c| {
let e = c.get();
assert!(e.is_entered(), "asked to exit when not entered");
c.set(EnterContext::NotEntered);
e
});
let _reset = Reset(was);
// dropping _reset after f() will reset ENTERED
f()
}
}
cfg_rt! {
/// Disallow blocking in the current runtime context until the guard is dropped.
pub(crate) fn disallow_blocking() -> DisallowBlockingGuard {
let reset = ENTERED.with(|c| {
if let EnterContext::Entered {
allow_blocking: true,
} = c.get()
{
c.set(EnterContext::Entered {
allow_blocking: false,
});
true
} else {
false
}
});
DisallowBlockingGuard(reset)
}
pub(crate) struct DisallowBlockingGuard(bool);
impl Drop for DisallowBlockingGuard {
fn drop(&mut self) {
if self.0 {
// XXX: Do we want some kind of assertion here, or is "best effort" okay?
ENTERED.with(|c| {
if let EnterContext::Entered {
allow_blocking: false,
} = c.get()
{
c.set(EnterContext::Entered {
allow_blocking: true,
});
}
})
}
}
}
}
cfg_rt_multi_thread! {
/// Returns true if in a runtime context.
pub(crate) fn context() -> EnterContext {
ENTERED.with(|c| c.get())
}
}
cfg_rt! {
impl Enter {
/// Blocks the thread on the specified future, returning the value with
/// which that future completes.
pub(crate) fn block_on<F>(&mut self, f: F) -> Result<F::Output, ParkError>
where
F: std::future::Future,
{
use crate::park::thread::CachedParkThread;
let mut park = CachedParkThread::new();
park.block_on(f)
}
/// Blocks the thread on the specified future for **at most** `timeout`
///
/// If the future completes before `timeout`, the result is returned. If
/// `timeout` elapses, then `Err` is returned.
pub(crate) fn block_on_timeout<F>(&mut self, f: F, timeout: Duration) -> Result<F::Output, ParkError>
where
F: std::future::Future,
{
use crate::park::Park;
use crate::park::thread::CachedParkThread;
use std::task::Context;
use std::task::Poll::Ready;
use std::time::Instant;
let mut park = CachedParkThread::new();
let waker = park.get_unpark()?.into_waker();
let mut cx = Context::from_waker(&waker);
pin!(f);
let when = Instant::now() + timeout;
loop {
if let Ready(v) = crate::coop::budget(|| f.as_mut().poll(&mut cx)) {
return Ok(v);
}
let now = Instant::now();
if now >= when {
return Err(());
}
park.park_timeout(when - now)?;
}
}
}
}
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().is_entered());
c.set(EnterContext::NotEntered);
});
}
}
|
