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use crate::loom::thread::AccessError;
use crate::runtime::coop;
use std::cell::Cell;
#[cfg(any(feature = "rt", feature = "macros"))]
use crate::util::rand::{FastRand, RngSeed};
cfg_rt! {
use crate::runtime::{scheduler, task::Id, Defer};
use std::cell::RefCell;
use std::marker::PhantomData;
use std::time::Duration;
}
struct Context {
/// Uniquely identifies the current thread
#[cfg(feature = "rt")]
thread_id: Cell<Option<ThreadId>>,
/// Handle to the runtime scheduler running on the current thread.
#[cfg(feature = "rt")]
handle: RefCell<Option<scheduler::Handle>>,
#[cfg(feature = "rt")]
current_task_id: Cell<Option<Id>>,
/// Tracks if the current thread is currently driving a runtime.
/// Note, that if this is set to "entered", the current scheduler
/// handle may not reference the runtime currently executing. This
/// is because other runtime handles may be set to current from
/// within a runtime.
#[cfg(feature = "rt")]
runtime: Cell<EnterRuntime>,
/// Yielded task wakers are stored here and notified after resource drivers
/// are polled.
#[cfg(feature = "rt")]
defer: RefCell<Option<Defer>>,
#[cfg(any(feature = "rt", feature = "macros"))]
rng: FastRand,
/// Tracks the amount of "work" a task may still do before yielding back to
/// the sheduler
budget: Cell<coop::Budget>,
}
tokio_thread_local! {
static CONTEXT: Context = {
Context {
#[cfg(feature = "rt")]
thread_id: Cell::new(None),
/// Tracks the current runtime handle to use when spawning,
/// accessing drivers, etc...
#[cfg(feature = "rt")]
handle: RefCell::new(None),
#[cfg(feature = "rt")]
current_task_id: Cell::new(None),
/// Tracks if the current thread is currently driving a runtime.
/// Note, that if this is set to "entered", the current scheduler
/// handle may not reference the runtime currently executing. This
/// is because other runtime handles may be set to current from
/// within a runtime.
#[cfg(feature = "rt")]
runtime: Cell::new(EnterRuntime::NotEntered),
#[cfg(feature = "rt")]
defer: RefCell::new(None),
#[cfg(any(feature = "rt", feature = "macros"))]
rng: FastRand::new(RngSeed::new()),
budget: Cell::new(coop::Budget::unconstrained()),
}
}
}
#[cfg(any(feature = "macros", all(feature = "sync", feature = "rt")))]
pub(crate) fn thread_rng_n(n: u32) -> u32 {
CONTEXT.with(|ctx| ctx.rng.fastrand_n(n))
}
pub(super) fn budget<R>(f: impl FnOnce(&Cell<coop::Budget>) -> R) -> Result<R, AccessError> {
CONTEXT.try_with(|ctx| f(&ctx.budget))
}
cfg_rt! {
use crate::runtime::{ThreadId, TryCurrentError};
use std::fmt;
pub(crate) fn thread_id() -> Result<ThreadId, AccessError> {
CONTEXT.try_with(|ctx| {
match ctx.thread_id.get() {
Some(id) => id,
None => {
let id = ThreadId::next();
ctx.thread_id.set(Some(id));
id
}
}
})
}
#[derive(Debug, Clone, Copy)]
#[must_use]
pub(crate) enum EnterRuntime {
/// Currently in a runtime context.
#[cfg_attr(not(feature = "rt"), allow(dead_code))]
Entered { allow_block_in_place: bool },
/// Not in a runtime context **or** a blocking region.
NotEntered,
}
#[derive(Debug)]
#[must_use]
pub(crate) struct SetCurrentGuard {
old_handle: Option<scheduler::Handle>,
old_seed: RngSeed,
}
/// Guard tracking that a caller has entered a runtime context.
#[must_use]
pub(crate) struct EnterRuntimeGuard {
/// Tracks that the current thread has entered a blocking function call.
pub(crate) blocking: BlockingRegionGuard,
#[allow(dead_code)] // Only tracking the guard.
pub(crate) handle: SetCurrentGuard,
/// If true, then this is the root runtime guard. It is possible to nest
/// runtime guards by using `block_in_place` between the calls. We need
/// to track the root guard as this is the guard responsible for freeing
/// the deferred task queue.
is_root: bool,
}
/// Guard tracking that a caller has entered a blocking region.
#[must_use]
pub(crate) struct BlockingRegionGuard {
_p: PhantomData<RefCell<()>>,
}
pub(crate) struct DisallowBlockInPlaceGuard(bool);
pub(crate) fn set_current_task_id(id: Option<Id>) -> Option<Id> {
CONTEXT.try_with(|ctx| ctx.current_task_id.replace(id)).unwrap_or(None)
}
pub(crate) fn current_task_id() -> Option<Id> {
CONTEXT.try_with(|ctx| ctx.current_task_id.get()).unwrap_or(None)
}
pub(crate) fn try_current() -> Result<scheduler::Handle, TryCurrentError> {
match CONTEXT.try_with(|ctx| ctx.handle.borrow().clone()) {
Ok(Some(handle)) => Ok(handle),
Ok(None) => Err(TryCurrentError::new_no_context()),
Err(_access_error) => Err(TryCurrentError::new_thread_local_destroyed()),
}
}
/// Sets this [`Handle`] as the current active [`Handle`].
///
/// [`Handle`]: crate::runtime::scheduler::Handle
pub(crate) fn try_set_current(handle: &scheduler::Handle) -> Option<SetCurrentGuard> {
CONTEXT.try_with(|ctx| ctx.set_current(handle)).ok()
}
/// Marks the current thread as being within the dynamic extent of an
/// executor.
#[track_caller]
pub(crate) fn enter_runtime(handle: &scheduler::Handle, allow_block_in_place: bool) -> EnterRuntimeGuard {
if let Some(enter) = try_enter_runtime(handle, allow_block_in_place) {
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.
fn try_enter_runtime(handle: &scheduler::Handle, allow_block_in_place: bool) -> Option<EnterRuntimeGuard> {
CONTEXT.with(|c| {
if c.runtime.get().is_entered() {
None
} else {
// Set the entered flag
c.runtime.set(EnterRuntime::Entered { allow_block_in_place });
// Initialize queue to track yielded tasks
let mut defer = c.defer.borrow_mut();
let is_root = if defer.is_none() {
*defer = Some(Defer::new());
true
} else {
false
};
Some(EnterRuntimeGuard {
blocking: BlockingRegionGuard::new(),
handle: c.set_current(handle),
is_root,
})
}
})
}
pub(crate) fn try_enter_blocking_region() -> Option<BlockingRegionGuard> {
CONTEXT.try_with(|c| {
if c.runtime.get().is_entered() {
None
} else {
Some(BlockingRegionGuard::new())
}
// If accessing the thread-local fails, the thread is terminating
// and thread-locals are being destroyed. Because we don't know if
// we are currently in a runtime or not, we default to being
// permissive.
}).unwrap_or_else(|_| Some(BlockingRegionGuard::new()))
}
/// Disallows blocking in the current runtime context until the guard is dropped.
pub(crate) fn disallow_block_in_place() -> DisallowBlockInPlaceGuard {
let reset = CONTEXT.with(|c| {
if let EnterRuntime::Entered {
allow_block_in_place: true,
} = c.runtime.get()
{
c.runtime.set(EnterRuntime::Entered {
allow_block_in_place: false,
});
true
} else {
false
}
});
DisallowBlockInPlaceGuard(reset)
}
pub(crate) fn with_defer<R>(f: impl FnOnce(&mut Defer) -> R) -> Option<R> {
CONTEXT.with(|c| {
let mut defer = c.defer.borrow_mut();
defer.as_mut().map(f)
})
}
impl Context {
fn set_current(&self, handle: &scheduler::Handle) -> SetCurrentGuard {
let rng_seed = handle.seed_generator().next_seed();
let old_handle = self.handle.borrow_mut().replace(handle.clone());
let old_seed = self.rng.replace_seed(rng_seed);
SetCurrentGuard {
old_handle,
old_seed,
}
}
}
impl Drop for SetCurrentGuard {
fn drop(&mut self) {
CONTEXT.with(|ctx| {
*ctx.handle.borrow_mut() = self.old_handle.take();
ctx.rng.replace_seed(self.old_seed.clone());
});
}
}
impl fmt::Debug for EnterRuntimeGuard {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Enter").finish()
}
}
impl Drop for EnterRuntimeGuard {
fn drop(&mut self) {
CONTEXT.with(|c| {
assert!(c.runtime.get().is_entered());
c.runtime.set(EnterRuntime::NotEntered);
if self.is_root {
*c.defer.borrow_mut() = None;
}
});
}
}
impl BlockingRegionGuard {
fn new() -> BlockingRegionGuard {
BlockingRegionGuard { _p: PhantomData }
}
/// 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, AccessError>
where
F: std::future::Future,
{
use crate::runtime::park::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, ()>
where
F: std::future::Future,
{
use crate::runtime::park::CachedParkThread;
use std::task::Context;
use std::task::Poll::Ready;
use std::time::Instant;
let mut park = CachedParkThread::new();
let waker = park.waker().map_err(|_| ())?;
let mut cx = Context::from_waker(&waker);
pin!(f);
let when = Instant::now() + timeout;
loop {
if let Ready(v) = crate::runtime::coop::budget(|| f.as_mut().poll(&mut cx)) {
return Ok(v);
}
let now = Instant::now();
if now >= when {
return Err(());
}
// Wake any yielded tasks before parking in order to avoid
// blocking.
with_defer(|defer| defer.wake());
park.park_timeout(when - now);
}
}
}
impl Drop for DisallowBlockInPlaceGuard {
fn drop(&mut self) {
if self.0 {
// XXX: Do we want some kind of assertion here, or is "best effort" okay?
CONTEXT.with(|c| {
if let EnterRuntime::Entered {
allow_block_in_place: false,
} = c.runtime.get()
{
c.runtime.set(EnterRuntime::Entered {
allow_block_in_place: true,
});
}
})
}
}
}
impl EnterRuntime {
pub(crate) fn is_entered(self) -> bool {
matches!(self, EnterRuntime::Entered { .. })
}
}
}
// 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! {
/// Returns true if in a runtime context.
pub(crate) fn current_enter_context() -> EnterRuntime {
CONTEXT.with(|c| c.runtime.get())
}
pub(crate) fn exit_runtime<F: FnOnce() -> R, R>(f: F) -> R {
// Reset in case the closure panics
struct Reset(EnterRuntime);
impl Drop for Reset {
fn drop(&mut self) {
CONTEXT.with(|c| {
assert!(!c.runtime.get().is_entered(), "closure claimed permanent executor");
c.runtime.set(self.0);
});
}
}
let was = CONTEXT.with(|c| {
let e = c.runtime.get();
assert!(e.is_entered(), "asked to exit when not entered");
c.runtime.set(EnterRuntime::NotEntered);
e
});
let _reset = Reset(was);
// dropping _reset after f() will reset ENTERED
f()
}
}