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//! Shuttle's implementation of [`std::thread`].
use crate::runtime::execution::ExecutionState;
use crate::runtime::task::TaskId;
use crate::runtime::thread;
use std::marker::PhantomData;
use std::time::Duration;
/// A unique identifier for a running thread
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct ThreadId {
// TODO Should we add an execution id here, like Loom does?
task_id: TaskId,
}
impl From<ThreadId> for usize {
fn from(id: ThreadId) -> usize {
id.task_id.into()
}
}
/// A handle to a thread.
#[derive(Debug, Clone)]
pub struct Thread {
name: Option<String>,
id: ThreadId,
}
impl Thread {
/// Gets the thread's name.
pub fn name(&self) -> Option<&str> {
self.name.as_deref()
}
/// Gets the thread's unique identifier
pub fn id(&self) -> ThreadId {
self.id
}
/// Atomically makes the handle's token available if it is not already.
pub fn unpark(&self) {
ExecutionState::with(|s| {
s.get_mut(self.id.task_id).unpark();
});
// Making the token available is a yield point
thread::switch();
}
}
/// Spawn a new thread, returning a JoinHandle for it.
///
/// The join handle can be used (via the `join` method) to block until the child thread has
/// finished.
pub fn spawn<F, T>(f: F) -> JoinHandle<T>
where
F: FnOnce() -> T,
F: Send + 'static,
T: Send + 'static,
{
spawn_named(f, None, None)
}
fn spawn_named<F, T>(f: F, name: Option<String>, stack_size: Option<usize>) -> JoinHandle<T>
where
F: FnOnce() -> T,
F: Send + 'static,
T: Send + 'static,
{
// TODO Check if it's worth avoiding the call to `ExecutionState::config()` if we're going
// TODO to use an existing continuation from the pool.
let stack_size = stack_size.unwrap_or_else(|| ExecutionState::with(|s| s.config.stack_size));
let result = std::sync::Arc::new(std::sync::Mutex::new(None));
let task_id = {
let result = std::sync::Arc::clone(&result);
let f = move || thread_fn(f, result);
ExecutionState::spawn_thread(f, stack_size, name.clone(), None)
};
thread::switch();
let thread = Thread {
id: ThreadId { task_id },
name,
};
JoinHandle {
task_id,
thread,
result,
}
}
/// Body of a Shuttle thread, that runs the given closure, handles thread-local destructors, and
/// stores the result of the thread in the given lock.
pub(crate) fn thread_fn<F, T>(f: F, result: std::sync::Arc<std::sync::Mutex<Option<std::thread::Result<T>>>>)
where
F: FnOnce() -> T,
F: Send + 'static,
T: Send + 'static,
{
let ret = f();
tracing::trace!("thread finished, dropping thread locals");
// Run thread-local destructors before publishing the result, because
// [`JoinHandle::join`] says join "waits for the associated thread to finish", but
// destructors must be run on the thread, so it can't be considered "finished" if the
// destructors haven't run yet.
// See `pop_local` for details on why this loop looks this slightly funky way.
while let Some(local) = ExecutionState::with(|state| state.current_mut().pop_local()) {
tracing::trace!("dropping thread local {:p}", local);
drop(local);
}
tracing::trace!("done dropping thread locals");
// Publish the result and unblock the waiter. We need to do this now, because once this
// closure completes, the Execution will consider this task Finished and invoke the
// scheduler.
*result.lock().unwrap() = Some(Ok(ret));
ExecutionState::with(|state| {
if let Some(waiter) = state.current_mut().take_waiter() {
state.get_mut(waiter).unblock();
}
});
}
/// An owned permission to join on a thread (block on its termination).
#[derive(Debug)]
pub struct JoinHandle<T> {
task_id: TaskId,
thread: Thread,
result: std::sync::Arc<std::sync::Mutex<Option<std::thread::Result<T>>>>,
}
impl<T> JoinHandle<T> {
/// Waits for the associated thread to finish.
pub fn join(self) -> std::thread::Result<T> {
ExecutionState::with(|state| {
let me = state.current().id();
let target = state.get_mut(self.task_id);
if target.set_waiter(me) {
state.current_mut().block(false);
}
});
// TODO can we soundly skip the yield if the target thread has already finished?
thread::switch();
// Waiting thread inherits the clock of the finished thread
ExecutionState::with(|state| {
let target = state.get_mut(self.task_id);
let clock = target.clock.clone();
state.update_clock(&clock);
});
self.result.lock().unwrap().take().expect("target should have finished")
}
/// Extracts a handle to the underlying thread.
pub fn thread(&self) -> &Thread {
&self.thread
}
}
/// Cooperatively gives up a timeslice to the Shuttle scheduler.
///
/// Some Shuttle schedulers use this as a hint to deprioritize the current thread in order for other
/// threads to make progress (e.g., in a spin loop).
pub fn yield_now() {
let waker = ExecutionState::with(|state| state.current().waker());
waker.wake_by_ref();
ExecutionState::request_yield();
thread::switch();
}
/// Puts the current thread to sleep for at least the specified amount of time.
// Note that Shuttle does not model time, so this behaves just like a context switch.
pub fn sleep(_dur: Duration) {
thread::switch();
}
/// Get a handle to the thread that invokes it
pub fn current() -> Thread {
let (task_id, name) = ExecutionState::with(|s| {
let me = s.current();
(me.id(), me.name())
});
Thread {
id: ThreadId { task_id },
name,
}
}
/// Blocks unless or until the current thread's token is made available (may wake spuriously).
pub fn park() {
let switch = ExecutionState::with(|s| s.current_mut().park());
// We only need to context switch if the park token was unavailable. If it was available, then
// any execution reachable by context switching here would also be reachable by having not
// chosen this thread at the last context switch, because the park state of a thread is only
// observable by the thread itself. We also mark it as an explicit yield request by the task,
// since otherwise some schedulers might prefer to to reschedule the current task, which in this
// context would result in spurious wakeups triggering nearly every time.
if switch {
ExecutionState::request_yield();
thread::switch();
}
}
/// Blocks unless or until the current thread's token is made available or the specified duration
/// has been reached (may wake spuriously).
///
/// Note that Shuttle does not model time, so this behaves identically to `park`. In particular,
/// Shuttle does not assume that the timeout will ever fire, so if all threads are blocked in a call
/// to `park_timeout` it will be treated as a deadlock.
pub fn park_timeout(_dur: Duration) {
park();
}
/// Thread factory, which can be used in order to configure the properties of a new thread.
#[derive(Debug, Default)]
pub struct Builder {
name: Option<String>,
stack_size: Option<usize>,
}
impl Builder {
/// Generates the base configuration for spawning a thread, from which configuration methods can be chained.
pub fn new() -> Self {
Self {
name: None,
stack_size: None,
}
}
/// Names the thread-to-be. Currently the name is used for identification only in panic messages.
pub fn name(mut self, name: String) -> Self {
self.name = Some(name);
self
}
/// Sets the size of the stack (in bytes) for the new thread.
pub fn stack_size(mut self, stack_size: usize) -> Self {
self.stack_size = Some(stack_size);
self
}
/// Spawns a new thread by taking ownership of the Builder, and returns an `io::Result` to its `JoinHandle`.
pub fn spawn<F, T>(self, f: F) -> std::io::Result<JoinHandle<T>>
where
F: FnOnce() -> T,
F: Send + 'static,
T: Send + 'static,
{
Ok(spawn_named(f, self.name, self.stack_size))
}
}
/// A thread local storage key which owns its contents
// Sadly, the fields of this thing need to be public because function pointers in const fns are
// unstable, so an explicit instantiation is the only way to construct this struct. User code should
// not rely on these fields.
pub struct LocalKey<T: 'static> {
#[doc(hidden)]
pub init: fn() -> T,
#[doc(hidden)]
pub _p: PhantomData<T>,
}
// Safety: `LocalKey` implements thread-local storage; each thread sees its own value of the type T.
unsafe impl<T> Send for LocalKey<T> {}
unsafe impl<T> Sync for LocalKey<T> {}
impl<T: 'static> std::fmt::Debug for LocalKey<T> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.debug_struct("LocalKey").finish_non_exhaustive()
}
}
impl<T: 'static> LocalKey<T> {
/// Acquires a reference to the value in this TLS key.
///
/// This will lazily initialize the value if this thread has not referenced this key yet.
pub fn with<F, R>(&'static self, f: F) -> R
where
F: FnOnce(&T) -> R,
{
self.try_with(f).expect(
"cannot access a Thread Local Storage value \
during or after destruction",
)
}
/// Acquires a reference to the value in this TLS key.
///
/// This will lazily initialize the value if this thread has not referenced this key yet. If the
/// key has been destroyed (which may happen if this is called in a destructor), this function
/// will return an AccessError.
pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
where
F: FnOnce(&T) -> R,
{
let value = self.get().unwrap_or_else(|| {
let value = (self.init)();
ExecutionState::with(move |state| {
state.current_mut().init_local(self, value);
});
self.get().unwrap()
})?;
Ok(f(value))
}
fn get(&'static self) -> Option<Result<&T, AccessError>> {
// Safety: see the usage below
unsafe fn extend_lt<'b, T>(t: &'_ T) -> &'b T {
std::mem::transmute(t)
}
ExecutionState::with(|state| {
if let Ok(value) = state.current().local(self)? {
// Safety: unfortunately the lifetime of a value in our thread-local storage is
// bound to the lifetime of `ExecutionState`, which has no visible relation to the
// lifetime of the thread we're running on. However, *we* know that the
// `ExecutionState` outlives any thread, including the caller, and so it's safe to
// give the caller the lifetime it's asking for here.
Some(Ok(unsafe { extend_lt(value) }))
} else {
// Slot has already been destructed
Some(Err(AccessError))
}
})
}
}
/// An error returned by [`LocalKey::try_with`]
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
#[non_exhaustive]
pub struct AccessError;
impl std::fmt::Display for AccessError {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
std::fmt::Display::fmt("already destroyed", f)
}
}
impl std::error::Error for AccessError {}