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//! This module defines an `IdleNotifiedSet`, which is a collection of elements.
//! Each element is intended to correspond to a task, and the collection will
//! keep track of which tasks have had their waker notified, and which have not.
//!
//! Each entry in the set holds some user-specified value. The value's type is
//! specified using the `T` parameter. It will usually be a `JoinHandle` or
//! similar.
use std::marker::PhantomPinned;
use std::mem::ManuallyDrop;
use std::ptr::NonNull;
use std::task::{Context, Waker};
use crate::loom::cell::UnsafeCell;
use crate::loom::sync::{Arc, Mutex};
use crate::util::linked_list::{self, Link};
use crate::util::{waker_ref, Wake};
type LinkedList<T> =
linked_list::LinkedList<ListEntry<T>, <ListEntry<T> as linked_list::Link>::Target>;
/// This is the main handle to the collection.
pub(crate) struct IdleNotifiedSet<T> {
lists: Arc<Lists<T>>,
length: usize,
}
/// A handle to an entry that is guaranteed to be stored in the idle or notified
/// list of its `IdleNotifiedSet`. This value borrows the `IdleNotifiedSet`
/// mutably to prevent the entry from being moved to the `Neither` list, which
/// only the `IdleNotifiedSet` may do.
///
/// The main consequence of being stored in one of the lists is that the `value`
/// field has not yet been consumed.
///
/// Note: This entry can be moved from the idle to the notified list while this
/// object exists by waking its waker.
pub(crate) struct EntryInOneOfTheLists<'a, T> {
entry: Arc<ListEntry<T>>,
set: &'a mut IdleNotifiedSet<T>,
}
type Lists<T> = Mutex<ListsInner<T>>;
/// The linked lists hold strong references to the `ListEntry` items, and the
/// `ListEntry` items also hold a strong reference back to the Lists object, but
/// the destructor of the `IdleNotifiedSet` will clear the two lists, so once
/// that object is destroyed, no ref-cycles will remain.
struct ListsInner<T> {
notified: LinkedList<T>,
idle: LinkedList<T>,
/// Whenever an element in the `notified` list is woken, this waker will be
/// notified and consumed, if it exists.
waker: Option<Waker>,
}
/// Which of the two lists in the shared Lists object is this entry stored in?
///
/// If the value is `Idle`, then an entry's waker may move it to the notified
/// list. Otherwise, only the `IdleNotifiedSet` may move it.
///
/// If the value is `Neither`, then it is still possible that the entry is in
/// some third external list (this happens in `drain`).
#[derive(Copy, Clone, Eq, PartialEq)]
enum List {
Notified,
Idle,
Neither,
}
/// An entry in the list.
///
/// # Safety
///
/// The `my_list` field must only be accessed while holding the mutex in
/// `parent`. It is an invariant that the value of `my_list` corresponds to
/// which linked list in the `parent` holds this entry. Once this field takes
/// the value `Neither`, then it may never be modified again.
///
/// If the value of `my_list` is `Notified` or `Idle`, then the `pointers` field
/// must only be accessed while holding the mutex. If the value of `my_list` is
/// `Neither`, then the `pointers` field may be accessed by the
/// `IdleNotifiedSet` (this happens inside `drain`).
///
/// The `value` field is owned by the `IdleNotifiedSet` and may only be accessed
/// by the `IdleNotifiedSet`. The operation that sets the value of `my_list` to
/// `Neither` assumes ownership of the `value`, and it must either drop it or
/// move it out from this entry to prevent it from getting leaked. (Since the
/// two linked lists are emptied in the destructor of `IdleNotifiedSet`, the
/// value should not be leaked.)
struct ListEntry<T> {
/// The linked list pointers of the list this entry is in.
pointers: linked_list::Pointers<ListEntry<T>>,
/// Pointer to the shared `Lists` struct.
parent: Arc<Lists<T>>,
/// The value stored in this entry.
value: UnsafeCell<ManuallyDrop<T>>,
/// Used to remember which list this entry is in.
my_list: UnsafeCell<List>,
/// Required by the `linked_list::Pointers` field.
_pin: PhantomPinned,
}
generate_addr_of_methods! {
impl<T> ListEntry<T> {
unsafe fn addr_of_pointers(self: NonNull<Self>) -> NonNull<linked_list::Pointers<ListEntry<T>>> {
&self.pointers
}
}
}
// With mutable access to the `IdleNotifiedSet`, you can get mutable access to
// the values.
unsafe impl<T: Send> Send for IdleNotifiedSet<T> {}
// With the current API we strictly speaking don't even need `T: Sync`, but we
// require it anyway to support adding &self APIs that access the values in the
// future.
unsafe impl<T: Sync> Sync for IdleNotifiedSet<T> {}
// These impls control when it is safe to create a Waker. Since the waker does
// not allow access to the value in any way (including its destructor), it is
// not necessary for `T` to be Send or Sync.
unsafe impl<T> Send for ListEntry<T> {}
unsafe impl<T> Sync for ListEntry<T> {}
impl<T> IdleNotifiedSet<T> {
/// Create a new `IdleNotifiedSet`.
pub(crate) fn new() -> Self {
let lists = Mutex::new(ListsInner {
notified: LinkedList::new(),
idle: LinkedList::new(),
waker: None,
});
IdleNotifiedSet {
lists: Arc::new(lists),
length: 0,
}
}
pub(crate) fn len(&self) -> usize {
self.length
}
pub(crate) fn is_empty(&self) -> bool {
self.length == 0
}
/// Insert the given value into the `idle` list.
pub(crate) fn insert_idle(&mut self, value: T) -> EntryInOneOfTheLists<'_, T> {
self.length += 1;
let entry = Arc::new(ListEntry {
parent: self.lists.clone(),
value: UnsafeCell::new(ManuallyDrop::new(value)),
my_list: UnsafeCell::new(List::Idle),
pointers: linked_list::Pointers::new(),
_pin: PhantomPinned,
});
{
let mut lock = self.lists.lock();
lock.idle.push_front(entry.clone());
}
// Safety: We just put the entry in the idle list, so it is in one of the lists.
EntryInOneOfTheLists { entry, set: self }
}
/// Pop an entry from the notified list to poll it. The entry is moved to
/// the idle list atomically.
pub(crate) fn pop_notified(&mut self, waker: &Waker) -> Option<EntryInOneOfTheLists<'_, T>> {
// We don't decrement the length because this call moves the entry to
// the idle list rather than removing it.
if self.length == 0 {
// Fast path.
return None;
}
let mut lock = self.lists.lock();
let should_update_waker = match lock.waker.as_mut() {
Some(cur_waker) => !waker.will_wake(cur_waker),
None => true,
};
if should_update_waker {
lock.waker = Some(waker.clone());
}
// Pop the entry, returning None if empty.
let entry = lock.notified.pop_back()?;
lock.idle.push_front(entry.clone());
// Safety: We are holding the lock.
entry.my_list.with_mut(|ptr| unsafe {
*ptr = List::Idle;
});
drop(lock);
// Safety: We just put the entry in the idle list, so it is in one of the lists.
Some(EntryInOneOfTheLists { entry, set: self })
}
/// Tries to pop an entry from the notified list to poll it. The entry is moved to
/// the idle list atomically.
pub(crate) fn try_pop_notified(&mut self) -> Option<EntryInOneOfTheLists<'_, T>> {
// We don't decrement the length because this call moves the entry to
// the idle list rather than removing it.
if self.length == 0 {
// Fast path.
return None;
}
let mut lock = self.lists.lock();
// Pop the entry, returning None if empty.
let entry = lock.notified.pop_back()?;
lock.idle.push_front(entry.clone());
// Safety: We are holding the lock.
entry.my_list.with_mut(|ptr| unsafe {
*ptr = List::Idle;
});
drop(lock);
// Safety: We just put the entry in the idle list, so it is in one of the lists.
Some(EntryInOneOfTheLists { entry, set: self })
}
/// Call a function on every element in this list.
pub(crate) fn for_each<F: FnMut(&mut T)>(&mut self, mut func: F) {
fn get_ptrs<T>(list: &mut LinkedList<T>, ptrs: &mut Vec<*mut T>) {
let mut node = list.last();
while let Some(entry) = node {
ptrs.push(entry.value.with_mut(|ptr| {
let ptr: *mut ManuallyDrop<T> = ptr;
let ptr: *mut T = ptr.cast();
ptr
}));
let prev = entry.pointers.get_prev();
node = prev.map(|prev| unsafe { &*prev.as_ptr() });
}
}
// Atomically get a raw pointer to the value of every entry.
//
// Since this only locks the mutex once, it is not possible for a value
// to get moved from the idle list to the notified list during the
// operation, which would otherwise result in some value being listed
// twice.
let mut ptrs = Vec::with_capacity(self.len());
{
let mut lock = self.lists.lock();
get_ptrs(&mut lock.idle, &mut ptrs);
get_ptrs(&mut lock.notified, &mut ptrs);
}
debug_assert_eq!(ptrs.len(), ptrs.capacity());
for ptr in ptrs {
// Safety: When we grabbed the pointers, the entries were in one of
// the two lists. This means that their value was valid at the time,
// and it must still be valid because we are the IdleNotifiedSet,
// and only we can remove an entry from the two lists. (It's
// possible that an entry is moved from one list to the other during
// this loop, but that is ok.)
func(unsafe { &mut *ptr });
}
}
/// Remove all entries in both lists, applying some function to each element.
///
/// The closure is called on all elements even if it panics. Having it panic
/// twice is a double-panic, and will abort the application.
pub(crate) fn drain<F: FnMut(T)>(&mut self, func: F) {
if self.length == 0 {
// Fast path.
return;
}
self.length = 0;
// The LinkedList is not cleared on panic, so we use a bomb to clear it.
//
// This value has the invariant that any entry in its `all_entries` list
// has `my_list` set to `Neither` and that the value has not yet been
// dropped.
struct AllEntries<T, F: FnMut(T)> {
all_entries: LinkedList<T>,
func: F,
}
impl<T, F: FnMut(T)> AllEntries<T, F> {
fn pop_next(&mut self) -> bool {
if let Some(entry) = self.all_entries.pop_back() {
// Safety: We just took this value from the list, so we can
// destroy the value in the entry.
entry
.value
.with_mut(|ptr| unsafe { (self.func)(ManuallyDrop::take(&mut *ptr)) });
true
} else {
false
}
}
}
impl<T, F: FnMut(T)> Drop for AllEntries<T, F> {
fn drop(&mut self) {
while self.pop_next() {}
}
}
let mut all_entries = AllEntries {
all_entries: LinkedList::new(),
func,
};
// Atomically move all entries to the new linked list in the AllEntries
// object.
{
let mut lock = self.lists.lock();
unsafe {
// Safety: We are holding the lock and `all_entries` is a new
// LinkedList.
move_to_new_list(&mut lock.idle, &mut all_entries.all_entries);
move_to_new_list(&mut lock.notified, &mut all_entries.all_entries);
}
}
// Keep destroying entries in the list until it is empty.
//
// If the closure panics, then the destructor of the `AllEntries` bomb
// ensures that we keep running the destructor on the remaining values.
// A second panic will abort the program.
while all_entries.pop_next() {}
}
}
/// # Safety
///
/// The mutex for the entries must be held, and the target list must be such
/// that setting `my_list` to `Neither` is ok.
unsafe fn move_to_new_list<T>(from: &mut LinkedList<T>, to: &mut LinkedList<T>) {
while let Some(entry) = from.pop_back() {
entry.my_list.with_mut(|ptr| {
*ptr = List::Neither;
});
to.push_front(entry);
}
}
impl<'a, T> EntryInOneOfTheLists<'a, T> {
/// Remove this entry from the list it is in, returning the value associated
/// with the entry.
///
/// This consumes the value, since it is no longer guaranteed to be in a
/// list.
pub(crate) fn remove(self) -> T {
self.set.length -= 1;
{
let mut lock = self.set.lists.lock();
// Safety: We are holding the lock so there is no race, and we will
// remove the entry afterwards to uphold invariants.
let old_my_list = self.entry.my_list.with_mut(|ptr| unsafe {
let old_my_list = *ptr;
*ptr = List::Neither;
old_my_list
});
let list = match old_my_list {
List::Idle => &mut lock.idle,
List::Notified => &mut lock.notified,
// An entry in one of the lists is in one of the lists.
List::Neither => unreachable!(),
};
unsafe {
// Safety: We just checked that the entry is in this particular
// list.
list.remove(ListEntry::as_raw(&self.entry)).unwrap();
}
}
// By setting `my_list` to `Neither`, we have taken ownership of the
// value. We return it to the caller.
//
// Safety: We have a mutable reference to the `IdleNotifiedSet` that
// owns this entry, so we can use its permission to access the value.
self.entry
.value
.with_mut(|ptr| unsafe { ManuallyDrop::take(&mut *ptr) })
}
/// Access the value in this entry together with a context for its waker.
pub(crate) fn with_value_and_context<F, U>(&mut self, func: F) -> U
where
F: FnOnce(&mut T, &mut Context<'_>) -> U,
T: 'static,
{
let waker = waker_ref(&self.entry);
let mut context = Context::from_waker(&waker);
// Safety: We have a mutable reference to the `IdleNotifiedSet` that
// owns this entry, so we can use its permission to access the value.
self.entry
.value
.with_mut(|ptr| unsafe { func(&mut *ptr, &mut context) })
}
}
impl<T> Drop for IdleNotifiedSet<T> {
fn drop(&mut self) {
// Clear both lists.
self.drain(drop);
#[cfg(debug_assertions)]
if !std::thread::panicking() {
let lock = self.lists.lock();
assert!(lock.idle.is_empty());
assert!(lock.notified.is_empty());
}
}
}
impl<T: 'static> Wake for ListEntry<T> {
fn wake_by_ref(me: &Arc<Self>) {
let mut lock = me.parent.lock();
// Safety: We are holding the lock and we will update the lists to
// maintain invariants.
let old_my_list = me.my_list.with_mut(|ptr| unsafe {
let old_my_list = *ptr;
if old_my_list == List::Idle {
*ptr = List::Notified;
}
old_my_list
});
if old_my_list == List::Idle {
// We move ourself to the notified list.
let me = unsafe {
// Safety: We just checked that we are in this particular list.
lock.idle.remove(ListEntry::as_raw(me)).unwrap()
};
lock.notified.push_front(me);
if let Some(waker) = lock.waker.take() {
drop(lock);
waker.wake();
}
}
}
fn wake(me: Arc<Self>) {
Self::wake_by_ref(&me);
}
}
/// # Safety
///
/// `ListEntry` is forced to be !Unpin.
unsafe impl<T> linked_list::Link for ListEntry<T> {
type Handle = Arc<ListEntry<T>>;
type Target = ListEntry<T>;
fn as_raw(handle: &Self::Handle) -> NonNull<ListEntry<T>> {
let ptr: *const ListEntry<T> = Arc::as_ptr(handle);
// Safety: We can't get a null pointer from `Arc::as_ptr`.
unsafe { NonNull::new_unchecked(ptr as *mut ListEntry<T>) }
}
unsafe fn from_raw(ptr: NonNull<ListEntry<T>>) -> Arc<ListEntry<T>> {
Arc::from_raw(ptr.as_ptr())
}
unsafe fn pointers(
target: NonNull<ListEntry<T>>,
) -> NonNull<linked_list::Pointers<ListEntry<T>>> {
ListEntry::addr_of_pointers(target)
}
}
#[cfg(all(test, not(loom)))]
mod tests {
use crate::runtime::Builder;
use crate::task::JoinSet;
// A test that runs under miri.
//
#[test]
fn join_set_test() {
let rt = Builder::new_current_thread().build().unwrap();
let mut set = JoinSet::new();
set.spawn_on(futures::future::ready(()), rt.handle());
rt.block_on(set.join_next()).unwrap().unwrap();
}
}