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//! An asynchronously awaitable semaphore for synchronization between concurrently
//! executing futures.
use crate::{
intrusive_double_linked_list::{LinkedList, ListNode},
utils::update_waker_ref,
NoopLock,
};
use core::pin::Pin;
use futures_core::{
future::{FusedFuture, Future},
task::{Context, Poll, Waker},
};
use lock_api::{Mutex as LockApiMutex, RawMutex};
/// Tracks how the future had interacted with the semaphore
#[derive(PartialEq)]
enum PollState {
/// The task has never interacted with the semaphore.
New,
/// The task was added to the wait queue at the semaphore.
Waiting,
/// The task had previously waited on the semaphore, but was notified
/// that the semaphore was released in the meantime and that the task
/// thereby could retry.
Notified,
/// The task had been polled to completion.
Done,
}
/// Tracks the SemaphoreAcquireFuture waiting state.
struct WaitQueueEntry {
/// The task handle of the waiting task
task: Option<Waker>,
/// Current polling state
state: PollState,
/// The amount of permits that should be obtained
required_permits: usize,
}
impl WaitQueueEntry {
/// Creates a new WaitQueueEntry
fn new(required_permits: usize) -> WaitQueueEntry {
WaitQueueEntry {
task: None,
state: PollState::New,
required_permits,
}
}
}
/// Internal state of the `Semaphore`
struct SemaphoreState {
is_fair: bool,
permits: usize,
waiters: LinkedList<WaitQueueEntry>,
}
impl SemaphoreState {
fn new(is_fair: bool, permits: usize) -> Self {
SemaphoreState {
is_fair,
permits,
waiters: LinkedList::new(),
}
}
/// Wakes up the last waiter and removes it from the wait queue
fn wakeup_waiters(&mut self) {
// Wake as many tasks as the permits allow
let mut available = self.permits;
loop {
match self.waiters.peek_last_mut() {
None => return,
Some(last_waiter) => {
// Check if enough permits are available for this waiter.
// If not then a wakeup attempt won't be successful.
if available < last_waiter.required_permits {
return;
}
available -= last_waiter.required_permits;
// Notify the waiter that it can try to acquire the semaphore again.
// The notification gets tracked inside the waiter.
// If the waiter aborts it's wait (drops the future), another task
// must be woken.
if last_waiter.state != PollState::Notified {
last_waiter.state = PollState::Notified;
let task = &last_waiter.task;
if let Some(ref handle) = task {
handle.wake_by_ref();
}
}
// In the case of a non-fair semaphore, the waiters are directly
// removed from the semaphores wait queue when woken.
// That avoids having to remove the wait element later.
if !self.is_fair {
self.waiters.remove_last();
} else {
// For a fair Semaphore we never wake more than 1 task.
// That one needs to acquire the Semaphore.
// TODO: We actually should be able to wake more, since
// it's guaranteed that both tasks could make progress.
// However the we currently can't peek iterate in reverse order.
return;
}
}
}
}
}
fn permits(&self) -> usize {
self.permits
}
/// Releases a certain amount of permits back to the semaphore
fn release(&mut self, permits: usize) {
if permits == 0 {
return;
}
// TODO: Overflow check
self.permits += permits;
// Wakeup the last waiter
self.wakeup_waiters();
}
/// Tries to acquire the given amount of permits synchronously.
///
/// Returns true if the permits were obtained and false otherwise.
fn try_acquire_sync(&mut self, required_permits: usize) -> bool {
// Permits can only be obtained synchronously if there are
// - enough permits available
// - the Semaphore is either not fair, or there are no waiters
// - required_permits == 0
if (self.permits >= required_permits)
&& (!self.is_fair
|| self.waiters.is_empty()
|| required_permits == 0)
{
self.permits -= required_permits;
true
} else {
false
}
}
/// Tries to acquire the Semaphore from a WaitQueueEntry.
/// If it isn't available, the WaitQueueEntry gets added to the wait
/// queue at the Semaphore, and will be signalled once ready.
/// This function is only safe as long as the `wait_node`s address is guaranteed
/// to be stable until it gets removed from the queue.
unsafe fn try_acquire(
&mut self,
wait_node: &mut ListNode<WaitQueueEntry>,
cx: &mut Context<'_>,
) -> Poll<()> {
match wait_node.state {
PollState::New => {
// The fast path - enough permits are available
if self.try_acquire_sync(wait_node.required_permits) {
wait_node.state = PollState::Done;
Poll::Ready(())
} else {
// Add the task to the wait queue
wait_node.task = Some(cx.waker().clone());
wait_node.state = PollState::Waiting;
self.waiters.add_front(wait_node);
Poll::Pending
}
}
PollState::Waiting => {
// The SemaphoreAcquireFuture is already in the queue.
if self.is_fair {
// The task needs to wait until it gets notified in order to
// maintain the ordering.
// However the caller might have passed a different `Waker`.
// In this case we need to update it.
update_waker_ref(&mut wait_node.task, cx);
Poll::Pending
} else {
// For throughput improvement purposes, check immediately
// if enough permits are available
if self.permits >= wait_node.required_permits {
self.permits -= wait_node.required_permits;
wait_node.state = PollState::Done;
// Since this waiter has been registered before, it must
// get removed from the waiter list.
// Safety: Due to the state, we know that the node must be part
// of the waiter list
self.force_remove_waiter(wait_node);
Poll::Ready(())
} else {
// The caller might have passed a different `Waker`.
// In this case we need to update it.
update_waker_ref(&mut wait_node.task, cx);
Poll::Pending
}
}
}
PollState::Notified => {
// We had been woken by the semaphore, since the semaphore is available again.
// The semaphore thereby removed us from the waiters list.
// Just try to lock again. If the semaphore isn't available,
// we need to add it to the wait queue again.
if self.permits >= wait_node.required_permits {
if self.is_fair {
// In a fair Semaphore, the WaitQueueEntry is kept in the
// linked list and must be removed here
// Safety: Due to the state, we know that the node must be part
// of the waiter list
self.force_remove_waiter(wait_node);
}
self.permits -= wait_node.required_permits;
if self.is_fair {
// There might be another task which is ready to run,
// but couldn't, since it was blocked behind the fair waiter.
self.wakeup_waiters();
}
wait_node.state = PollState::Done;
Poll::Ready(())
} else {
// A fair semaphore should never end up in that branch, since
// it's only notified when it's permits are guaranteed to
// be available. assert! in order to find logic bugs
assert!(
!self.is_fair,
"Fair semaphores should always be ready when notified"
);
// Add to queue
wait_node.task = Some(cx.waker().clone());
wait_node.state = PollState::Waiting;
self.waiters.add_front(wait_node);
Poll::Pending
}
}
PollState::Done => {
// The future had been polled to completion before
panic!("polled Mutex after completion");
}
}
}
/// Tries to remove a waiter from the wait queue, and panics if the
/// waiter is no longer valid.
unsafe fn force_remove_waiter(
&mut self,
wait_node: &mut ListNode<WaitQueueEntry>,
) {
if !self.waiters.remove(wait_node) {
// Panic if the address isn't found. This can only happen if the contract was
// violated, e.g. the WaitQueueEntry got moved after the initial poll.
panic!("Future could not be removed from wait queue");
}
}
/// Removes the waiter from the list.
/// This function is only safe as long as the reference that is passed here
/// equals the reference/address under which the waiter was added.
/// The waiter must not have been moved in between.
fn remove_waiter(&mut self, wait_node: &mut ListNode<WaitQueueEntry>) {
// SemaphoreAcquireFuture only needs to get removed if it had been added to
// the wait queue of the Semaphore. This has happened in the PollState::Waiting case.
// If the current waiter was notified, another waiter must get notified now.
match wait_node.state {
PollState::Notified => {
if self.is_fair {
// In a fair Mutex, the WaitQueueEntry is kept in the
// linked list and must be removed here
// Safety: Due to the state, we know that the node must be part
// of the waiter list
unsafe { self.force_remove_waiter(wait_node) };
}
wait_node.state = PollState::Done;
// Wakeup more waiters
self.wakeup_waiters();
}
PollState::Waiting => {
// Remove the WaitQueueEntry from the linked list
// Safety: Due to the state, we know that the node must be part
// of the waiter list
unsafe { self.force_remove_waiter(wait_node) };
wait_node.state = PollState::Done;
}
PollState::New | PollState::Done => {}
}
}
}
/// An RAII guard returned by the `acquire` and `try_acquire` methods.
///
/// When this structure is dropped (falls out of scope),
/// the amount of permits that was used in the `acquire()` call will be released
/// back to the Semaphore.
pub struct GenericSemaphoreReleaser<'a, MutexType: RawMutex> {
/// The Semaphore which is associated with this Releaser
semaphore: &'a GenericSemaphore<MutexType>,
/// The amount of permits to release
permits: usize,
}
impl<MutexType: RawMutex> core::fmt::Debug
for GenericSemaphoreReleaser<'_, MutexType>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("GenericSemaphoreReleaser").finish()
}
}
impl<MutexType: RawMutex> GenericSemaphoreReleaser<'_, MutexType> {
/// Prevents the SemaphoreReleaser from automatically releasing the permits
/// when it gets dropped.
/// This is helpful if the permits must be acquired for a longer lifetime
/// than the one of the SemaphoreReleaser.
/// If this method is used it is important to release the acquired permits
/// manually back to the Semaphore.
pub fn disarm(&mut self) -> usize {
let permits = self.permits;
self.permits = 0;
permits
}
}
impl<MutexType: RawMutex> Drop for GenericSemaphoreReleaser<'_, MutexType> {
fn drop(&mut self) {
// Release the requested amount of permits to the semaphore
if self.permits != 0 {
self.semaphore.state.lock().release(self.permits);
}
}
}
/// A future which resolves when the target semaphore has been successfully acquired.
#[must_use = "futures do nothing unless polled"]
pub struct GenericSemaphoreAcquireFuture<'a, MutexType: RawMutex> {
/// The Semaphore which should get acquired trough this Future
semaphore: Option<&'a GenericSemaphore<MutexType>>,
/// Node for waiting at the semaphore
wait_node: ListNode<WaitQueueEntry>,
/// Whether the obtained permits should automatically be released back
/// to the semaphore.
auto_release: bool,
}
// Safety: Futures can be sent between threads as long as the underlying
// semaphore is thread-safe (Sync), which allows to poll/register/unregister from
// a different thread.
unsafe impl<'a, MutexType: RawMutex + Sync> Send
for GenericSemaphoreAcquireFuture<'a, MutexType>
{
}
impl<'a, MutexType: RawMutex> core::fmt::Debug
for GenericSemaphoreAcquireFuture<'a, MutexType>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("GenericSemaphoreAcquireFuture").finish()
}
}
impl<'a, MutexType: RawMutex> Future
for GenericSemaphoreAcquireFuture<'a, MutexType>
{
type Output = GenericSemaphoreReleaser<'a, MutexType>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// Safety: The next operations are safe, because Pin promises us that
// the address of the wait queue entry inside GenericSemaphoreAcquireFuture is stable,
// and we don't move any fields inside the future until it gets dropped.
let mut_self: &mut GenericSemaphoreAcquireFuture<MutexType> =
unsafe { Pin::get_unchecked_mut(self) };
let semaphore = mut_self
.semaphore
.expect("polled GenericSemaphoreAcquireFuture after completion");
let mut semaphore_state = semaphore.state.lock();
let poll_res =
unsafe { semaphore_state.try_acquire(&mut mut_self.wait_node, cx) };
match poll_res {
Poll::Pending => Poll::Pending,
Poll::Ready(()) => {
// The semaphore was acquired.
mut_self.semaphore = None;
let to_release = match mut_self.auto_release {
true => mut_self.wait_node.required_permits,
false => 0,
};
Poll::Ready(GenericSemaphoreReleaser::<'a, MutexType> {
semaphore,
permits: to_release,
})
}
}
}
}
impl<'a, MutexType: RawMutex> FusedFuture
for GenericSemaphoreAcquireFuture<'a, MutexType>
{
fn is_terminated(&self) -> bool {
self.semaphore.is_none()
}
}
impl<'a, MutexType: RawMutex> Drop
for GenericSemaphoreAcquireFuture<'a, MutexType>
{
fn drop(&mut self) {
// If this GenericSemaphoreAcquireFuture has been polled and it was added to the
// wait queue at the semaphore, it must be removed before dropping.
// Otherwise the semaphore would access invalid memory.
if let Some(semaphore) = self.semaphore {
let mut semaphore_state = semaphore.state.lock();
// Analysis: Does the number of permits play a role here?
// The future was notified because there was a certain amount of permits
// available.
// Removing the waiter will wake up as many tasks as there are permits
// available inside the Semaphore now. If this is bigger than the
// amount of permits required for this task, then additional new
// tasks might get woken. However that isn't bad, since
// those tasks should get into the wait state anyway.
semaphore_state.remove_waiter(&mut self.wait_node);
}
}
}
/// A futures-aware semaphore.
pub struct GenericSemaphore<MutexType: RawMutex> {
state: LockApiMutex<MutexType, SemaphoreState>,
}
// It is safe to send semaphores between threads, as long as they are not used and
// thereby borrowed
unsafe impl<MutexType: RawMutex + Send> Send for GenericSemaphore<MutexType> {}
// The Semaphore is thread-safe as long as the utilized Mutex is thread-safe
unsafe impl<MutexType: RawMutex + Sync> Sync for GenericSemaphore<MutexType> {}
impl<MutexType: RawMutex> core::fmt::Debug for GenericSemaphore<MutexType> {
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("Semaphore")
.field("permits", &self.permits())
.finish()
}
}
impl<MutexType: RawMutex> GenericSemaphore<MutexType> {
/// Creates a new futures-aware semaphore.
///
/// `is_fair` defines whether the `Semaphore` should behave be fair regarding the
/// order of waiters. A fair `Semaphore` will only allow the oldest waiter on
/// a `Semaphore` to retry acquiring it once it's available again.
/// Other waiters must wait until either this acquire attempt completes, and
/// the `Semaphore` has enough permits after that, or until the
/// [`SemaphoreAcquireFuture`] which tried to acquire the `Semaphore` is dropped.
///
/// If the `Semaphore` isn't fair, waiters that wait for a high amount of
/// permits might never succeed since the permits might be stolen in between
/// by other waiters. Therefore use-cases which make use of very different
/// amount of permits per acquire should use fair semaphores.
/// For use-cases where each `acquire()` tries to acquire the same amount of
/// permits an unfair `Semaphore` might provide throughput advantages.
///
/// `permits` is the amount of permits that a semaphore should hold when
/// created.
pub fn new(is_fair: bool, permits: usize) -> GenericSemaphore<MutexType> {
GenericSemaphore::<MutexType> {
state: LockApiMutex::new(SemaphoreState::new(is_fair, permits)),
}
}
/// Acquire a certain amount of permits on a semaphore asynchronously.
///
/// This method returns a future that will resolve once the given amount of
/// permits have been acquired.
/// The Future will resolve to a [`GenericSemaphoreReleaser`], which will
/// release all acquired permits automatically when dropped.
pub fn acquire(
&self,
nr_permits: usize,
) -> GenericSemaphoreAcquireFuture<'_, MutexType> {
GenericSemaphoreAcquireFuture::<MutexType> {
semaphore: Some(&self),
wait_node: ListNode::new(WaitQueueEntry::new(nr_permits)),
auto_release: true,
}
}
/// Tries to acquire a certain amount of permits on a semaphore.
///
/// If acquiring the permits is successful, a [`GenericSemaphoreReleaser`]
/// will be returned, which will release all acquired permits automatically
/// when dropped.
///
/// Otherwise `None` will be returned.
pub fn try_acquire(
&self,
nr_permits: usize,
) -> Option<GenericSemaphoreReleaser<'_, MutexType>> {
if self.state.lock().try_acquire_sync(nr_permits) {
Some(GenericSemaphoreReleaser {
semaphore: self,
permits: nr_permits,
})
} else {
None
}
}
/// Releases the given amount of permits back to the semaphore.
///
/// This method should in most cases not be used, since the
/// [`GenericSemaphoreReleaser`] which is obtained when acquiring a Semaphore
/// will automatically release the obtained permits again.
///
/// Therefore this method should only be used if the automatic release was
/// disabled by calling [`GenericSemaphoreReleaser::disarm`],
/// or when the amount of permits in the Semaphore
/// should increase from the initial amount.
pub fn release(&self, nr_permits: usize) {
self.state.lock().release(nr_permits)
}
/// Returns the amount of permits that are available on the semaphore
pub fn permits(&self) -> usize {
self.state.lock().permits()
}
}
// Export a non thread-safe version using NoopLock
/// A [`GenericSemaphore`] which is not thread-safe.
pub type LocalSemaphore = GenericSemaphore<NoopLock>;
/// A [`GenericSemaphoreReleaser`] for [`LocalSemaphore`].
pub type LocalSemaphoreReleaser<'a> = GenericSemaphoreReleaser<'a, NoopLock>;
/// A [`GenericSemaphoreAcquireFuture`] for [`LocalSemaphore`].
pub type LocalSemaphoreAcquireFuture<'a> =
GenericSemaphoreAcquireFuture<'a, NoopLock>;
#[cfg(feature = "std")]
mod if_std {
use super::*;
// Export a thread-safe version using parking_lot::RawMutex
/// A [`GenericSemaphore`] backed by [`parking_lot`].
pub type Semaphore = GenericSemaphore<parking_lot::RawMutex>;
/// A [`GenericSemaphoreReleaser`] for [`Semaphore`].
pub type SemaphoreReleaser<'a> =
GenericSemaphoreReleaser<'a, parking_lot::RawMutex>;
/// A [`GenericSemaphoreAcquireFuture`] for [`Semaphore`].
pub type SemaphoreAcquireFuture<'a> =
GenericSemaphoreAcquireFuture<'a, parking_lot::RawMutex>;
}
#[cfg(feature = "std")]
pub use self::if_std::*;
#[cfg(feature = "alloc")]
mod if_alloc {
use super::*;
use alloc::sync::Arc;
/// An RAII guard returned by the `acquire` and `try_acquire` methods.
///
/// When this structure is dropped (falls out of scope),
/// the amount of permits that was used in the `acquire()` call will be released
/// back to the Semaphore.
pub struct GenericSharedSemaphoreReleaser<MutexType: RawMutex> {
/// The Semaphore which is associated with this Releaser
semaphore: GenericSharedSemaphore<MutexType>,
/// The amount of permits to release
permits: usize,
}
impl<MutexType: RawMutex> core::fmt::Debug
for GenericSharedSemaphoreReleaser<MutexType>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("GenericSharedSemaphoreReleaser").finish()
}
}
impl<MutexType: RawMutex> GenericSharedSemaphoreReleaser<MutexType> {
/// Prevents the SharedSemaphoreReleaser from automatically releasing the permits
/// when it gets dropped.
///
/// This is helpful if the permits must be acquired for a longer lifetime
/// than the one of the SemaphoreReleaser.
///
/// If this method is used it is important to release the acquired permits
/// manually back to the Semaphore.
pub fn disarm(&mut self) -> usize {
let permits = self.permits;
self.permits = 0;
permits
}
}
impl<MutexType: RawMutex> Drop for GenericSharedSemaphoreReleaser<MutexType> {
fn drop(&mut self) {
// Release the requested amount of permits to the semaphore
if self.permits != 0 {
self.semaphore.state.lock().release(self.permits);
}
}
}
/// A future which resolves when the target semaphore has been successfully acquired.
#[must_use = "futures do nothing unless polled"]
pub struct GenericSharedSemaphoreAcquireFuture<MutexType: RawMutex> {
/// The Semaphore which should get acquired trough this Future
semaphore: Option<GenericSharedSemaphore<MutexType>>,
/// Node for waiting at the semaphore
wait_node: ListNode<WaitQueueEntry>,
/// Whether the obtained permits should automatically be released back
/// to the semaphore.
auto_release: bool,
}
// Safety: Futures can be sent between threads as long as the underlying
// semaphore is thread-safe (Sync), which allows to poll/register/unregister from
// a different thread.
unsafe impl<MutexType: RawMutex + Sync> Send
for GenericSharedSemaphoreAcquireFuture<MutexType>
{
}
impl<MutexType: RawMutex> core::fmt::Debug
for GenericSharedSemaphoreAcquireFuture<MutexType>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("GenericSharedSemaphoreAcquireFuture")
.finish()
}
}
impl<MutexType: RawMutex> Future
for GenericSharedSemaphoreAcquireFuture<MutexType>
{
type Output = GenericSharedSemaphoreReleaser<MutexType>;
fn poll(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
) -> Poll<Self::Output> {
// Safety: The next operations are safe, because Pin promises us that
// the address of the wait queue entry inside
// GenericSharedSemaphoreAcquireFuture is stable,
// and we don't move any fields inside the future until it gets dropped.
let mut_self: &mut GenericSharedSemaphoreAcquireFuture<MutexType> =
unsafe { Pin::get_unchecked_mut(self) };
let semaphore = mut_self.semaphore.take().expect(
"polled GenericSharedSemaphoreAcquireFuture after completion",
);
let poll_res = unsafe {
let mut semaphore_state = semaphore.state.lock();
semaphore_state.try_acquire(&mut mut_self.wait_node, cx)
};
match poll_res {
Poll::Pending => {
mut_self.semaphore.replace(semaphore);
Poll::Pending
}
Poll::Ready(()) => {
let to_release = match mut_self.auto_release {
true => mut_self.wait_node.required_permits,
false => 0,
};
Poll::Ready(GenericSharedSemaphoreReleaser::<MutexType> {
semaphore,
permits: to_release,
})
}
}
}
}
impl<MutexType: RawMutex> FusedFuture
for GenericSharedSemaphoreAcquireFuture<MutexType>
{
fn is_terminated(&self) -> bool {
self.semaphore.is_none()
}
}
impl<MutexType: RawMutex> Drop
for GenericSharedSemaphoreAcquireFuture<MutexType>
{
fn drop(&mut self) {
// If this GenericSharedSemaphoreAcquireFuture has been polled and it was added to the
// wait queue at the semaphore, it must be removed before dropping.
// Otherwise the semaphore would access invalid memory.
if let Some(semaphore) = self.semaphore.take() {
let mut semaphore_state = semaphore.state.lock();
// Analysis: Does the number of permits play a role here?
// The future was notified because there was a certain amount of permits
// available.
// Removing the waiter will wake up as many tasks as there are permits
// available inside the Semaphore now. If this is bigger than the
// amount of permits required for this task, then additional new
// tasks might get woken. However that isn't bad, since
// those tasks should get into the wait state anyway.
semaphore_state.remove_waiter(&mut self.wait_node);
}
}
}
/// A futures-aware shared semaphore.
pub struct GenericSharedSemaphore<MutexType: RawMutex> {
state: Arc<LockApiMutex<MutexType, SemaphoreState>>,
}
impl<MutexType: RawMutex> Clone for GenericSharedSemaphore<MutexType> {
fn clone(&self) -> Self {
Self {
state: self.state.clone(),
}
}
}
// It is safe to send semaphores between threads, as long as they are not used and
// thereby borrowed
unsafe impl<MutexType: RawMutex + Send + Sync> Send
for GenericSharedSemaphore<MutexType>
{
}
// The Semaphore is thread-safe as long as the utilized Mutex is thread-safe
unsafe impl<MutexType: RawMutex + Sync> Sync
for GenericSharedSemaphore<MutexType>
{
}
impl<MutexType: RawMutex> core::fmt::Debug
for GenericSharedSemaphore<MutexType>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("Semaphore")
.field("permits", &self.permits())
.finish()
}
}
impl<MutexType: RawMutex> GenericSharedSemaphore<MutexType> {
/// Creates a new futures-aware shared semaphore.
///
/// See `GenericSharedSemaphore` for more information.
pub fn new(
is_fair: bool,
permits: usize,
) -> GenericSharedSemaphore<MutexType> {
GenericSharedSemaphore::<MutexType> {
state: Arc::new(LockApiMutex::new(SemaphoreState::new(
is_fair, permits,
))),
}
}
/// Acquire a certain amount of permits on a semaphore asynchronously.
///
/// This method returns a future that will resolve once the given amount of
/// permits have been acquired.
/// The Future will resolve to a [`GenericSharedSemaphoreReleaser`], which will
/// release all acquired permits automatically when dropped.
pub fn acquire(
&self,
nr_permits: usize,
) -> GenericSharedSemaphoreAcquireFuture<MutexType> {
GenericSharedSemaphoreAcquireFuture::<MutexType> {
semaphore: Some(self.clone()),
wait_node: ListNode::new(WaitQueueEntry::new(nr_permits)),
auto_release: true,
}
}
/// Tries to acquire a certain amount of permits on a semaphore.
///
/// If acquiring the permits is successful, a [`GenericSharedSemaphoreReleaser`]
/// will be returned, which will release all acquired permits automatically
/// when dropped.
///
/// Otherwise `None` will be returned.
pub fn try_acquire(
&self,
nr_permits: usize,
) -> Option<GenericSharedSemaphoreReleaser<MutexType>> {
if self.state.lock().try_acquire_sync(nr_permits) {
Some(GenericSharedSemaphoreReleaser {
semaphore: self.clone(),
permits: nr_permits,
})
} else {
None
}
}
/// Releases the given amount of permits back to the semaphore.
///
/// This method should in most cases not be used, since the
/// [`GenericSharedSemaphoreReleaser`] which is obtained when acquiring a Semaphore
/// will automatically release the obtained permits again.
///
/// Therefore this method should only be used if the automatic release was
/// disabled by calling [`GenericSharedSemaphoreReleaser::disarm`],
/// or when the amount of permits in the Semaphore
/// should increase from the initial amount.
pub fn release(&self, nr_permits: usize) {
self.state.lock().release(nr_permits)
}
/// Returns the amount of permits that are available on the semaphore
pub fn permits(&self) -> usize {
self.state.lock().permits()
}
}
// Export parking_lot based shared semaphores in std mode
#[cfg(feature = "std")]
mod if_std {
use super::*;
/// A [`GenericSharedSemaphore`] backed by [`parking_lot`].
pub type SharedSemaphore =
GenericSharedSemaphore<parking_lot::RawMutex>;
/// A [`GenericSharedSemaphoreReleaser`] for [`SharedSemaphore`].
pub type SharedSemaphoreReleaser =
GenericSharedSemaphoreReleaser<parking_lot::RawMutex>;
/// A [`GenericSharedSemaphoreAcquireFuture`] for [`SharedSemaphore`].
pub type SharedSemaphoreAcquireFuture =
GenericSharedSemaphoreAcquireFuture<parking_lot::RawMutex>;
}
#[cfg(feature = "std")]
pub use self::if_std::*;
}
#[cfg(feature = "alloc")]
pub use self::if_alloc::*;