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//! Zero-capacity channel.
//!
//! This kind of channel is also known as *rendezvous* channel.
use std::boxed::Box;
use std::cell::UnsafeCell;
use std::marker::PhantomData;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Mutex;
use std::time::Instant;
use std::{fmt, ptr};
use crossbeam_utils::Backoff;
use crate::context::Context;
use crate::err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
use crate::select::{Operation, SelectHandle, Selected, Token};
use crate::waker::Waker;
/// A pointer to a packet.
pub(crate) struct ZeroToken(*mut ());
impl Default for ZeroToken {
fn default() -> Self {
Self(ptr::null_mut())
}
}
impl fmt::Debug for ZeroToken {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&(self.0 as usize), f)
}
}
/// A slot for passing one message from a sender to a receiver.
struct Packet<T> {
/// Equals `true` if the packet is allocated on the stack.
on_stack: bool,
/// Equals `true` once the packet is ready for reading or writing.
ready: AtomicBool,
/// The message.
msg: UnsafeCell<Option<T>>,
}
impl<T> Packet<T> {
/// Creates an empty packet on the stack.
fn empty_on_stack() -> Packet<T> {
Packet {
on_stack: true,
ready: AtomicBool::new(false),
msg: UnsafeCell::new(None),
}
}
/// Creates an empty packet on the heap.
fn empty_on_heap() -> Box<Packet<T>> {
Box::new(Packet {
on_stack: false,
ready: AtomicBool::new(false),
msg: UnsafeCell::new(None),
})
}
/// Creates a packet on the stack, containing a message.
fn message_on_stack(msg: T) -> Packet<T> {
Packet {
on_stack: true,
ready: AtomicBool::new(false),
msg: UnsafeCell::new(Some(msg)),
}
}
/// Waits until the packet becomes ready for reading or writing.
fn wait_ready(&self) {
let backoff = Backoff::new();
while !self.ready.load(Ordering::Acquire) {
backoff.snooze();
}
}
}
/// Inner representation of a zero-capacity channel.
struct Inner {
/// Senders waiting to pair up with a receive operation.
senders: Waker,
/// Receivers waiting to pair up with a send operation.
receivers: Waker,
/// Equals `true` when the channel is disconnected.
is_disconnected: bool,
}
/// Zero-capacity channel.
pub(crate) struct Channel<T> {
/// Inner representation of the channel.
inner: Mutex<Inner>,
/// Indicates that dropping a `Channel<T>` may drop values of type `T`.
_marker: PhantomData<T>,
}
impl<T> Channel<T> {
/// Constructs a new zero-capacity channel.
pub(crate) fn new() -> Self {
Channel {
inner: Mutex::new(Inner {
senders: Waker::new(),
receivers: Waker::new(),
is_disconnected: false,
}),
_marker: PhantomData,
}
}
/// Returns a receiver handle to the channel.
pub(crate) fn receiver(&self) -> Receiver<'_, T> {
Receiver(self)
}
/// Returns a sender handle to the channel.
pub(crate) fn sender(&self) -> Sender<'_, T> {
Sender(self)
}
/// Attempts to reserve a slot for sending a message.
fn start_send(&self, token: &mut Token) -> bool {
let mut inner = self.inner.lock().unwrap();
// If there's a waiting receiver, pair up with it.
if let Some(operation) = inner.receivers.try_select() {
token.zero.0 = operation.packet;
true
} else if inner.is_disconnected {
token.zero.0 = ptr::null_mut();
true
} else {
false
}
}
/// Writes a message into the packet.
pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
// If there is no packet, the channel is disconnected.
if token.zero.0.is_null() {
return Err(msg);
}
let packet = &*(token.zero.0 as *const Packet<T>);
packet.msg.get().write(Some(msg));
packet.ready.store(true, Ordering::Release);
Ok(())
}
/// Attempts to pair up with a sender.
fn start_recv(&self, token: &mut Token) -> bool {
let mut inner = self.inner.lock().unwrap();
// If there's a waiting sender, pair up with it.
if let Some(operation) = inner.senders.try_select() {
token.zero.0 = operation.packet;
true
} else if inner.is_disconnected {
token.zero.0 = ptr::null_mut();
true
} else {
false
}
}
/// Reads a message from the packet.
pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
// If there is no packet, the channel is disconnected.
if token.zero.0.is_null() {
return Err(());
}
let packet = &*(token.zero.0 as *const Packet<T>);
if packet.on_stack {
// The message has been in the packet from the beginning, so there is no need to wait
// for it. However, after reading the message, we need to set `ready` to `true` in
// order to signal that the packet can be destroyed.
let msg = packet.msg.get().replace(None).unwrap();
packet.ready.store(true, Ordering::Release);
Ok(msg)
} else {
// Wait until the message becomes available, then read it and destroy the
// heap-allocated packet.
packet.wait_ready();
let msg = packet.msg.get().replace(None).unwrap();
drop(Box::from_raw(token.zero.0.cast::<Packet<T>>()));
Ok(msg)
}
}
/// Attempts to send a message into the channel.
pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
let token = &mut Token::default();
let mut inner = self.inner.lock().unwrap();
// If there's a waiting receiver, pair up with it.
if let Some(operation) = inner.receivers.try_select() {
token.zero.0 = operation.packet;
drop(inner);
unsafe {
self.write(token, msg).ok().unwrap();
}
Ok(())
} else if inner.is_disconnected {
Err(TrySendError::Disconnected(msg))
} else {
Err(TrySendError::Full(msg))
}
}
/// Sends a message into the channel.
pub(crate) fn send(
&self,
msg: T,
deadline: Option<Instant>,
) -> Result<(), SendTimeoutError<T>> {
let token = &mut Token::default();
let mut inner = self.inner.lock().unwrap();
// If there's a waiting receiver, pair up with it.
if let Some(operation) = inner.receivers.try_select() {
token.zero.0 = operation.packet;
drop(inner);
unsafe {
self.write(token, msg).ok().unwrap();
}
return Ok(());
}
if inner.is_disconnected {
return Err(SendTimeoutError::Disconnected(msg));
}
Context::with(|cx| {
// Prepare for blocking until a receiver wakes us up.
let oper = Operation::hook(token);
let mut packet = Packet::<T>::message_on_stack(msg);
inner
.senders
.register_with_packet(oper, &mut packet as *mut Packet<T> as *mut (), cx);
inner.receivers.notify();
drop(inner);
// Block the current thread.
let sel = cx.wait_until(deadline);
match sel {
Selected::Waiting => unreachable!(),
Selected::Aborted => {
self.inner.lock().unwrap().senders.unregister(oper).unwrap();
let msg = unsafe { packet.msg.get().replace(None).unwrap() };
Err(SendTimeoutError::Timeout(msg))
}
Selected::Disconnected => {
self.inner.lock().unwrap().senders.unregister(oper).unwrap();
let msg = unsafe { packet.msg.get().replace(None).unwrap() };
Err(SendTimeoutError::Disconnected(msg))
}
Selected::Operation(_) => {
// Wait until the message is read, then drop the packet.
packet.wait_ready();
Ok(())
}
}
})
}
/// Attempts to receive a message without blocking.
pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
let token = &mut Token::default();
let mut inner = self.inner.lock().unwrap();
// If there's a waiting sender, pair up with it.
if let Some(operation) = inner.senders.try_select() {
token.zero.0 = operation.packet;
drop(inner);
unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
} else if inner.is_disconnected {
Err(TryRecvError::Disconnected)
} else {
Err(TryRecvError::Empty)
}
}
/// Receives a message from the channel.
pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
let token = &mut Token::default();
let mut inner = self.inner.lock().unwrap();
// If there's a waiting sender, pair up with it.
if let Some(operation) = inner.senders.try_select() {
token.zero.0 = operation.packet;
drop(inner);
unsafe {
return self.read(token).map_err(|_| RecvTimeoutError::Disconnected);
}
}
if inner.is_disconnected {
return Err(RecvTimeoutError::Disconnected);
}
Context::with(|cx| {
// Prepare for blocking until a sender wakes us up.
let oper = Operation::hook(token);
let mut packet = Packet::<T>::empty_on_stack();
inner.receivers.register_with_packet(
oper,
&mut packet as *mut Packet<T> as *mut (),
cx,
);
inner.senders.notify();
drop(inner);
// Block the current thread.
let sel = cx.wait_until(deadline);
match sel {
Selected::Waiting => unreachable!(),
Selected::Aborted => {
self.inner
.lock()
.unwrap()
.receivers
.unregister(oper)
.unwrap();
Err(RecvTimeoutError::Timeout)
}
Selected::Disconnected => {
self.inner
.lock()
.unwrap()
.receivers
.unregister(oper)
.unwrap();
Err(RecvTimeoutError::Disconnected)
}
Selected::Operation(_) => {
// Wait until the message is provided, then read it.
packet.wait_ready();
unsafe { Ok(packet.msg.get().replace(None).unwrap()) }
}
}
})
}
/// Disconnects the channel and wakes up all blocked senders and receivers.
///
/// Returns `true` if this call disconnected the channel.
pub(crate) fn disconnect(&self) -> bool {
let mut inner = self.inner.lock().unwrap();
if !inner.is_disconnected {
inner.is_disconnected = true;
inner.senders.disconnect();
inner.receivers.disconnect();
true
} else {
false
}
}
/// Returns the current number of messages inside the channel.
pub(crate) fn len(&self) -> usize {
0
}
/// Returns the capacity of the channel.
pub(crate) fn capacity(&self) -> Option<usize> {
Some(0)
}
/// Returns `true` if the channel is empty.
pub(crate) fn is_empty(&self) -> bool {
true
}
/// Returns `true` if the channel is full.
pub(crate) fn is_full(&self) -> bool {
true
}
}
/// Receiver handle to a channel.
pub(crate) struct Receiver<'a, T>(&'a Channel<T>);
/// Sender handle to a channel.
pub(crate) struct Sender<'a, T>(&'a Channel<T>);
impl<T> SelectHandle for Receiver<'_, T> {
fn try_select(&self, token: &mut Token) -> bool {
self.0.start_recv(token)
}
fn deadline(&self) -> Option<Instant> {
None
}
fn register(&self, oper: Operation, cx: &Context) -> bool {
let packet = Box::into_raw(Packet::<T>::empty_on_heap());
let mut inner = self.0.inner.lock().unwrap();
inner
.receivers
.register_with_packet(oper, packet.cast::<()>(), cx);
inner.senders.notify();
inner.senders.can_select() || inner.is_disconnected
}
fn unregister(&self, oper: Operation) {
if let Some(operation) = self.0.inner.lock().unwrap().receivers.unregister(oper) {
unsafe {
drop(Box::from_raw(operation.packet.cast::<Packet<T>>()));
}
}
}
fn accept(&self, token: &mut Token, cx: &Context) -> bool {
token.zero.0 = cx.wait_packet();
true
}
fn is_ready(&self) -> bool {
let inner = self.0.inner.lock().unwrap();
inner.senders.can_select() || inner.is_disconnected
}
fn watch(&self, oper: Operation, cx: &Context) -> bool {
let mut inner = self.0.inner.lock().unwrap();
inner.receivers.watch(oper, cx);
inner.senders.can_select() || inner.is_disconnected
}
fn unwatch(&self, oper: Operation) {
let mut inner = self.0.inner.lock().unwrap();
inner.receivers.unwatch(oper);
}
}
impl<T> SelectHandle for Sender<'_, T> {
fn try_select(&self, token: &mut Token) -> bool {
self.0.start_send(token)
}
fn deadline(&self) -> Option<Instant> {
None
}
fn register(&self, oper: Operation, cx: &Context) -> bool {
let packet = Box::into_raw(Packet::<T>::empty_on_heap());
let mut inner = self.0.inner.lock().unwrap();
inner
.senders
.register_with_packet(oper, packet.cast::<()>(), cx);
inner.receivers.notify();
inner.receivers.can_select() || inner.is_disconnected
}
fn unregister(&self, oper: Operation) {
if let Some(operation) = self.0.inner.lock().unwrap().senders.unregister(oper) {
unsafe {
drop(Box::from_raw(operation.packet.cast::<Packet<T>>()));
}
}
}
fn accept(&self, token: &mut Token, cx: &Context) -> bool {
token.zero.0 = cx.wait_packet();
true
}
fn is_ready(&self) -> bool {
let inner = self.0.inner.lock().unwrap();
inner.receivers.can_select() || inner.is_disconnected
}
fn watch(&self, oper: Operation, cx: &Context) -> bool {
let mut inner = self.0.inner.lock().unwrap();
inner.senders.watch(oper, cx);
inner.receivers.can_select() || inner.is_disconnected
}
fn unwatch(&self, oper: Operation) {
let mut inner = self.0.inner.lock().unwrap();
inner.senders.unwatch(oper);
}
}