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/// Shared channels.
///
/// This is the flavor of channels which are not necessarily optimized for any
/// particular use case, but are the most general in how they are used. Shared
/// channels are cloneable allowing for multiple senders.
///
/// High level implementation details can be found in the comment of the parent
/// module. You'll also note that the implementation of the shared and stream
/// channels are quite similar, and this is no coincidence!
pub use self::Failure::*;
use self::StartResult::*;
use core::cmp;
use core::intrinsics::abort;
use crate::cell::UnsafeCell;
use crate::ptr;
use crate::sync::atomic::{AtomicBool, AtomicIsize, AtomicUsize, Ordering};
use crate::sync::mpsc::blocking::{self, SignalToken};
use crate::sync::mpsc::mpsc_queue as mpsc;
use crate::sync::{Mutex, MutexGuard};
use crate::thread;
use crate::time::Instant;
const DISCONNECTED: isize = isize::MIN;
const FUDGE: isize = 1024;
const MAX_REFCOUNT: usize = (isize::MAX) as usize;
#[cfg(test)]
const MAX_STEALS: isize = 5;
#[cfg(not(test))]
const MAX_STEALS: isize = 1 << 20;
pub struct Packet<T> {
queue: mpsc::Queue<T>,
cnt: AtomicIsize, // How many items are on this channel
steals: UnsafeCell<isize>, // How many times has a port received without blocking?
to_wake: AtomicUsize, // SignalToken for wake up
// The number of channels which are currently using this packet.
channels: AtomicUsize,
// See the discussion in Port::drop and the channel send methods for what
// these are used for
port_dropped: AtomicBool,
sender_drain: AtomicIsize,
// this lock protects various portions of this implementation during
// select()
select_lock: Mutex<()>,
}
pub enum Failure {
Empty,
Disconnected,
}
#[derive(PartialEq, Eq)]
enum StartResult {
Installed,
Abort,
}
impl<T> Packet<T> {
// Creation of a packet *must* be followed by a call to postinit_lock
// and later by inherit_blocker
pub fn new() -> Packet<T> {
Packet {
queue: mpsc::Queue::new(),
cnt: AtomicIsize::new(0),
steals: UnsafeCell::new(0),
to_wake: AtomicUsize::new(0),
channels: AtomicUsize::new(2),
port_dropped: AtomicBool::new(false),
sender_drain: AtomicIsize::new(0),
select_lock: Mutex::new(()),
}
}
// This function should be used after newly created Packet
// was wrapped with an Arc
// In other case mutex data will be duplicated while cloning
// and that could cause problems on platforms where it is
// represented by opaque data structure
pub fn postinit_lock(&self) -> MutexGuard<'_, ()> {
self.select_lock.lock().unwrap()
}
// This function is used at the creation of a shared packet to inherit a
// previously blocked thread. This is done to prevent spurious wakeups of
// threads in select().
//
// This can only be called at channel-creation time
pub fn inherit_blocker(&self, token: Option<SignalToken>, guard: MutexGuard<'_, ()>) {
if let Some(token) = token {
assert_eq!(self.cnt.load(Ordering::SeqCst), 0);
assert_eq!(self.to_wake.load(Ordering::SeqCst), 0);
self.to_wake.store(unsafe { token.cast_to_usize() }, Ordering::SeqCst);
self.cnt.store(-1, Ordering::SeqCst);
// This store is a little sketchy. What's happening here is that
// we're transferring a blocker from a oneshot or stream channel to
// this shared channel. In doing so, we never spuriously wake them
// up and rather only wake them up at the appropriate time. This
// implementation of shared channels assumes that any blocking
// recv() will undo the increment of steals performed in try_recv()
// once the recv is complete. This thread that we're inheriting,
// however, is not in the middle of recv. Hence, the first time we
// wake them up, they're going to wake up from their old port, move
// on to the upgraded port, and then call the block recv() function.
//
// When calling this function, they'll find there's data immediately
// available, counting it as a steal. This in fact wasn't a steal
// because we appropriately blocked them waiting for data.
//
// To offset this bad increment, we initially set the steal count to
// -1. You'll find some special code in abort_selection() as well to
// ensure that this -1 steal count doesn't escape too far.
unsafe {
*self.steals.get() = -1;
}
}
// When the shared packet is constructed, we grabbed this lock. The
// purpose of this lock is to ensure that abort_selection() doesn't
// interfere with this method. After we unlock this lock, we're
// signifying that we're done modifying self.cnt and self.to_wake and
// the port is ready for the world to continue using it.
drop(guard);
}
pub fn send(&self, t: T) -> Result<(), T> {
// See Port::drop for what's going on
if self.port_dropped.load(Ordering::SeqCst) {
return Err(t);
}
// Note that the multiple sender case is a little trickier
// semantically than the single sender case. The logic for
// incrementing is "add and if disconnected store disconnected".
// This could end up leading some senders to believe that there
// wasn't a disconnect if in fact there was a disconnect. This means
// that while one thread is attempting to re-store the disconnected
// states, other threads could walk through merrily incrementing
// this very-negative disconnected count. To prevent senders from
// spuriously attempting to send when the channels is actually
// disconnected, the count has a ranged check here.
//
// This is also done for another reason. Remember that the return
// value of this function is:
//
// `true` == the data *may* be received, this essentially has no
// meaning
// `false` == the data will *never* be received, this has a lot of
// meaning
//
// In the SPSC case, we have a check of 'queue.is_empty()' to see
// whether the data was actually received, but this same condition
// means nothing in a multi-producer context. As a result, this
// preflight check serves as the definitive "this will never be
// received". Once we get beyond this check, we have permanently
// entered the realm of "this may be received"
if self.cnt.load(Ordering::SeqCst) < DISCONNECTED + FUDGE {
return Err(t);
}
self.queue.push(t);
match self.cnt.fetch_add(1, Ordering::SeqCst) {
-1 => {
self.take_to_wake().signal();
}
// In this case, we have possibly failed to send our data, and
// we need to consider re-popping the data in order to fully
// destroy it. We must arbitrate among the multiple senders,
// however, because the queues that we're using are
// single-consumer queues. In order to do this, all exiting
// pushers will use an atomic count in order to count those
// flowing through. Pushers who see 0 are required to drain as
// much as possible, and then can only exit when they are the
// only pusher (otherwise they must try again).
n if n < DISCONNECTED + FUDGE => {
// see the comment in 'try' for a shared channel for why this
// window of "not disconnected" is ok.
self.cnt.store(DISCONNECTED, Ordering::SeqCst);
if self.sender_drain.fetch_add(1, Ordering::SeqCst) == 0 {
loop {
// drain the queue, for info on the thread yield see the
// discussion in try_recv
loop {
match self.queue.pop() {
mpsc::Data(..) => {}
mpsc::Empty => break,
mpsc::Inconsistent => thread::yield_now(),
}
}
// maybe we're done, if we're not the last ones
// here, then we need to go try again.
if self.sender_drain.fetch_sub(1, Ordering::SeqCst) == 1 {
break;
}
}
// At this point, there may still be data on the queue,
// but only if the count hasn't been incremented and
// some other sender hasn't finished pushing data just
// yet. That sender in question will drain its own data.
}
}
// Can't make any assumptions about this case like in the SPSC case.
_ => {}
}
Ok(())
}
pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure> {
// This code is essentially the exact same as that found in the stream
// case (see stream.rs)
match self.try_recv() {
Err(Empty) => {}
data => return data,
}
let (wait_token, signal_token) = blocking::tokens();
if self.decrement(signal_token) == Installed {
if let Some(deadline) = deadline {
let timed_out = !wait_token.wait_max_until(deadline);
if timed_out {
self.abort_selection(false);
}
} else {
wait_token.wait();
}
}
match self.try_recv() {
data @ Ok(..) => unsafe {
*self.steals.get() -= 1;
data
},
data => data,
}
}
// Essentially the exact same thing as the stream decrement function.
// Returns true if blocking should proceed.
fn decrement(&self, token: SignalToken) -> StartResult {
unsafe {
assert_eq!(
self.to_wake.load(Ordering::SeqCst),
0,
"This is a known bug in the Rust standard library. See https://github.com/rust-lang/rust/issues/39364"
);
let ptr = token.cast_to_usize();
self.to_wake.store(ptr, Ordering::SeqCst);
let steals = ptr::replace(self.steals.get(), 0);
match self.cnt.fetch_sub(1 + steals, Ordering::SeqCst) {
DISCONNECTED => {
self.cnt.store(DISCONNECTED, Ordering::SeqCst);
}
// If we factor in our steals and notice that the channel has no
// data, we successfully sleep
n => {
assert!(n >= 0);
if n - steals <= 0 {
return Installed;
}
}
}
self.to_wake.store(0, Ordering::SeqCst);
drop(SignalToken::cast_from_usize(ptr));
Abort
}
}
pub fn try_recv(&self) -> Result<T, Failure> {
let ret = match self.queue.pop() {
mpsc::Data(t) => Some(t),
mpsc::Empty => None,
// This is a bit of an interesting case. The channel is reported as
// having data available, but our pop() has failed due to the queue
// being in an inconsistent state. This means that there is some
// pusher somewhere which has yet to complete, but we are guaranteed
// that a pop will eventually succeed. In this case, we spin in a
// yield loop because the remote sender should finish their enqueue
// operation "very quickly".
//
// Avoiding this yield loop would require a different queue
// abstraction which provides the guarantee that after M pushes have
// succeeded, at least M pops will succeed. The current queues
// guarantee that if there are N active pushes, you can pop N times
// once all N have finished.
mpsc::Inconsistent => {
let data;
loop {
thread::yield_now();
match self.queue.pop() {
mpsc::Data(t) => {
data = t;
break;
}
mpsc::Empty => panic!("inconsistent => empty"),
mpsc::Inconsistent => {}
}
}
Some(data)
}
};
match ret {
// See the discussion in the stream implementation for why we
// might decrement steals.
Some(data) => unsafe {
if *self.steals.get() > MAX_STEALS {
match self.cnt.swap(0, Ordering::SeqCst) {
DISCONNECTED => {
self.cnt.store(DISCONNECTED, Ordering::SeqCst);
}
n => {
let m = cmp::min(n, *self.steals.get());
*self.steals.get() -= m;
self.bump(n - m);
}
}
assert!(*self.steals.get() >= 0);
}
*self.steals.get() += 1;
Ok(data)
},
// See the discussion in the stream implementation for why we try
// again.
None => {
match self.cnt.load(Ordering::SeqCst) {
n if n != DISCONNECTED => Err(Empty),
_ => {
match self.queue.pop() {
mpsc::Data(t) => Ok(t),
mpsc::Empty => Err(Disconnected),
// with no senders, an inconsistency is impossible.
mpsc::Inconsistent => unreachable!(),
}
}
}
}
}
}
// Prepares this shared packet for a channel clone, essentially just bumping
// a refcount.
pub fn clone_chan(&self) {
let old_count = self.channels.fetch_add(1, Ordering::SeqCst);
// See comments on Arc::clone() on why we do this (for `mem::forget`).
if old_count > MAX_REFCOUNT {
abort();
}
}
// Decrement the reference count on a channel. This is called whenever a
// Chan is dropped and may end up waking up a receiver. It's the receiver's
// responsibility on the other end to figure out that we've disconnected.
pub fn drop_chan(&self) {
match self.channels.fetch_sub(1, Ordering::SeqCst) {
1 => {}
n if n > 1 => return,
n => panic!("bad number of channels left {}", n),
}
match self.cnt.swap(DISCONNECTED, Ordering::SeqCst) {
-1 => {
self.take_to_wake().signal();
}
DISCONNECTED => {}
n => {
assert!(n >= 0);
}
}
}
// See the long discussion inside of stream.rs for why the queue is drained,
// and why it is done in this fashion.
pub fn drop_port(&self) {
self.port_dropped.store(true, Ordering::SeqCst);
let mut steals = unsafe { *self.steals.get() };
while {
match self.cnt.compare_exchange(
steals,
DISCONNECTED,
Ordering::SeqCst,
Ordering::SeqCst,
) {
Ok(_) => false,
Err(old) => old != DISCONNECTED,
}
} {
// See the discussion in 'try_recv' for why we yield
// control of this thread.
loop {
match self.queue.pop() {
mpsc::Data(..) => {
steals += 1;
}
mpsc::Empty | mpsc::Inconsistent => break,
}
}
}
}
// Consumes ownership of the 'to_wake' field.
fn take_to_wake(&self) -> SignalToken {
let ptr = self.to_wake.load(Ordering::SeqCst);
self.to_wake.store(0, Ordering::SeqCst);
assert!(ptr != 0);
unsafe { SignalToken::cast_from_usize(ptr) }
}
////////////////////////////////////////////////////////////////////////////
// select implementation
////////////////////////////////////////////////////////////////////////////
// increment the count on the channel (used for selection)
fn bump(&self, amt: isize) -> isize {
match self.cnt.fetch_add(amt, Ordering::SeqCst) {
DISCONNECTED => {
self.cnt.store(DISCONNECTED, Ordering::SeqCst);
DISCONNECTED
}
n => n,
}
}
// Cancels a previous thread waiting on this port, returning whether there's
// data on the port.
//
// This is similar to the stream implementation (hence fewer comments), but
// uses a different value for the "steals" variable.
pub fn abort_selection(&self, _was_upgrade: bool) -> bool {
// Before we do anything else, we bounce on this lock. The reason for
// doing this is to ensure that any upgrade-in-progress is gone and
// done with. Without this bounce, we can race with inherit_blocker
// about looking at and dealing with to_wake. Once we have acquired the
// lock, we are guaranteed that inherit_blocker is done.
{
let _guard = self.select_lock.lock().unwrap();
}
// Like the stream implementation, we want to make sure that the count
// on the channel goes non-negative. We don't know how negative the
// stream currently is, so instead of using a steal value of 1, we load
// the channel count and figure out what we should do to make it
// positive.
let steals = {
let cnt = self.cnt.load(Ordering::SeqCst);
if cnt < 0 && cnt != DISCONNECTED { -cnt } else { 0 }
};
let prev = self.bump(steals + 1);
if prev == DISCONNECTED {
assert_eq!(self.to_wake.load(Ordering::SeqCst), 0);
true
} else {
let cur = prev + steals + 1;
assert!(cur >= 0);
if prev < 0 {
drop(self.take_to_wake());
} else {
while self.to_wake.load(Ordering::SeqCst) != 0 {
thread::yield_now();
}
}
unsafe {
// if the number of steals is -1, it was the pre-emptive -1 steal
// count from when we inherited a blocker. This is fine because
// we're just going to overwrite it with a real value.
let old = self.steals.get();
assert!(*old == 0 || *old == -1);
*old = steals;
prev >= 0
}
}
}
}
impl<T> Drop for Packet<T> {
fn drop(&mut self) {
// Note that this load is not only an assert for correctness about
// disconnection, but also a proper fence before the read of
// `to_wake`, so this assert cannot be removed with also removing
// the `to_wake` assert.
assert_eq!(self.cnt.load(Ordering::SeqCst), DISCONNECTED);
assert_eq!(self.to_wake.load(Ordering::SeqCst), 0);
assert_eq!(self.channels.load(Ordering::SeqCst), 0);
}
}