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std: drop all messages in bounded channel when destroying the last receiver

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This commit is contained in:
joboet 2023-02-17 15:47:58 +01:00
parent a64ef7d07d
commit 746331edf3
2 changed files with 109 additions and 27 deletions
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132
library/std/src/sync/mpmc/array.rs
View File

@ -15,7 +15,7 @@
use super::waker::SyncWaker;
use crate::cell::UnsafeCell;
use crate::mem::MaybeUninit;
use crate::mem::{self, MaybeUninit};
use crate::ptr;
use crate::sync::atomic::{self, AtomicUsize, Ordering};
use crate::time::Instant;
@ -25,7 +25,8 @@ struct Slot<T> {
/// The current stamp.
stamp: AtomicUsize,
/// The message in this slot.
/// The message in this slot. Either read out in `read` or dropped through
/// `discard_all_messages`.
msg: UnsafeCell<MaybeUninit<T>>,
}
@ -439,14 +440,13 @@ pub(crate) fn capacity(&self) -> Option<usize> {
Some(self.cap)
}
/// Disconnects the channel and wakes up all blocked senders and receivers.
/// Disconnects senders and wakes up all blocked receivers.
///
/// Returns `true` if this call disconnected the channel.
pub(crate) fn disconnect(&self) -> bool {
pub(crate) fn disconnect_senders(&self) -> bool {
let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
if tail & self.mark_bit == 0 {
self.senders.disconnect();
self.receivers.disconnect();
true
} else {
@ -454,6 +454,108 @@ pub(crate) fn disconnect(&self) -> bool {
}
}
/// Disconnects receivers and wakes up all blocked senders.
///
/// Returns `true` if this call disconnected the channel.
///
/// # Safety
/// May only be called once upon dropping the last receiver. The
/// destruction of all other receivers must have been observed with acquire
/// ordering or stronger.
pub(crate) unsafe fn disconnect_receivers(&self) -> bool {
let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
self.discard_all_messages(tail);
if tail & self.mark_bit == 0 {
self.senders.disconnect();
true
} else {
false
}
}
/// Discards all messages.
///
/// `tail` should be the current (and therefore last) value of `tail`.
///
/// # Safety
/// This method must only be called when dropping the last receiver. The
/// destruction of all other receivers must have been observed with acquire
/// ordering or stronger.
unsafe fn discard_all_messages(&self, tail: usize) {
debug_assert!(self.is_disconnected());
/// Use a helper struct with a custom `Drop` to ensure all messages are
/// dropped, even if a destructor panicks.
struct DiscardState<'a, T> {
channel: &'a Channel<T>,
head: usize,
tail: usize,
backoff: Backoff,
}
impl<'a, T> DiscardState<'a, T> {
fn discard(&mut self) {
loop {
// Deconstruct the head.
let index = self.head & (self.channel.mark_bit - 1);
let lap = self.head & !(self.channel.one_lap - 1);
// Inspect the corresponding slot.
debug_assert!(index < self.channel.buffer.len());
let slot = unsafe { self.channel.buffer.get_unchecked(index) };
let stamp = slot.stamp.load(Ordering::Acquire);
// If the stamp is ahead of the head by 1, we may drop the message.
if self.head + 1 == stamp {
self.head = if index + 1 < self.channel.cap {
// Same lap, incremented index.
// Set to `{ lap: lap, mark: 0, index: index + 1 }`.
self.head + 1
} else {
// One lap forward, index wraps around to zero.
// Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
lap.wrapping_add(self.channel.one_lap)
};
// We updated the head, so even if this descrutor panics,
// we will not attempt to destroy the slot again.
unsafe {
(*slot.msg.get()).assume_init_drop();
}
// If the tail equals the head, that means the channel is empty.
} else if self.tail == self.head {
return;
// Otherwise, a sender is about to write into the slot, so we need
// to wait for it to update the stamp.
} else {
self.backoff.spin_heavy();
}
}
}
}
impl<'a, T> Drop for DiscardState<'a, T> {
fn drop(&mut self) {
self.discard();
}
}
let mut state = DiscardState {
channel: self,
// Only receivers modify `head`, so since we are the last one,
// this value will not change and will not be observed (since
// no new messages can be sent after disconnection).
head: self.head.load(Ordering::Relaxed),
tail: tail & !self.mark_bit,
backoff: Backoff::new(),
};
state.discard();
// This point is only reached if no destructor panics, so all messages
// have already been dropped.
mem::forget(state);
}
/// Returns `true` if the channel is disconnected.
pub(crate) fn is_disconnected(&self) -> bool {
self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
@ -483,23 +585,3 @@ pub(crate) fn is_full(&self) -> bool {
head.wrapping_add(self.one_lap) == tail & !self.mark_bit
}
}
impl<T> Drop for Channel<T> {
fn drop(&mut self) {
// Get the index of the head.
let hix = self.head.load(Ordering::Relaxed) & (self.mark_bit - 1);
// Loop over all slots that hold a message and drop them.
for i in 0..self.len() {
// Compute the index of the next slot holding a message.
let index = if hix + i < self.cap { hix + i } else { hix + i - self.cap };
unsafe {
debug_assert!(index < self.buffer.len());
let slot = self.buffer.get_unchecked_mut(index);
let msg = &mut *slot.msg.get();
msg.as_mut_ptr().drop_in_place();
}
}
}
}

4
library/std/src/sync/mpmc/mod.rs
View File

@ -227,7 +227,7 @@ impl<T> Drop for Sender<T> {
fn drop(&mut self) {
unsafe {
match &self.flavor {
SenderFlavor::Array(chan) => chan.release(|c| c.disconnect()),
SenderFlavor::Array(chan) => chan.release(|c| c.disconnect_senders()),
SenderFlavor::List(chan) => chan.release(|c| c.disconnect_senders()),
SenderFlavor::Zero(chan) => chan.release(|c| c.disconnect()),
}
@ -403,7 +403,7 @@ impl<T> Drop for Receiver<T> {
fn drop(&mut self) {
unsafe {
match &self.flavor {
ReceiverFlavor::Array(chan) => chan.release(|c| c.disconnect()),
ReceiverFlavor::Array(chan) => chan.release(|c| c.disconnect_receivers()),
ReceiverFlavor::List(chan) => chan.release(|c| c.disconnect_receivers()),
ReceiverFlavor::Zero(chan) => chan.release(|c| c.disconnect()),
}