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Fix memory leak of MpMcQueue #483

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Jun 30, 2024
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Fix memory leak of MpMcQueue #483

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1 change: 1 addition & 0 deletions CHANGELOG.md
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Expand Up @@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
- Fixed the list of implemented data structures in the crate docs, by adding `Deque`,
`HistoryBuffer` and `SortedLinkedList` to the list.
- Fixed `MpMcQueue` with `mpmc_large` feature.
- Fix missing `Drop` for `MpMcQueue`

## [v0.8.0] - 2023-11-07

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19 changes: 19 additions & 0 deletions src/mpmc.rs
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Expand Up @@ -194,6 +194,13 @@ impl<T, const N: usize> Default for MpMcQueue<T, N> {
}
}

impl<T, const N: usize> Drop for MpMcQueue<T, N> {
fn drop(&mut self) {
// drop all contents currently in the queue
while self.dequeue().is_some() {}
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This is copying the elements to the stack before dropping them, and running the complex dequeue() logic.

it'd be more efficient to read enqueue_pos/dequeue_pos, iterate, then .assume_init_drop() all the items in place. No copies, no atomic ops. This is sound because on drop we have &mut self so we know no other thread can be concurrently accessing the queue at this point.

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I would like to implement that but I'm not 100% certain I understand the layout of the Queue which is not documented, and which items are actually valid. I'm certain that the dequeue approach is correct so this is the approach I took.

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ah yes, it's true it's not that trivial. We can optimize later if someone complains.

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Let's open an Issue for that

}
}

unsafe impl<T, const N: usize> Sync for MpMcQueue<T, N> where T: Send {}

struct Cell<T> {
Expand Down Expand Up @@ -306,6 +313,18 @@ mod tests {
// Ensure a `MpMcQueue` containing `!Send` values stays `!Send` itself.
assert_not_impl_any!(MpMcQueue<*const (), 4>: Send);

#[test]
fn memory_leak() {
droppable!();

let q = Q2::new();
q.enqueue(Droppable::new()).unwrap_or_else(|_| panic!());
q.enqueue(Droppable::new()).unwrap_or_else(|_| panic!());
drop(q);

assert_eq!(Droppable::count(), 0);
}

#[test]
fn sanity() {
let q = Q2::new();
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