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Simplify WaitGroup implementation #958

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Simplify WaitGroup implementation #958

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41 changes: 20 additions & 21 deletions src/storage.rs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
//! Public API facades for the implementation details of [`Zalsa`] and [`ZalsaLocal`].
use std::marker::PhantomData;
use std::mem;
use std::panic::RefUnwindSafe;

use crate::database::RawDatabase;
Expand All @@ -25,8 +26,6 @@ pub struct StorageHandle<Db> {

impl<Db> Clone for StorageHandle<Db> {
fn clone(&self) -> Self {
*self.coordinate.clones.lock() += 1;

Self {
zalsa_impl: self.zalsa_impl.clone(),
coordinate: CoordinateDrop(Arc::clone(&self.coordinate)),
Expand All @@ -53,7 +52,7 @@ impl<Db: Database> StorageHandle<Db> {
Self {
zalsa_impl: Arc::new(Zalsa::new::<Db>(event_callback, jars)),
coordinate: CoordinateDrop(Arc::new(Coordinate {
clones: Mutex::new(1),
coordinate_lock: Mutex::default(),
cvar: Default::default(),
})),
phantom: PhantomData,
Expand Down Expand Up @@ -95,17 +94,6 @@ impl<Db> Drop for Storage<Db> {
}
}

struct Coordinate {
/// Counter of the number of clones of actor. Begins at 1.
/// Incremented when cloned, decremented when dropped.
clones: Mutex<usize>,
cvar: Condvar,
}

// We cannot panic while holding a lock to `clones: Mutex<usize>` and therefore we cannot enter an
// inconsistent state.
impl RefUnwindSafe for Coordinate {}

impl<Db: Database> Default for Storage<Db> {
fn default() -> Self {
Self::new(None)
Expand Down Expand Up @@ -168,12 +156,14 @@ impl<Db: Database> Storage<Db> {
.zalsa_impl
.event(&|| Event::new(EventKind::DidSetCancellationFlag));

let mut clones = self.handle.coordinate.clones.lock();
while *clones != 1 {
clones = self.handle.coordinate.cvar.wait(clones);
}
// The ref count on the `Arc` should now be 1
let zalsa = Arc::get_mut(&mut self.handle.zalsa_impl).unwrap();
let mut coordinate_lock = self.handle.coordinate.coordinate_lock.lock();
let zalsa = loop {
if let Some(zalsa) = Arc::get_mut(&mut self.handle.zalsa_impl) {
// SAFETY: Polonius when ... https://github.com/rust-lang/rfcs/blob/master/text/2094-nll.md#problem-case-3-conditional-control-flow-across-functions
break unsafe { mem::transmute::<&mut Zalsa, &mut Zalsa>(zalsa) };
}
Comment on lines +161 to +164
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@ibraheemdev ibraheemdev Aug 5, 2025 •
edited
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Suggested change
if let Some(zalsa) = Arc::get_mut(&mut self.handle.zalsa_impl) {
// SAFETY: Polonius when ... https://github.com/rust-lang/rfcs/blob/master/text/2094-nll.md#problem-case-3-conditional-control-flow-across-functions
break unsafe { mem::transmute::<&mut Zalsa, &mut Zalsa>(zalsa) };
}
if Arc::strong_count(&self.handle.zalsa_impl) == 1 {
// SAFETY: The strong count is 1, and we never create any weak pointers,
// so we have a unique reference.
break unsafe { &mut *(Arc::as_ptr(&self.handle.zalsa_impl) as *mut Zalsa) };
}

This should avoid the Polonius issue and is a bit cheaper. There is a little question about writing through Arc::as_ptr, but it is valid in the current implementation and nothing is really blocking it being guaranteed (see rust-lang/rust#104337).

coordinate_lock = self.handle.coordinate.cvar.wait(coordinate_lock);
};
// cancellation is done, so reset the flag
zalsa.runtime_mut().reset_cancellation_flag();
zalsa
Expand Down Expand Up @@ -260,6 +250,16 @@ impl<Db: Database> Clone for Storage<Db> {
}
}

/// A simplified `WaitGroup`, this is used together with `Arc<Zalsa>` as the actual counter
struct Coordinate {
coordinate_lock: Mutex<()>,
cvar: Condvar,
}

// We cannot panic while holding a lock to `clones: Mutex<usize>` and therefore we cannot enter an
// inconsistent state.
impl RefUnwindSafe for Coordinate {}

struct CoordinateDrop(Arc<Coordinate>);

impl std::ops::Deref for CoordinateDrop {
Expand All @@ -272,7 +272,6 @@ impl std::ops::Deref for CoordinateDrop {

impl Drop for CoordinateDrop {
fn drop(&mut self) {
*self.0.clones.lock() -= 1;
self.0.cvar.notify_all();
}
}
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