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chore: removing unnecessary arc in scoped threads implementation #125731

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chore: removing unnecessary arc in scoped threads implementation #125731

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13 changes: 4 additions & 9 deletions library/std/src/thread/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -164,7 +164,6 @@ use crate::env;
use crate::ffi::{CStr, CString};
use crate::fmt;
use crate::io;
use crate::marker::PhantomData;
use crate::mem::{self, forget};
use crate::num::NonZero;
use crate::panic;
Expand Down Expand Up @@ -459,7 +458,7 @@ impl Builder {
unsafe fn spawn_unchecked_<'a, 'scope, F, T>(
self,
f: F,
scope_data: Option<Arc<scoped::ScopeData>>,
scope_data: Option<&'scope scoped::ScopeData>,
) -> io::Result<JoinInner<'scope, T>>
where
F: FnOnce() -> T,
Expand Down Expand Up @@ -494,11 +493,8 @@ impl Builder {
});
let their_thread = my_thread.clone();

let my_packet: Arc<Packet<'scope, T>> = Arc::new(Packet {
scope: scope_data,
result: UnsafeCell::new(None),
_marker: PhantomData,
});
let my_packet: Arc<Packet<'scope, T>> =
Arc::new(Packet { scope: scope_data, result: UnsafeCell::new(None) });
let their_packet = my_packet.clone();

let output_capture = crate::io::set_output_capture(None);
Expand Down Expand Up @@ -1535,9 +1531,8 @@ pub type Result<T> = crate::result::Result<T, Box<dyn Any + Send + 'static>>;
// An Arc to the packet is stored into a `JoinInner` which in turns is placed
// in `JoinHandle`.
struct Packet<'scope, T> {
scope: Option<Arc<scoped::ScopeData>>,
scope: Option<&'scope scoped::ScopeData>,
result: UnsafeCell<Option<Result<T>>>,
_marker: PhantomData<Option<&'scope scoped::ScopeData>>,
}

// Due to the usage of `UnsafeCell` we need to manually implement Sync.
Expand Down
10 changes: 4 additions & 6 deletions library/std/src/thread/scoped.rs
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Expand Up @@ -11,7 +11,7 @@ use crate::sync::Arc;
/// See [`scope`] for details.
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub struct Scope<'scope, 'env: 'scope> {
data: Arc<ScopeData>,
data: ScopeData,
/// Invariance over 'scope, to make sure 'scope cannot shrink,
/// which is necessary for soundness.
///
Expand Down Expand Up @@ -136,14 +136,12 @@ pub fn scope<'env, F, T>(f: F) -> T
where
F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T,
{
// We put the `ScopeData` into an `Arc` so that other threads can finish their
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that justification seems fairly clear, so you'd need to show why it's incorrect

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Idk, it wasn't clear to me when I read it. The way I saw it, the scoped data outlives the place where it is joined, so it seemed reasonable a reference should outlive it.

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improving the comment would be a nice improvement then :)

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@workingjubilee workingjubilee May 29, 2024 •
edited
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for an example of how the lifetimes involved can be very complex and counterintuitive even in the scoped thread case, see:

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improving the comment would be a nice improvement then :)

Yeah, definitely.

// `decrement_num_running_threads` even after this function returns.
let scope = Scope {
data: Arc::new(ScopeData {
data: ScopeData {
num_running_threads: AtomicUsize::new(0),
main_thread: current(),
a_thread_panicked: AtomicBool::new(false),
}),
},
env: PhantomData,
scope: PhantomData,
};
Expand Down Expand Up @@ -258,7 +256,7 @@ impl Builder {
F: FnOnce() -> T + Send + 'scope,
T: Send + 'scope,
{
Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(f, Some(scope.data.clone())) }?))
Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(f, Some(&scope.data)) }?))
}
}

Expand Down
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