chore: don't use OnceCell on LTS 1.18.x

tokio/Cargo.toml

@@ -56,7 +56,6 @@ net = [
 ]
 process = [
   "bytes",
-  "once_cell",
   "libc",
   "mio/os-poll",
   "mio/os-ext",
@@ -65,13 +64,12 @@ process = [
   "winapi/threadpoollegacyapiset",
 ]
 # Includes basic task execution capabilities
-rt = ["once_cell"]
+rt = []
 rt-multi-thread = [
   "num_cpus",
   "rt",
 ]
 signal = [
-  "once_cell",
   "libc",
   "mio/os-poll",
   "mio/net",
@@ -95,7 +93,6 @@ pin-project-lite = "0.2.0"
 
 # Everything else is optional...
 bytes = { version = "1.0.0", optional = true }
-once_cell = { version = "1.5.2", optional = true }
 memchr = { version = "2.2", optional = true }
 mio = { version = "0.8.1", optional = true }
 socket2 = { version = "0.4.4", optional = true, features = [ "all" ] }

tokio/src/loom/std/parking_lot.rs

@@ -52,7 +52,7 @@ impl<T> Mutex<T> {
     }
 
     #[inline]
-    #[cfg(all(feature = "parking_lot", not(all(loom, test)),))]
+    #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
     #[cfg_attr(docsrs, doc(cfg(all(feature = "parking_lot",))))]
     pub(crate) const fn const_new(t: T) -> Mutex<T> {
         Mutex(PhantomData, parking_lot::const_mutex(t))

tokio/src/process/unix/mod.rs

@@ -34,10 +34,10 @@ use crate::process::kill::Kill;
 use crate::process::SpawnedChild;
 use crate::signal::unix::driver::Handle as SignalHandle;
 use crate::signal::unix::{signal, Signal, SignalKind};
+use crate::util::once_cell::OnceCell;
 
 use mio::event::Source;
 use mio::unix::SourceFd;
-use once_cell::sync::Lazy;
 use std::fmt;
 use std::fs::File;
 use std::future::Future;
@@ -64,25 +64,29 @@ impl Kill for StdChild {
     }
 }
 
-static ORPHAN_QUEUE: Lazy<OrphanQueueImpl<StdChild>> = Lazy::new(OrphanQueueImpl::new);
+fn get_orphan_queue() -> &'static OrphanQueueImpl<StdChild> {
+    static ORPHAN_QUEUE: OnceCell<OrphanQueueImpl<StdChild>> = OnceCell::new();
+
+    ORPHAN_QUEUE.get(OrphanQueueImpl::new)
+}
 
 pub(crate) struct GlobalOrphanQueue;
 
 impl fmt::Debug for GlobalOrphanQueue {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        ORPHAN_QUEUE.fmt(fmt)
+        get_orphan_queue().fmt(fmt)
     }
 }
 
 impl GlobalOrphanQueue {
     fn reap_orphans(handle: &SignalHandle) {
-        ORPHAN_QUEUE.reap_orphans(handle)
+        get_orphan_queue().reap_orphans(handle)
     }
 }
 
 impl OrphanQueue<StdChild> for GlobalOrphanQueue {
     fn push_orphan(&self, orphan: StdChild) {
-        ORPHAN_QUEUE.push_orphan(orphan)
+        get_orphan_queue().push_orphan(orphan)
     }
 }
 

tokio/src/runtime/task/mod.rs

@@ -500,11 +500,17 @@ impl Id {
 
     cfg_not_has_atomic_u64! {
         pub(crate) fn next() -> Self {
-            use once_cell::sync::Lazy;
+            use crate::util::once_cell::OnceCell;
             use crate::loom::sync::Mutex;
 
-            static NEXT_ID: Lazy<Mutex<u64>> = Lazy::new(|| Mutex::new(1));
-            let mut lock = NEXT_ID.lock();
+            fn init_next_id() -> Mutex<u64> {
+                Mutex::new(1)
+            }
+
+            static NEXT_ID: OnceCell<Mutex<u64>> = OnceCell::new();
+
+            let next_id = NEXT_ID.get(init_next_id);
+            let mut lock = next_id.lock();
             let id = *lock;
             *lock += 1;
             Self(id)

tokio/src/signal/registry.rs

@@ -1,10 +1,9 @@
 #![allow(clippy::unit_arg)]
 
 use crate::signal::os::{OsExtraData, OsStorage};
-
 use crate::sync::watch;
+use crate::util::once_cell::OnceCell;
 
-use once_cell::sync::Lazy;
 use std::ops;
 use std::pin::Pin;
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -152,19 +151,25 @@ impl Globals {
     }
 }
 
+fn globals_init() -> Globals
+where
+    OsExtraData: 'static + Send + Sync + Init,
+    OsStorage: 'static + Send + Sync + Init,
+{
+    Globals {
+        extra: OsExtraData::init(),
+        registry: Registry::new(OsStorage::init()),
+    }
+}
+
 pub(crate) fn globals() -> Pin<&'static Globals>
 where
     OsExtraData: 'static + Send + Sync + Init,
     OsStorage: 'static + Send + Sync + Init,
 {
-    static GLOBALS: Lazy<Pin<Box<Globals>>> = Lazy::new(|| {
-        Box::pin(Globals {
-            extra: OsExtraData::init(),
-            registry: Registry::new(OsStorage::init()),
-        })
-    });
-
-    GLOBALS.as_ref()
+    static GLOBALS: OnceCell<Globals> = OnceCell::new();
+
+    Pin::new(GLOBALS.get(globals_init))
 }
 
 #[cfg(all(test, not(loom)))]

tokio/src/util/mod.rs

@@ -6,6 +6,9 @@ cfg_io_driver! {
 #[cfg(feature = "rt")]
 pub(crate) mod atomic_cell;
 
+#[cfg(any(feature = "rt", feature = "signal", feature = "process"))]
+pub(crate) mod once_cell;
+
 #[cfg(any(
     // io driver uses `WakeList` directly
     feature = "net",

tokio/src/util/once_cell.rs

@@ -0,0 +1,69 @@
+use std::cell::UnsafeCell;
+use std::mem::MaybeUninit;
+use std::sync::Once;
+
+pub(crate) struct OnceCell<T> {
+    once: Once,
+    value: UnsafeCell<MaybeUninit<T>>,
+}
+
+unsafe impl<T: Send + Sync> Send for OnceCell<T> {}
+unsafe impl<T: Send + Sync> Sync for OnceCell<T> {}
+
+impl<T> OnceCell<T> {
+    pub(crate) const fn new() -> Self {
+        Self {
+            once: Once::new(),
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+        }
+    }
+
+    /// Get the value inside this cell, intiailizing it using the provided
+    /// function if necessary.
+    ///
+    /// If the `init` closure panics, then the `OnceCell` is poisoned and all
+    /// future calls to `get` will panic.
+    #[inline]
+    pub(crate) fn get(&self, init: fn() -> T) -> &T {
+        if !self.once.is_completed() {
+            self.do_init(init);
+        }
+
+        // Safety: The `std::sync::Once` guarantees that we can only reach this
+        // line if a `call_once` closure has been run exactly once and without
+        // panicking. Thus, the value is not uninitialized.
+        //
+        // There is also no race because the only `&self` method that modifies
+        // `value` is `do_init`, but if the `call_once` closure is still
+        // running, then no thread has gotten past the `call_once`.
+        unsafe { &*(self.value.get() as *const T) }
+    }
+
+    #[cold]
+    fn do_init(&self, init: fn() -> T) {
+        let value_ptr = self.value.get() as *mut T;
+
+        self.once.call_once(|| {
+            let set_to = init();
+
+            // Safety: The `std::sync::Once` guarantees that this initialization
+            // will run at most once, and that no thread can get past the
+            // `call_once` until it has run exactly once. Thus, we have
+            // exclusive access to `value`.
+            unsafe {
+                std::ptr::write(value_ptr, set_to);
+            }
+        });
+    }
+}
+
+impl<T> Drop for OnceCell<T> {
+    fn drop(&mut self) {
+        if self.once.is_completed() {
+            let value_ptr = self.value.get() as *mut T;
+            unsafe {
+                std::ptr::drop_in_place(value_ptr);
+            }
+        }
+    }
+}