rt: use internal ThreadId implementation (#5329)

The version provided by `std` has limitations, including no way to try
to get a thread ID without panicking.

tokio/src/runtime/context.rs

@@ -15,6 +15,10 @@ cfg_rt! {
 }
 
 struct Context {
+    /// Uniquely identifies the current thread
+    #[cfg(feature = "rt")]
+    thread_id: Cell<Option<ThreadId>>,
+
     /// Handle to the runtime scheduler running on the current thread.
     #[cfg(feature = "rt")]
     handle: RefCell<Option<scheduler::Handle>>,
@@ -46,6 +50,9 @@ struct Context {
 tokio_thread_local! {
     static CONTEXT: Context = {
         Context {
+            #[cfg(feature = "rt")]
+            thread_id: Cell::new(None),
+
             /// Tracks the current runtime handle to use when spawning,
             /// accessing drivers, etc...
             #[cfg(feature = "rt")]
@@ -82,10 +89,23 @@ pub(super) fn budget<R>(f: impl FnOnce(&Cell<coop::Budget>) -> R) -> Result<R, A
 }
 
 cfg_rt! {
-    use crate::runtime::TryCurrentError;
+    use crate::runtime::{ThreadId, TryCurrentError};
 
     use std::fmt;
 
+    pub(crate) fn thread_id() -> Result<ThreadId, AccessError> {
+        CONTEXT.try_with(|ctx| {
+            match ctx.thread_id.get() {
+                Some(id) => id,
+                None => {
+                    let id = ThreadId::next();
+                    ctx.thread_id.set(Some(id));
+                    id
+                }
+            }
+        })
+    }
+
     #[derive(Debug, Clone, Copy)]
     #[must_use]
     pub(crate) enum EnterRuntime {

tokio/src/runtime/mod.rs

@@ -237,6 +237,9 @@ cfg_rt! {
     mod runtime;
     pub use runtime::{Runtime, RuntimeFlavor};
 
+    mod thread_id;
+    pub(crate) use thread_id::ThreadId;
+
     cfg_metrics! {
         mod metrics;
         pub use metrics::RuntimeMetrics;

tokio/src/runtime/thread_id.rs

@@ -0,0 +1,31 @@
+use std::num::NonZeroU64;
+
+#[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
+pub(crate) struct ThreadId(NonZeroU64);
+
+impl ThreadId {
+    pub(crate) fn next() -> Self {
+        use crate::loom::sync::atomic::{Ordering::Relaxed, StaticAtomicU64};
+
+        static NEXT_ID: StaticAtomicU64 = StaticAtomicU64::new(0);
+
+        let mut last = NEXT_ID.load(Relaxed);
+        loop {
+            let id = match last.checked_add(1) {
+                Some(id) => id,
+                None => exhausted(),
+            };
+
+            match NEXT_ID.compare_exchange_weak(last, id, Relaxed, Relaxed) {
+                Ok(_) => return ThreadId(NonZeroU64::new(id).unwrap()),
+                Err(id) => last = id,
+            }
+        }
+    }
+}
+
+#[cold]
+#[allow(dead_code)]
+fn exhausted() -> ! {
+    panic!("failed to generate unique thread ID: bitspace exhausted")
+}

tokio/src/task/local.rs

@@ -1,8 +1,8 @@
 //! Runs `!Send` futures on the current thread.
 use crate::loom::cell::UnsafeCell;
 use crate::loom::sync::{Arc, Mutex};
-use crate::loom::thread::{self, ThreadId};
 use crate::runtime::task::{self, JoinHandle, LocalOwnedTasks, Task};
+use crate::runtime::{context, ThreadId};
 use crate::sync::AtomicWaker;
 use crate::util::RcCell;
 
@@ -277,12 +277,10 @@ pin_project! {
 }
 
 tokio_thread_local!(static CURRENT: LocalData = const { LocalData {
-    thread_id: Cell::new(None),
     ctx: RcCell::new(),
 } });
 
 struct LocalData {
-    thread_id: Cell<Option<ThreadId>>,
     ctx: RcCell<Context>,
 }
 
@@ -379,12 +377,14 @@ impl fmt::Debug for LocalEnterGuard {
 impl LocalSet {
     /// Returns a new local task set.
     pub fn new() -> LocalSet {
+        let owner = context::thread_id().expect("cannot create LocalSet during thread shutdown");
+
         LocalSet {
             tick: Cell::new(0),
             context: Rc::new(Context {
                 shared: Arc::new(Shared {
                     local_state: LocalState {
-                        owner: thread_id().expect("cannot create LocalSet during thread shutdown"),
+                        owner,
                         owned: LocalOwnedTasks::new(),
                         local_queue: UnsafeCell::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
                     },
@@ -949,7 +949,7 @@ impl Shared {
 
                 // We are on the thread that owns the `LocalSet`, so we can
                 // wake to the local queue.
-                _ if localdata.get_id() == Some(self.local_state.owner) => {
+                _ if context::thread_id().ok() == Some(self.local_state.owner) => {
                     unsafe {
                         // Safety: we just checked that the thread ID matches
                         // the localset's owner, so this is safe.
@@ -1093,7 +1093,9 @@ impl LocalState {
             // if we couldn't get the thread ID because we're dropping the local
             // data, skip the assertion --- the `Drop` impl is not going to be
             // called from another thread, because `LocalSet` is `!Send`
-            thread_id().map(|id| id == self.owner).unwrap_or(true),
+            context::thread_id()
+                .map(|id| id == self.owner)
+                .unwrap_or(true),
             "`LocalSet`'s local run queue must not be accessed by another thread!"
         );
     }
@@ -1103,26 +1105,6 @@ impl LocalState {
 // ensure they are on the same thread that owns the `LocalSet`.
 unsafe impl Send for LocalState {}
 
-impl LocalData {
-    fn get_id(&self) -> Option<ThreadId> {
-        self.thread_id.get()
-    }
-
-    fn get_or_insert_id(&self) -> ThreadId {
-        self.thread_id.get().unwrap_or_else(|| {
-            let id = thread::current().id();
-            self.thread_id.set(Some(id));
-            id
-        })
-    }
-}
-
-fn thread_id() -> Option<ThreadId> {
-    CURRENT
-        .try_with(|localdata| localdata.get_or_insert_id())
-        .ok()
-}
-
 #[cfg(all(test, not(loom)))]
 mod tests {
     use super::*;

tokio/tests/rt_metrics.rs

@@ -141,7 +141,7 @@ fn worker_noop_count() {
         time::sleep(Duration::from_millis(1)).await;
     });
     drop(rt);
-    assert!(2 <= metrics.worker_noop_count(0));
+    assert!(0 < metrics.worker_noop_count(0));
 
     let rt = threaded();
     let metrics = rt.metrics();
@@ -149,8 +149,8 @@ fn worker_noop_count() {
         time::sleep(Duration::from_millis(1)).await;
     });
     drop(rt);
-    assert!(1 <= metrics.worker_noop_count(0));
-    assert!(1 <= metrics.worker_noop_count(1));
+    assert!(0 < metrics.worker_noop_count(0));
+    assert!(0 < metrics.worker_noop_count(1));
 }
 
 #[test]