avoid tls access while iterating through mpsc thread entries

library/std/src/sync/mpmc/waker.rs

@@ -66,11 +66,16 @@ impl Waker {
     /// Attempts to find another thread's entry, select the operation, and wake it up.
     #[inline]
     pub(crate) fn try_select(&mut self) -> Option<Entry> {
+        if self.selectors.is_empty() {
+            None
+        } else {
+            let thread_id = current_thread_id();
+
             self.selectors
                 .iter()
                 .position(|selector| {
                     // Does the entry belong to a different thread?
-                selector.cx.thread_id() != current_thread_id()
+                    selector.cx.thread_id() != thread_id
                         && selector // Try selecting this operation.
                             .cx
                             .try_select(Selected::Operation(selector.oper))
@@ -87,6 +92,7 @@ impl Waker {
                 // performance.
                 .map(|pos| self.selectors.remove(pos))
         }
+    }
 
     /// Notifies all operations waiting to be ready.
     #[inline]