Use atomics in join_orders_after_tls_destructors test

std::sync::mpsc uses thread locals and depending on the order TLS dtors
are run `rx.recv()` can panic when used in a TLS dtor.

library/std/src/thread/local/tests.rs

@@ -1,6 +1,6 @@
 use crate::cell::{Cell, UnsafeCell};
-use crate::sync::atomic::{AtomicBool, Ordering};
-use crate::sync::mpsc::{self, channel, Sender};
+use crate::sync::atomic::{AtomicU8, Ordering};
+use crate::sync::mpsc::{channel, Sender};
 use crate::thread::{self, LocalKey};
 use crate::thread_local;
 
@@ -217,46 +217,100 @@ fn dtors_in_dtors_in_dtors_const_init() {
 // thread::yield_now and running the test several times.
 #[test]
 fn join_orders_after_tls_destructors() {
-    static THREAD2_LAUNCHED: AtomicBool = AtomicBool::new(false);
+    // We emulate a synchronous MPSC rendezvous channel using only atomics and
+    // thread::yield_now. We can't use std::mpsc as the implementation itself
+    // may rely on thread locals.
+    //
+    // The basic state machine for an SPSC rendezvous channel is:
+    //           FRESH -> THREAD1_WAITING -> MAIN_THREAD_RENDEZVOUS
+    // where the first transition is done by the “receiving” thread and the 2nd
+    // transition is done by the “sending” thread.
+    //
+    // We add an additional state `THREAD2_LAUNCHED` between `FRESH` and
+    // `THREAD1_WAITING` to block until all threads are actually running.
+    //
+    // A thread that joins on the “receiving” thread completion should never
+    // observe the channel in the `THREAD1_WAITING` state. If this does occur,
+    // we switch to the “poison” state `THREAD2_JOINED` and panic all around.
+    // (This is equivalent to “sending” from an alternate producer thread.)
+    const FRESH: u8 = 0;
+    const THREAD2_LAUNCHED: u8 = 1;
+    const THREAD1_WAITING: u8 = 2;
+    const MAIN_THREAD_RENDEZVOUS: u8 = 3;
+    const THREAD2_JOINED: u8 = 4;
+    static SYNC_STATE: AtomicU8 = AtomicU8::new(FRESH);
 
     for _ in 0..10 {
-        let (tx, rx) = mpsc::sync_channel(0);
-        THREAD2_LAUNCHED.store(false, Ordering::SeqCst);
+        SYNC_STATE.store(FRESH, Ordering::SeqCst);
+
+        let jh = thread::Builder::new()
+            .name("thread1".into())
+            .spawn(move || {
+                struct TlDrop;
+
+                impl Drop for TlDrop {
+                    fn drop(&mut self) {
+                        loop {
+                            match SYNC_STATE.load(Ordering::SeqCst) {
+                                FRESH => thread::yield_now(),
+                                THREAD2_LAUNCHED => break,
+                                v => unreachable!("sync state: {}", v),
+                            }
+                        }
+                        let mut sync_state = SYNC_STATE.swap(THREAD1_WAITING, Ordering::SeqCst);
+                        loop {
+                            match sync_state {
+                                THREAD2_LAUNCHED | THREAD1_WAITING => thread::yield_now(),
+                                MAIN_THREAD_RENDEZVOUS => break,
+                                THREAD2_JOINED => panic!(
+                                    "Thread 1 still running after thread 2 joined on thread 1"
+                                ),
+                                v => unreachable!("sync state: {}", v),
+                            }
+                            sync_state = SYNC_STATE.load(Ordering::SeqCst);
+                        }
+                    }
+                }
 
-        let jh = thread::spawn(move || {
-            struct RecvOnDrop(Cell<Option<mpsc::Receiver<()>>>);
+                thread_local! {
+                    static TL_DROP: TlDrop = TlDrop;
+                }
 
-            impl Drop for RecvOnDrop {
-                fn drop(&mut self) {
-                    let rx = self.0.take().unwrap();
-                    while !THREAD2_LAUNCHED.load(Ordering::SeqCst) {
-                        thread::yield_now();
+                TL_DROP.with(|_| {})
+            })
+            .unwrap();
+
+        let jh2 = thread::Builder::new()
+            .name("thread2".into())
+            .spawn(move || {
+                assert_eq!(SYNC_STATE.swap(THREAD2_LAUNCHED, Ordering::SeqCst), FRESH);
+                jh.join().unwrap();
+                match SYNC_STATE.swap(THREAD2_JOINED, Ordering::SeqCst) {
+                    MAIN_THREAD_RENDEZVOUS => return,
+                    THREAD2_LAUNCHED | THREAD1_WAITING => {
+                        panic!("Thread 2 running after thread 1 join before main thread rendezvous")
                     }
-                    rx.recv().unwrap();
+                    v => unreachable!("sync state: {:?}", v),
                 }
+            })
+            .unwrap();
+
+        loop {
+            match SYNC_STATE.compare_exchange_weak(
+                THREAD1_WAITING,
+                MAIN_THREAD_RENDEZVOUS,
+                Ordering::SeqCst,
+                Ordering::SeqCst,
+            ) {
+                Ok(_) => break,
+                Err(FRESH) => thread::yield_now(),
+                Err(THREAD2_LAUNCHED) => thread::yield_now(),
+                Err(THREAD2_JOINED) => {
+                    panic!("Main thread rendezvous after thread 2 joined thread 1")
+                }
+                v => unreachable!("sync state: {:?}", v),
             }
-
-            thread_local! {
-                static TL_RX: RecvOnDrop = RecvOnDrop(Cell::new(None));
-            }
-
-            TL_RX.with(|v| v.0.set(Some(rx)))
-        });
-
-        let tx_clone = tx.clone();
-        let jh2 = thread::spawn(move || {
-            THREAD2_LAUNCHED.store(true, Ordering::SeqCst);
-            jh.join().unwrap();
-            tx_clone.send(()).expect_err(
-                "Expecting channel to be closed because thread 1 TLS destructors must've run",
-            );
-        });
-
-        while !THREAD2_LAUNCHED.load(Ordering::SeqCst) {
-            thread::yield_now();
         }
-        thread::yield_now();
-        tx.send(()).expect("Expecting channel to be live because thread 2 must block on join");
         jh2.join().unwrap();
     }
 }