add stdlib test for TLS dtor order

Author: RalfJung

tests/pass/concurrency/tls_lib_drop.rs

@@ -71,7 +71,116 @@ fn check_blocking() {
     thread::yield_now();
 }
 
+// This test tests that TLS destructors have run before the thread joins. The
+// test has no false positives (meaning: if the test fails, there's actually
+// an ordering problem). It may have false negatives, where the test passes but
+// join is not guaranteed to be after the TLS destructors. However, false
+// negatives should be exceedingly rare due to judicious use of
+// thread::yield_now and running the test several times.
+fn join_orders_after_tls_destructors() {
+    use std::sync::atomic::{AtomicU8, Ordering};
+
+    // 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 {
+        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) {
+                        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);
+                        }
+                    }
+                }
+
+                thread_local! {
+                    static TL_DROP: TlDrop = TlDrop;
+                }
+
+                TL_DROP.with(|_| {});
+
+                loop {
+                    match SYNC_STATE.load(Ordering::SeqCst) {
+                        FRESH => thread::yield_now(),
+                        THREAD2_LAUNCHED => break,
+                        v => unreachable!("sync state: {}", v),
+                    }
+                }
+            })
+            .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")
+                    }
+                    v => unreachable!("sync state: {:?}", v),
+                }
+            })
+            .unwrap();
+
+        loop {
+            match SYNC_STATE.compare_exchange(
+                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),
+            }
+        }
+        jh2.join().unwrap();
+    }
+}
+
 fn main() {
     check_destructors();
     check_blocking();
+    join_orders_after_tls_destructors();
 }