From 49edaf14fd1ef0c190e67998ad299db67d19b739 Mon Sep 17 00:00:00 2001
From: Jacques-Henri Jourdan <jacques-henri.jourdan@normalesup.org>
Date: Tue, 18 Jul 2017 19:31:22 +0200
Subject: [PATCH 1/2] Fix in weak_count in Arc.

In the case the weak count was locked, the weak_count function could
return usize::MAX. We need to test this condition manually.
---
 src/liballoc/arc.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index d9edf50b9c8ec..0205f3f3553e2 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -453,7 +453,10 @@ impl<T: ?Sized> Arc<T> {
     #[inline]
     #[stable(feature = "arc_counts", since = "1.15.0")]
     pub fn weak_count(this: &Self) -> usize {
-        this.inner().weak.load(SeqCst) - 1
+        let cnt = this.inner().weak.load(SeqCst);
+        // If the weak count is currently locked, the value of the
+        // count was 0 just before taking the lock.
+        if cnt == usize::MAX { 0 } else { cnt - 1 }
     }
 
     /// Gets the number of strong (`Arc`) pointers to this value.

From 8416713240e11e898872f5028f927997f3754696 Mon Sep 17 00:00:00 2001
From: Jacques-Henri Jourdan <jacques-henri.jourdan@normalesup.org>
Date: Thu, 20 Jul 2017 01:59:01 +0200
Subject: [PATCH 2/2] Add test test_weak_count_locked

---
 src/liballoc/arc.rs | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index 0205f3f3553e2..85c7efb7ac50c 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -1501,6 +1501,25 @@ mod tests {
         assert!(Arc::ptr_eq(&five, &same_five));
         assert!(!Arc::ptr_eq(&five, &other_five));
     }
+
+    #[test]
+    #[cfg_attr(target_os = "emscripten", ignore)]
+    fn test_weak_count_locked() {
+        let mut a = Arc::new(atomic::AtomicBool::new(false));
+        let a2 = a.clone();
+        let t = thread::spawn(move || {
+            for _i in 0..1000000 {
+                Arc::get_mut(&mut a);
+            }
+            a.store(true, SeqCst);
+        });
+
+        while !a2.load(SeqCst) {
+            let n = Arc::weak_count(&a2);
+            assert!(n < 2, "bad weak count: {}", n);
+        }
+        t.join().unwrap();
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]