Fix flaky tests caused by relaxed load in C++ 'shared_ptr::use_count()' (#450)

Fixes: #284

Author: piscisaureus

src/support.rs

@@ -1,9 +1,11 @@
+use std::any::type_name;
 use std::any::Any;
 use std::borrow::Borrow;
 use std::borrow::BorrowMut;
 use std::convert::identity;
 use std::convert::AsMut;
 use std::convert::AsRef;
+use std::convert::TryFrom;
 use std::marker::PhantomData;
 use std::mem::align_of;
 use std::mem::forget;
@@ -18,6 +20,9 @@ use std::ptr::null_mut;
 use std::ptr::NonNull;
 use std::rc::Rc;
 use std::sync::Arc;
+use std::thread::yield_now;
+use std::time::Duration;
+use std::time::Instant;
 
 // TODO use libc::intptr_t when stable.
 // https://doc.rust-lang.org/1.7.0/libc/type.intptr_t.html
@@ -209,12 +214,20 @@ impl<T: Shared> Drop for SharedPtrBase<T> {
 pub struct SharedPtr<T: Shared>(SharedPtrBase<T>);
 
 impl<T: Shared> SharedPtr<T> {
-  pub fn is_null(&self) -> bool {
-    <T as Shared>::get(&self.0).is_null()
+  /// Asserts that the number of references to the shared inner value is equal
+  /// to the `expected` count.
+  ///
+  /// This function relies on the C++ method `std::shared_ptr::use_count()`,
+  /// which usually performs a relaxed load. This function will repeatedly call
+  /// `use_count()` until it returns the expected value, for up to one second.
+  /// Therefore it should probably not be used in performance critical code.
+  #[track_caller]
+  pub fn assert_use_count_eq(&self, expected: usize) {
+    assert_shared_ptr_use_count_eq("SharedPtr", &self.0, expected);
   }
 
-  pub fn use_count(&self) -> long {
-    <T as Shared>::use_count(&self.0)
+  pub fn is_null(&self) -> bool {
+    <T as Shared>::get(&self.0).is_null()
   }
 
   pub fn take(&mut self) -> Option<SharedRef<T>> {
@@ -267,8 +280,16 @@ impl<T: Shared> From<SharedRef<T>> for SharedPtr<T> {
 pub struct SharedRef<T: Shared>(SharedPtrBase<T>);
 
 impl<T: Shared> SharedRef<T> {
-  pub fn use_count(&self) -> long {
-    <T as Shared>::use_count(&self.0)
+  /// Asserts that the number of references to the shared inner value is equal
+  /// to the `expected` count.
+  ///
+  /// This function relies on the C++ method `std::shared_ptr::use_count()`,
+  /// which usually performs a relaxed load. This function will repeatedly call
+  /// `use_count()` until it returns the expected value, for up to one second.
+  /// Therefore it should probably not be used in performance critical code.
+  #[track_caller]
+  pub fn assert_use_count_eq(&self, expected: usize) {
+    assert_shared_ptr_use_count_eq("SharedRef", &self.0, expected);
   }
 }
 
@@ -303,6 +324,42 @@ impl<T: Shared> Borrow<T> for SharedRef<T> {
   }
 }
 
+#[track_caller]
+fn assert_shared_ptr_use_count_eq<T: Shared>(
+  wrapper_type_name: &str,
+  shared_ptr: &SharedPtrBase<T>,
+  expected: usize,
+) {
+  let mut actual = T::use_count(shared_ptr);
+  let ok = match long::try_from(expected) {
+    Err(_) => false, // Non-`long` value can never match actual use count.
+    Ok(expected) if actual == expected => true, // Fast path.
+    Ok(expected) => {
+      pub const RETRY_TIMEOUT: Duration = Duration::from_secs(1);
+      let start = Instant::now();
+      loop {
+        yield_now();
+        actual = T::use_count(shared_ptr);
+        if actual == expected {
+          break true;
+        } else if start.elapsed() > RETRY_TIMEOUT {
+          break false;
+        }
+      }
+    }
+  };
+  assert!(
+    ok,
+    "assertion failed: `{}<{}>` reference count does not match expectation\
+       \n   actual: {}\
+       \n expected: {}",
+    wrapper_type_name,
+    type_name::<T>(),
+    actual,
+    expected
+  );
+}
+
 /// A trait for values with static lifetimes that are allocated at a fixed
 /// address in memory. Practically speaking, that means they're either a
 /// `&'static` reference, or they're heap-allocated in a `Arc`, `Box`, `Rc`,
@@ -672,38 +729,69 @@ mod tests {
       forget(take(p));
     }
 
-    fn use_count(_: &SharedPtrBase<Self>) -> long {
-      unimplemented!()
+    fn use_count(p: &SharedPtrBase<Self>) -> long {
+      match p {
+        &Self::SHARED_PTR_BASE_A => 1,
+        &Self::SHARED_PTR_BASE_B => 2,
+        p if p == &Default::default() => 0,
+        _ => unreachable!(),
+      }
     }
   }
 
   #[test]
   fn shared_ptr_and_shared_ref() {
     let mut shared_ptr_a1 = SharedPtr(MockSharedObj::SHARED_PTR_BASE_A);
     assert!(!shared_ptr_a1.is_null());
+    shared_ptr_a1.assert_use_count_eq(1);
 
     let shared_ref_a: SharedRef<_> = shared_ptr_a1.take().unwrap();
     assert_eq!(shared_ref_a.inner, 11111);
+    shared_ref_a.assert_use_count_eq(1);
 
     assert!(shared_ptr_a1.is_null());
+    shared_ptr_a1.assert_use_count_eq(0);
 
     let shared_ptr_a2: SharedPtr<_> = shared_ref_a.into();
     assert!(!shared_ptr_a2.is_null());
+    shared_ptr_a2.assert_use_count_eq(1);
     assert_eq!(shared_ptr_a2.unwrap().inner, 11111);
 
     let mut shared_ptr_b1 = SharedPtr(MockSharedObj::SHARED_PTR_BASE_B);
     assert!(!shared_ptr_b1.is_null());
+    shared_ptr_b1.assert_use_count_eq(2);
 
     let shared_ref_b: SharedRef<_> = shared_ptr_b1.take().unwrap();
     assert_eq!(shared_ref_b.inner, 22222);
+    shared_ref_b.assert_use_count_eq(2);
 
     assert!(shared_ptr_b1.is_null());
+    shared_ptr_b1.assert_use_count_eq(0);
 
     let shared_ptr_b2: SharedPtr<_> = shared_ref_b.into();
     assert!(!shared_ptr_b2.is_null());
+    shared_ptr_b2.assert_use_count_eq(2);
     assert_eq!(shared_ptr_b2.unwrap().inner, 22222);
   }
 
+  #[test]
+  #[should_panic(expected = "assertion failed: \
+      `SharedPtr<rusty_v8::support::tests::MockSharedObj>` reference count \
+      does not match expectation")]
+  fn shared_ptr_use_count_assertion_failed() {
+    let shared_ptr: SharedPtr<MockSharedObj> = Default::default();
+    shared_ptr.assert_use_count_eq(3);
+  }
+
+  #[test]
+  #[should_panic(expected = "assertion failed: \
+      `SharedRef<rusty_v8::support::tests::MockSharedObj>` reference count \
+      does not match expectation")]
+  fn shared_ref_use_count_assertion_failed() {
+    let shared_ref = SharedRef(MockSharedObj::SHARED_PTR_BASE_B);
+    shared_ref.assert_use_count_eq(7);
+  }
+
   static TEST_OBJ_DROPPED: AtomicBool = AtomicBool::new(false);
 
   struct TestObj {

tests/test_api.rs

@@ -459,44 +459,44 @@ fn backing_store_segfault() {
   let _setup_guard = setup();
   let array_buffer_allocator = v8::new_default_allocator().make_shared();
   let shared_bs = {
-    assert_eq!(1, v8::SharedRef::use_count(&array_buffer_allocator));
+    array_buffer_allocator.assert_use_count_eq(1);
     let params = v8::Isolate::create_params()
       .array_buffer_allocator(array_buffer_allocator.clone());
-    assert_eq!(2, v8::SharedRef::use_count(&array_buffer_allocator));
+    array_buffer_allocator.assert_use_count_eq(2);
     let isolate = &mut v8::Isolate::new(params);
-    assert_eq!(2, v8::SharedRef::use_count(&array_buffer_allocator));
+    array_buffer_allocator.assert_use_count_eq(2);
     let scope = &mut v8::HandleScope::new(isolate);
     let context = v8::Context::new(scope);
     let scope = &mut v8::ContextScope::new(scope, context);
     let ab = v8::ArrayBuffer::new(scope, 10);
     let shared_bs = ab.get_backing_store();
-    assert_eq!(3, v8::SharedRef::use_count(&array_buffer_allocator));
+    array_buffer_allocator.assert_use_count_eq(3);
     shared_bs
   };
-  assert_eq!(1, v8::SharedRef::use_count(&shared_bs));
-  assert_eq!(2, v8::SharedRef::use_count(&array_buffer_allocator));
+  shared_bs.assert_use_count_eq(1);
+  array_buffer_allocator.assert_use_count_eq(2);
   drop(array_buffer_allocator);
   drop(shared_bs); // Error occurred here.
 }
 
 #[test]
 fn shared_array_buffer_allocator() {
   let alloc1 = v8::new_default_allocator().make_shared();
-  assert_eq!(1, v8::SharedRef::use_count(&alloc1));
+  alloc1.assert_use_count_eq(1);
 
   let alloc2 = alloc1.clone();
-  assert_eq!(2, v8::SharedRef::use_count(&alloc1));
-  assert_eq!(2, v8::SharedRef::use_count(&alloc2));
+  alloc1.assert_use_count_eq(2);
+  alloc2.assert_use_count_eq(2);
 
   let mut alloc2 = v8::SharedPtr::from(alloc2);
-  assert_eq!(2, v8::SharedRef::use_count(&alloc1));
-  assert_eq!(2, v8::SharedPtr::use_count(&alloc2));
+  alloc1.assert_use_count_eq(2);
+  alloc2.assert_use_count_eq(2);
 
   drop(alloc1);
-  assert_eq!(1, v8::SharedPtr::use_count(&alloc2));
+  alloc2.assert_use_count_eq(1);
 
   alloc2.take();
-  assert_eq!(0, v8::SharedPtr::use_count(&alloc2));
+  alloc2.assert_use_count_eq(0);
 }
 
 #[test]
@@ -514,46 +514,46 @@ fn array_buffer_with_shared_backing_store() {
 
     let bs1 = ab1.get_backing_store();
     assert_eq!(ab1.byte_length(), bs1.byte_length());
-    assert_eq!(2, v8::SharedRef::use_count(&bs1));
+    bs1.assert_use_count_eq(2);
 
     let bs2 = ab1.get_backing_store();
     assert_eq!(ab1.byte_length(), bs2.byte_length());
-    assert_eq!(3, v8::SharedRef::use_count(&bs1));
-    assert_eq!(3, v8::SharedRef::use_count(&bs2));
+    bs1.assert_use_count_eq(3);
+    bs2.assert_use_count_eq(3);
 
     let bs3 = ab1.get_backing_store();
     assert_eq!(ab1.byte_length(), bs3.byte_length());
-    assert_eq!(4, v8::SharedRef::use_count(&bs1));
-    assert_eq!(4, v8::SharedRef::use_count(&bs2));
-    assert_eq!(4, v8::SharedRef::use_count(&bs3));
+    bs1.assert_use_count_eq(4);
+    bs2.assert_use_count_eq(4);
+    bs3.assert_use_count_eq(4);
 
     drop(bs2);
-    assert_eq!(3, v8::SharedRef::use_count(&bs1));
-    assert_eq!(3, v8::SharedRef::use_count(&bs3));
+    bs1.assert_use_count_eq(3);
+    bs3.assert_use_count_eq(3);
 
     drop(bs1);
-    assert_eq!(2, v8::SharedRef::use_count(&bs3));
+    bs3.assert_use_count_eq(2);
 
     let ab2 = v8::ArrayBuffer::with_backing_store(scope, &bs3);
     assert_eq!(ab1.byte_length(), ab2.byte_length());
-    assert_eq!(3, v8::SharedRef::use_count(&bs3));
+    bs3.assert_use_count_eq(3);
 
     let bs4 = ab2.get_backing_store();
     assert_eq!(ab1.byte_length(), bs4.byte_length());
-    assert_eq!(4, v8::SharedRef::use_count(&bs3));
-    assert_eq!(4, v8::SharedRef::use_count(&bs4));
+    bs3.assert_use_count_eq(4);
+    bs4.assert_use_count_eq(4);
 
     let bs5 = bs4.clone();
-    assert_eq!(5, v8::SharedRef::use_count(&bs3));
-    assert_eq!(5, v8::SharedRef::use_count(&bs4));
-    assert_eq!(5, v8::SharedRef::use_count(&bs5));
+    bs3.assert_use_count_eq(5);
+    bs4.assert_use_count_eq(5);
+    bs5.assert_use_count_eq(5);
 
     drop(bs3);
-    assert_eq!(4, v8::SharedRef::use_count(&bs4));
-    assert_eq!(4, v8::SharedRef::use_count(&bs4));
+    bs4.assert_use_count_eq(4);
+    bs5.assert_use_count_eq(4);
 
     drop(bs4);
-    assert_eq!(3, v8::SharedRef::use_count(&bs5));
+    bs5.assert_use_count_eq(3);
   }
 }