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Minimal test - lines (35, 104)

Code

namespace TestThrottledEventQueue {

// A simple queue of runnables, to serve as the base target of
// ThrottledEventQueues in tests.
//
// This is much simpler than mozilla::TaskQueue, and so better for unit tests.
// It's about the same as mozilla::EventQueue, but that doesn't implement
// nsIEventTarget, so it can't be the base target of a ThrottledEventQueue.
struct RunnableQueue : nsISerialEventTarget {
  std::queue> runnables;

  bool IsEmpty() { return runnables.empty(); }
  size_t Length() { return runnables.size(); }

  [[nodiscard]] nsresult Run() {
    while (!runnables.empty()) {
      auto runnable = move(runnables.front());
      runnables.pop();
      nsresult rv = runnable->Run();
      if (NS_FAILED(rv)) return rv;
    }

    return NS_OK;
  }

  // nsIEventTarget methods

  [[nodiscard]] NS_IMETHODIMP Dispatch(already_AddRefed aRunnable,
                                       uint32_t aFlags) override {
    MOZ_ALWAYS_TRUE(aFlags == nsIEventTarget::DISPATCH_NORMAL);
    runnables.push(aRunnable);
    return NS_OK;
  }

  [[nodiscard]] NS_IMETHODIMP DispatchFromScript(nsIRunnable* aRunnable,
                                                 uint32_t aFlags) override {
    RefPtr r = aRunnable;
    return Dispatch(r.forget(), aFlags);
  }

  NS_IMETHOD_(bool)
  IsOnCurrentThreadInfallible(void) override { return NS_IsMainThread(); }

  [[nodiscard]] NS_IMETHOD IsOnCurrentThread(bool* retval) override {
    *retval = IsOnCurrentThreadInfallible();
    return NS_OK;
  }

  [[nodiscard]] NS_IMETHODIMP DelayedDispatch(
      already_AddRefed aEvent, uint32_t aDelay) override {
    return NS_ERROR_NOT_IMPLEMENTED;
  }

  // nsISupports methods

  NS_DECL_THREADSAFE_ISUPPORTS

 private:
  virtual ~RunnableQueue() = default;
};

NS_IMPL_ISUPPORTS(RunnableQueue, nsIEventTarget, nsISerialEventTarget)

static void Enqueue(nsIEventTarget* target, function&& aCallable) {
  nsresult rv =
      target->Dispatch(NS_NewRunnableFunction("TEQ GTest", move(aCallable)));
  MOZ_ALWAYS_TRUE(NS_SUCCEEDED(rv));
}

}  // namespace TestThrottledEventQueue

Minimal test - lines (138, 182)

Code

TEST(ThrottledEventQueue, MixedDispatch)
{
  string log;

  auto base = MakeRefPtr();
  RefPtr throttled =
      ThrottledEventQueue::Create(base, "test queue 2");

  // A ThrottledEventQueue limits its impact on the base target by only queuing
  // its next event on the base once the prior event has been run. What it
  // actually queues on the base is a sort of proxy event called an
  // "executor": the base running the executor draws an event from the
  // ThrottledEventQueue and runs that. If the ThrottledEventQueue has further
  // events, it re-queues the executor on the base, effectively "going to the
  // back of the line".

  // Queue an event on the ThrottledEventQueue. This also queues the "executor"
  // event on the base.
  Enqueue(throttled, [&]() { log += 'a'; });
  ASSERT_EQ(throttled->Length(), 1U);
  ASSERT_EQ(base->Length(), 1U);

  // Add a second event to the throttled queue. The executor is already queued.
  Enqueue(throttled, [&]() { log += 'b'; });
  ASSERT_EQ(throttled->Length(), 2U);
  ASSERT_EQ(base->Length(), 1U);

  // Add an event directly to the base, after the executor.
  Enqueue(base, [&]() { log += 'c'; });
  ASSERT_EQ(throttled->Length(), 2U);
  ASSERT_EQ(base->Length(), 2U);

  // Run the base target. This runs:
  // - the executor, which runs the first event from the ThrottledEventQueue,
  //   and re-enqueues itself
  // - the event queued directly on the base
  // - the executor again, which runs the second event from the
  // ThrottledEventQueue.
  ASSERT_EQ(log, "");
  ASSERT_TRUE(NS_SUCCEEDED(base->Run()));
  ASSERT_EQ(log, "acb");

  ASSERT_TRUE(base->IsEmpty());
  ASSERT_TRUE(throttled->IsEmpty());
}

Minimal test - lines (108, 120)

Code

TEST(ThrottledEventQueue, RunnableQueue)
{
  string log;

  RefPtr queue = MakeRefPtr();
  Enqueue(queue, [&]() { log += 'a'; });
  Enqueue(queue, [&]() { log += 'b'; });
  Enqueue(queue, [&]() { log += 'c'; });

  ASSERT_EQ(log, "");
  ASSERT_TRUE(NS_SUCCEEDED(queue->Run()));
  ASSERT_EQ(log, "abc");
}

Minimal test - lines (122, 136)

Code

TEST(ThrottledEventQueue, SimpleDispatch)
{
  string log;

  auto base = MakeRefPtr();
  RefPtr throttled =
      ThrottledEventQueue::Create(base, "test queue 1");

  Enqueue(throttled, [&]() { log += 'a'; });
  ASSERT_TRUE(NS_SUCCEEDED(base->Run()));
  ASSERT_EQ(log, "a");

  ASSERT_TRUE(base->IsEmpty());
  ASSERT_TRUE(throttled->IsEmpty());
}