enforce alignment on channels::detail::AtomicQueue consumer types

Summary:
Class template `folly::channels::detail::AtomicQueue` is parameterized by a consumer type template. The class holds pointers to consumers but, in the interest of minimizing memory usage, may steal the two lowest bits of the pointers.

A unit-test for `AtomicQueue` uses a consumer type holding only a `bool`, giving that consumer type alignment 1 and therefore pointers to it no bits allowable to steal.

Enforce a minimum alignment on the consumer type that allows stealing the two lowest bits of pointers, and update the unit-test.

Reviewed By: dmm-fb

Differential Revision: D64287317

fbshipit-source-id: fb54e15559f12d82717cbea20075c01adfa6e8d1

folly/channels/detail/AtomicQueue.h

@@ -82,7 +82,7 @@ class AtomicQueue {
  public:
   using MessageQueue = Queue<Message>;
 
-  AtomicQueue() {}
+  AtomicQueue() { static_assert(alignof(Consumer) > kTypeMask); }
   ~AtomicQueue() {
     auto storage = storage_.load(std::memory_order_acquire);
     auto type = static_cast<Type>(storage & kTypeMask);

folly/channels/detail/test/AtomicQueueTest.cpp

@@ -98,7 +98,7 @@ TEST(AtomicQueueTest, Basic) {
 }
 
 TEST(AtomicQueueTest, Canceled) {
-  struct Consumer {
+  struct alignas(int) Consumer {
     void consume(int*) { ADD_FAILURE() << "consume() shouldn't be called"; }
     void canceled(int* consumerParam) {
       EXPECT_EQ(consumerParam, getConsumerParam());