Fixed issue #546 by adding an "is_empty" method to the queue instead of the buggy approx_size

include/spdlog/details/async_log_helper.h

@@ -384,7 +384,7 @@ inline void spdlog::details::async_log_helper::sleep_or_yield(const spdlog::log_
 inline void spdlog::details::async_log_helper::wait_empty_q()
 {
     auto last_op = details::os::now();
-    while (_q.approx_size() > 0)
+    while (!_q.is_empty())
     {
         sleep_or_yield(details::os::now(), last_op);
     }

include/spdlog/details/mpmc_bounded_q.h

@@ -84,22 +84,18 @@ public:
 			{
 				cell_t* cell;
 				size_t pos = enqueue_pos_.load(std::memory_order_relaxed);
-        for (;;)
-        {
+				for (;;) {
 					cell = &buffer_[pos & buffer_mask_];
 					size_t seq = cell->sequence_.load(std::memory_order_acquire);
 					intptr_t dif = static_cast<intptr_t>(seq) - static_cast<intptr_t>(pos);
-            if (dif == 0)
-            {
+					if (dif == 0) {
 						if (enqueue_pos_.compare_exchange_weak(pos, pos + 1, std::memory_order_relaxed))
 							break;
 					}
-            else if (dif < 0)
-            {
+					else if (dif < 0) {
 						return false;
 					}
-            else
-            {
+					else {
 						pos = enqueue_pos_.load(std::memory_order_relaxed);
 					}
 				}
@@ -112,14 +108,12 @@ public:
 			{
 				cell_t* cell;
 				size_t pos = dequeue_pos_.load(std::memory_order_relaxed);
-        for (;;)
-        {
+				for (;;) {
 					cell = &buffer_[pos & buffer_mask_];
 					size_t seq =
 						cell->sequence_.load(std::memory_order_acquire);
 					intptr_t dif = static_cast<intptr_t>(seq) - static_cast<intptr_t>(pos + 1);
-            if (dif == 0)
-            {
+					if (dif == 0) {
 						if (dequeue_pos_.compare_exchange_weak(pos, pos + 1, std::memory_order_relaxed))
 							break;
 					}
@@ -133,14 +127,16 @@ public:
 				return true;
 			}
 
-    size_t approx_size()
+			bool is_empty()
 			{				
-        size_t first_pos = dequeue_pos_.load(std::memory_order_relaxed);
-        size_t last_pos = enqueue_pos_.load(std::memory_order_relaxed);
-        if (last_pos <= first_pos)
-            return 0;
-        auto size = last_pos - first_pos;
-        return size < max_size_ ? size : max_size_;
+				unsigned front, front1, back;
+				// try to take a consistent snapshot of front/tail.
+				do {					
+					front = enqueue_pos_.load(std::memory_order_acquire);
+					back = dequeue_pos_.load(std::memory_order_acquire);
+					front1 = enqueue_pos_.load(std::memory_order_relaxed);
+				} while (front != front1);
+				return back == front;
 			}
 
 		private: