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

@@ -50,22 +50,22 @@ Distributed under the MIT License (http://opensource.org/licenses/MIT)
 
 namespace spdlog
 {
-namespace details
-{
+	namespace details
+	{
 
-template<typename T>
-class mpmc_bounded_queue
-{
-public:
+		template<typename T>
+		class mpmc_bounded_queue
+		{
+		public:
 
 			using item_type = T;
 			mpmc_bounded_queue(size_t buffer_size)
 				:max_size_(buffer_size),
-         buffer_(new cell_t [buffer_size]),
+				buffer_(new cell_t[buffer_size]),
 				buffer_mask_(buffer_size - 1)
 			{
 				//queue size must be power of two
-        if(!((buffer_size >= 2) && ((buffer_size & (buffer_size - 1)) == 0)))
+				if (!((buffer_size >= 2) && ((buffer_size & (buffer_size - 1)) == 0)))
 					throw spdlog_ex("async logger queue size must be power of two");
 
 				for (size_t i = 0; i != buffer_size; i += 1)
@@ -76,7 +76,7 @@ public:
 
 			~mpmc_bounded_queue()
 			{
-        delete [] buffer_;
+				delete[] buffer_;
 			}
 
 
@@ -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;
 					}
@@ -133,17 +127,19 @@ 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:
+		private:
 			struct cell_t
 			{
 				std::atomic<size_t>   sequence_;
@@ -153,7 +149,7 @@ private:
 			size_t const max_size_;
 
 			static size_t const     cacheline_size = 64;
-    typedef char            cacheline_pad_t [cacheline_size];
+			typedef char            cacheline_pad_t[cacheline_size];
 
 			cacheline_pad_t         pad0_;
 			cell_t* const           buffer_;
@@ -166,7 +162,7 @@ private:
 
 			mpmc_bounded_queue(mpmc_bounded_queue const&) = delete;
 			void operator= (mpmc_bounded_queue const&) = delete;
-};
+		};
 
-} // ns details
+	} // ns details
 } // ns spdlog