async queue - overrun oldsest policy option

include/spdlog/common.h

@@ -126,8 +126,8 @@ using level_hasher = std::hash<int>;
 //
 enum class async_overflow_policy
 {
-    block_retry,    // Block / yield / sleep until message can be enqueued
-    discard_log_msg // Discard the message it enqueue fails
+    block_retry,    // Block until message can be enqueued
+    overrun_oldeset // Discard oldest message in the queue if full when trying to add new item.
 };
 
 //

include/spdlog/details/circular_q.h

@@ -15,20 +15,20 @@ public:
     using item_type = T;
 
     explicit circular_q(size_t max_items)
-        : max_items_(max_items + 1)
+        : max_items_(max_items + 1) // one item is reserved as marker for full q
         , v_(max_items_)
     {
     }
 
-    // push back, overrun last item if no room left
+    // push back, overrun (oldest) item if no room left
     void push_back(T &&item)
     {
-        v_[head_] = std::move(item);
-        head_ = (head_ + 1) % max_items_;
+        v_[tail_] = std::move(item);
+        tail_ = (tail_ + 1) % max_items_;
 
-        if (head_ == tail_)
+        if (tail_ == head_) // overrun last item if full
         {
-            tail_ = (tail_ + 1) % max_items_;
+            head_ = (head_ + 1) % max_items_;
         }
     }
 
@@ -36,25 +36,26 @@ public:
     // If there are no elements in the container, the behavior is undefined.
     void pop_front(T &popped_item)
     {
-        popped_item = std::move(v_[tail_]);
-        tail_ = (tail_ + 1) % max_items_;
+        popped_item = std::move(v_[head_]);
+        head_ = (head_ + 1) % max_items_;
     }
 
     bool empty()
     {
-        return head_ == tail_;
+        return tail_ == head_;
     }
 
     bool full()
     {
-        // tail is ahead of the head by 1
-        return ((head_ + 1) % max_items_) == tail_;
+        // head is ahead of the tail by 1
+        return ((tail_ + 1) % max_items_) == head_;
     }
 
 private:
     size_t max_items_;
     typename std::vector<T>::size_type head_ = 0;
     typename std::vector<T>::size_type tail_ = 0;
+
     std::vector<T> v_;
 };
 } // namespace details

include/spdlog/details/mpmc_blocking_q.h

@@ -39,19 +39,14 @@ public:
         push_cv_.notify_one();
     }
 
-    // try to enqueue and return immediately false if no room left
-    bool enqueue_nowait(T &&item)
+    // enqueue immediately. overrun oldest message in the queue if no room left.
+    void enqueue_nowait(T &&item)
     {
         {
             std::unique_lock<std::mutex> lock(queue_mutex_);
-            if (q_.full())
-            {
-                return false;
-            }
-            q_.push_back(std::forward<T>(item));
+            q_.push_back(std::move(item));
         }
         push_cv_.notify_one();
-        return true;
     }
 
     // try to dequeue item. if no item found. wait upto timeout and try again

tests/test_async.cpp

@@ -30,7 +30,7 @@ TEST_CASE("discard policy ", "[async]")
     size_t messages = 1024;
     {
         auto tp = std::make_shared<details::thread_pool>(queue_size, 1);
-        auto logger = std::make_shared<async_logger>("as", test_sink, tp, async_overflow_policy::discard_log_msg);
+        auto logger = std::make_shared<async_logger>("as", test_sink, tp, async_overflow_policy::overrun_oldeset);
         for (size_t i = 0; i < messages; i++)
         {
             logger->info("Hello message #{}", i);