feat(mpmc_blocking_q): add blocking dequeue without timeout (#2588)

Use the new blocking dequeue to avoid unnecessarily waking up the
thread pool every 10s.

Fixes #2587 by replacing std::condition_variable::wait_for with
std::condition_variable::wait as a workaroung for gcc 11.3 issue 101978.

Co-authored-by: Alok Priyadarshi <alokp@dexterity.ai>

include/spdlog/details/mpmc_blocking_q.h

@@ -49,7 +49,7 @@ public:
         push_cv_.notify_one();
     }
 
-    // try to dequeue item. if no item found. wait up to timeout and try again
+    // dequeue with a timeout.
     // Return true, if succeeded dequeue item, false otherwise
     bool dequeue_for(T &popped_item, std::chrono::milliseconds wait_duration)
     {
@@ -66,6 +66,18 @@ public:
         return true;
     }
 
+    // blocking dequeue without a timeout.
+    void dequeue(T &popped_item)
+    {
+        {
+            std::unique_lock<std::mutex> lock(queue_mutex_);
+            push_cv_.wait(lock, [this] { return !this->q_.empty(); });
+            popped_item = std::move(q_.front());
+            q_.pop_front();
+        }
+        pop_cv_.notify_one();
+    }
+
 #else
     // apparently mingw deadlocks if the mutex is released before cv.notify_one(),
     // so release the mutex at the very end each function.
@@ -87,7 +99,7 @@ public:
         push_cv_.notify_one();
     }
 
-    // try to dequeue item. if no item found. wait up to timeout and try again
+    // dequeue with a timeout.
     // Return true, if succeeded dequeue item, false otherwise
     bool dequeue_for(T &popped_item, std::chrono::milliseconds wait_duration)
     {
@@ -102,6 +114,16 @@ public:
         return true;
     }
 
+    // blocking dequeue without a timeout.
+    void dequeue(T &popped_item)
+    {
+        std::unique_lock<std::mutex> lock(queue_mutex_);
+        push_cv_.wait(lock, [this] { return !this->q_.empty(); });
+        popped_item = std::move(q_.front());
+        q_.pop_front();
+        pop_cv_.notify_one();
+    }
+
 #endif
 
     size_t overrun_counter()

include/spdlog/details/thread_pool-inl.h

@@ -108,11 +108,7 @@ void SPDLOG_INLINE thread_pool::worker_loop_()
 bool SPDLOG_INLINE thread_pool::process_next_msg_()
 {
     async_msg incoming_async_msg;
-    bool dequeued = q_.dequeue_for(incoming_async_msg, std::chrono::seconds(10));
-    if (!dequeued)
-    {
-        return true;
-    }
+    q_.dequeue(incoming_async_msg);
 
     switch (incoming_async_msg.msg_type)
     {

tests/test_mpmc_q.cpp

@@ -43,6 +43,26 @@ TEST_CASE("dequeue-empty-wait", "[mpmc_blocking_q]")
     REQUIRE(delta_ms <= wait_ms + tolerance_wait);
 }
 
+TEST_CASE("dequeue-full-nowait", "[mpmc_blocking_q]")
+{
+    spdlog::details::mpmc_blocking_queue<int> q(1);
+    q.enqueue(42);
+
+    int item = 0;
+    q.dequeue_for(item, milliseconds::zero());
+    REQUIRE(item == 42);
+}
+
+TEST_CASE("dequeue-full-wait", "[mpmc_blocking_q]")
+{
+    spdlog::details::mpmc_blocking_queue<int> q(1);
+    q.enqueue(42);
+
+    int item = 0;
+    q.dequeue(item);
+    REQUIRE(item == 42);
+}
+
 TEST_CASE("enqueue_nowait", "[mpmc_blocking_q]")
 {
 
@@ -95,12 +115,12 @@ TEST_CASE("full_queue", "[mpmc_blocking_q]")
     for (int i = 1; i < static_cast<int>(q_size); i++)
     {
         int item = -1;
-        q.dequeue_for(item, milliseconds(0));
+        q.dequeue(item);
         REQUIRE(item == i);
     }
 
     // last item pushed has overridden the oldest.
     int item = -1;
-    q.dequeue_for(item, milliseconds(0));
+    q.dequeue(item);
     REQUIRE(item == 123456);
 }