substitute hardcoded sleep with conditiona variables in thread_loop

include/spdlog/details/thread_pool.h

@@ -42,6 +42,8 @@ private:
     // synchronization
     std::mutex _mutex;
 
+    std::condition_variable _condition;
+
     std::function<void()> _worker_warmup_cb;
 
     std::function<void()> _worker_teardown_cb;
@@ -68,15 +70,12 @@ inline thread_pool::thread_pool(size_t num_threads,
             if( _worker_warmup_cb) _worker_warmup_cb();
             while( !_stop)
             {
-                if(  _loop_handles.empty()){
-                    std::this_thread::sleep_for( std::chrono::microseconds(100) );
-                }
-                else
                 {
-                    shared_function_ptr loop;
+                    shared_function_ptr handle;
                     size_t index = 0;
-                    bool busy = false;
+                    bool is_busy = false;
 
+                    // find an handle
                     {
                         std::unique_lock<std::mutex> lock(_mutex);
 
@@ -84,24 +83,25 @@ inline thread_pool::thread_pool(size_t num_threads,
                         for (size_t count=0; count<_loop_handles.size(); count++)
                         {
                             _index  = (_index+1) % _loop_handles.size();
-                            loop = _loop_handles[index].lock();
-                            busy = _busy[index];
+                            handle  = _loop_handles[index].lock();
+                            is_busy = _busy[index];
 
                             // if the weak pointer points to a delated handle, remove it
-                            if(!loop ){
+                            if(!handle ){
                                 _loop_handles.erase( _loop_handles.begin() + index);
                                 _busy.erase ( _busy.begin()  + index);
+                                _index  = (_index) % _loop_handles.size();
                             }
                             else{
                                 _busy[index] = true;
                             }
-                            if( loop && !busy) break;
+                            if( handle && !is_busy) break;
                         }
                     }
 
-                    if(loop && !busy)
+                    if(handle && !is_busy)
                     {
-                        bool continue_loop = (*loop)();
+                        bool continue_loop = (*handle)();
                         if(!continue_loop){
                             _loop_handles.erase( _loop_handles.begin() + index);
                             _busy.erase ( _busy.begin()  + index);
@@ -110,10 +110,13 @@ inline thread_pool::thread_pool(size_t num_threads,
                                 std::unique_lock<std::mutex> lock(_mutex);
                                 _busy[index] = false;
                         }
+                        // not busy anymore. notify to other threads
+                        _condition.notify_one();
                     }
                     else{
-                        // this happens if we haven't find any handle that needs our work
-                        std::this_thread::sleep_for( std::chrono::milliseconds(1) );
+                        // this happens when you didn't find an handle that is not busy
+                        std::unique_lock<std::mutex> lock(_mutex);
+                        _condition.wait(lock);
                     }
                 }
             }
@@ -124,10 +127,12 @@ inline thread_pool::thread_pool(size_t num_threads,
 
 inline void thread_pool::subscribe_handle(const shared_function_ptr &loop_handle)
 {
+    {
         std::unique_lock<std::mutex> lock(_mutex);
         _loop_handles.push_back( loop_handle );
         _busy.push_back( false );
-    assert( _loop_handles.size() == 1);
+    }
+    _condition.notify_one();
 }
 
 
@@ -138,6 +143,8 @@ inline thread_pool::~thread_pool()
         std::unique_lock<std::mutex> lock(_mutex);
         _stop = true;
     }
+    _condition.notify_all();
+
     for(std::thread &worker: _workers)
         worker.join();
 }