Allow custom callback to be executed by thread-pool's threads before joining them.

This is similar to a change that was made a while ago ( https://github.com/gabime/spdlog/pull/208 ).

include/spdlog/details/thread_pool-inl.h

@@ -13,7 +13,7 @@
 namespace spdlog {
 namespace details {
 
-SPDLOG_INLINE thread_pool::thread_pool(size_t q_max_items, size_t threads_n, std::function<void()> on_thread_start)
+SPDLOG_INLINE thread_pool::thread_pool(size_t q_max_items, size_t threads_n, std::function<void()> on_thread_start, std::function<void()> on_thread_stop)
     : q_(q_max_items)
 {
     if (threads_n == 0 || threads_n > 1000)
@@ -23,15 +23,20 @@ SPDLOG_INLINE thread_pool::thread_pool(size_t q_max_items, size_t threads_n, std
     }
     for (size_t i = 0; i < threads_n; i++)
     {
-        threads_.emplace_back([this, on_thread_start] {
+        threads_.emplace_back([this, on_thread_start, on_thread_stop] {
             on_thread_start();
             this->thread_pool::worker_loop_();
+            on_thread_stop();
         });
     }
 }
 
+SPDLOG_INLINE thread_pool::thread_pool(size_t q_max_items, size_t threads_n, std::function<void()> on_thread_start)
+    : thread_pool(q_max_items, threads_n, on_thread_start, [] {})
+{}
+
 SPDLOG_INLINE thread_pool::thread_pool(size_t q_max_items, size_t threads_n)
-    : thread_pool(q_max_items, threads_n, [] {})
+    : thread_pool(q_max_items, threads_n, [] {}, [] {})
 {}
 
 // message all threads to terminate gracefully join them

include/spdlog/details/thread_pool.h

@@ -84,6 +84,7 @@ public:
     using item_type = async_msg;
     using q_type = details::mpmc_blocking_queue<item_type>;
 
+    thread_pool(size_t q_max_items, size_t threads_n, std::function<void()> on_thread_start, std::function<void()> on_thread_stop);
     thread_pool(size_t q_max_items, size_t threads_n, std::function<void()> on_thread_start);
     thread_pool(size_t q_max_items, size_t threads_n);