pool: add call_once to initialize_pool() mem. fun

* also protect set_id() member function with a std::lock_guard, which
  holds mtx for the time of initialization

pool.cpp

@@ -15,6 +15,7 @@ namespace fortuna {
 namespace accumulator {
 
 auto Pool::set_id(const unsigned int& id) noexcept -> void {
+	std::lock_guard<std::mutex> lg(mtx);
 	this->pool_id = id;
 }
 
@@ -23,7 +24,8 @@ auto Pool::get_id() const noexcept -> unsigned int {
 }
 
 auto Pool::initialize_pool(const unsigned int& id) -> void {
-	set_id(id);
+	// ref: https://stackoverflow.com/a/23204682
+	std::call_once(init, [this, id]{ this->set_id(id); } );
 }
 
 auto Pool::add_entropy(const unsigned int& source,

pool.h

@@ -43,6 +43,7 @@ private:
 	uint64_t size{0};
 	std::mutex mtx; // mutex for write locks
 	mutable std::recursive_mutex ro_mtx_s; // mtx for readers of s
+	std::once_flag init;
 
 }; // class Pool