lib: allow shrinking of resource_pools

References: DESK-1497

include/gromox/resource_pool.hpp

@@ -17,6 +17,11 @@ namespace gromox {
  * of sense to construct them a-priori, so the API only supports lazy
  * construction. For this, get_wait() needs to know some ctor args for when a
  * new T needs construction.
+ *
+ * @m_numslots: maximum number of objects the pool is willing to hand out
+ * @m_list:     previously-constructed objects that the pool can immediately
+ *              hand out again (if m_list.size() < m_numslots, the pool will
+ *              create objects on-the-fly)
  */
 template<typename Tp> class resource_pool {
 	public:
@@ -92,8 +97,15 @@ template<typename Tp> class resource_pool {
 
 	public:
 	void resize(size_t n) {
-		if (m_numslots < n)
-			m_numslots = n;
+		/*
+		 * Lock-free: Updating m_numslots is atomic; and n>o check can
+		 * happen unguarded because the "erroneously" woken-up thread
+		 * itself will check m_numslots.
+		 */
+		size_t o = m_numslots.load();
+		m_numslots = n;
+		if (n > o)
+			m_cv.notify_one();
 	}
 	void clear() {
 		std::lock_guard lk(m_mtx);