MDEV-17441 - InnoDB transition to C++11 atomics

purge_sys_t::m_enabled transition to std::atomic.

enabled_latched() doesn't make much sense: in this particular case it is
as fast as atomic load. The sole caller has to reload it's value anyway,
due to rw_lock_x_lock(&purge_sys.latch) issuing acquire memory barrier.

When purge_sys_t::close() is reached, m_enabled must be false, otherwise
we may free members, which are still in use by the coordinator thread.

storage/innobase/include/trx0purge.h

@@ -147,8 +147,8 @@ class purge_sys_t
 	MY_ALIGNED(CACHE_LINE_SIZE)
 	rw_lock_t	latch;
 private:
-	/** whether purge is enabled; protected by latch and my_atomic */
-	int32_t		m_enabled;
+	/** whether purge is enabled; protected by latch and std::atomic */
+	std::atomic<bool>		m_enabled;
 	/** number of pending stop() calls without resume() */
 	Atomic_counter<int32_t>		m_paused;
 public:
@@ -242,16 +242,7 @@ class purge_sys_t
   void close();
 
   /** @return whether purge is enabled */
-  bool enabled()
-  {
-    return my_atomic_load32_explicit(&m_enabled, MY_MEMORY_ORDER_RELAXED);
-  }
-  /** @return whether purge is enabled */
-  bool enabled_latched()
-  {
-    ut_ad(rw_lock_own_flagged(&latch, RW_LOCK_FLAG_X | RW_LOCK_FLAG_S));
-    return bool(m_enabled);
-  }
+  bool enabled() { return m_enabled.load(std::memory_order_relaxed); }
   /** @return whether the purge coordinator is paused */
   bool paused()
   { return m_paused != 0; }
@@ -261,14 +252,14 @@ class purge_sys_t
   void coordinator_startup()
   {
     ut_ad(!enabled());
-    my_atomic_store32_explicit(&m_enabled, true, MY_MEMORY_ORDER_RELAXED);
+    m_enabled.store(true, std::memory_order_relaxed);
   }
 
   /** Disable purge at shutdown */
   void coordinator_shutdown()
   {
     ut_ad(enabled());
-    my_atomic_store32_explicit(&m_enabled, false, MY_MEMORY_ORDER_RELAXED);
+    m_enabled.store(false, std::memory_order_relaxed);
   }
 
   /** @return whether the purge coordinator thread is active */

storage/innobase/trx/trx0purge.cc

@@ -184,7 +184,7 @@ void purge_sys_t::close()
   ut_ad(this == &purge_sys);
   if (!event) return;
 
-  m_enabled= false;
+  ut_ad(!enabled());
   trx_t* trx = query->trx;
   que_graph_free(query);
   ut_ad(!trx->id);
@@ -1351,7 +1351,7 @@ void purge_sys_t::stop()
 {
   rw_lock_x_lock(&latch);
 
-  if (!enabled_latched())
+  if (!enabled())
   {
     /* Shutdown must have been initiated during FLUSH TABLES FOR EXPORT. */
     ut_ad(!srv_undo_sources);