MDEV-11026 Make InnoDB number of IO write/read threads dynamic

Fix concurrency error  - avoid accessing deleted memory, when io_slots is
resized. the deleted memory in this case was vftable pointer in
aiocb::m_internal_task

The fix avoids calling dummy release function, via a flag in task_group.

storage/innobase/os/os0file.cc

@@ -84,8 +84,7 @@ class io_slots
 	int m_max_aio;
 public:
 	io_slots(int max_submitted_io, int max_callback_concurrency) :
-		m_cache(max_submitted_io),
-		m_group(max_callback_concurrency),
+		m_cache(max_submitted_io), m_group(max_callback_concurrency, false),
 		m_max_aio(max_submitted_io)
 	{
 	}

tpool/task_group.cc

@@ -25,11 +25,26 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02111 - 1301 USA*/
 #endif
 namespace tpool
 {
-  task_group::task_group(unsigned int max_concurrency) :
+
+  /**
+    Task_group constructor
+
+     @param max_threads - maximum number of threads allowed to execute
+     tasks from the group at the same time.
+
+     @param enable_task_release - if true (default), task::release() will be
+     called after task execution.'false' should only be used in rare cases
+     when accessing memory, pointed by task structures, would be unsafe after.
+     the callback. Also 'false' is only possible ,if task::release() is a trivial function
+  */
+  task_group::task_group(unsigned int max_concurrency,
+                       bool enable_task_release)
+    :
     m_queue(8),
     m_mtx(),
     m_tasks_running(),
-    m_max_concurrent_tasks(max_concurrency)
+    m_max_concurrent_tasks(max_concurrency),
+    m_enable_task_release(enable_task_release)
   {};
 
   void task_group::set_max_tasks(unsigned int max_concurrency)
@@ -53,7 +68,8 @@ namespace tpool
       if (t)
       {
         t->m_func(t->m_arg);
-        t->release();
+        if (m_enable_task_release)
+          t->release();
       }
       lk.lock();
 

tpool/tpool.h

@@ -68,8 +68,10 @@ class task_group
   std::condition_variable m_cv;
   unsigned int m_tasks_running;
   unsigned int m_max_concurrent_tasks;
+  const bool m_enable_task_release;
+
 public:
-  task_group(unsigned int max_concurrency= 100000);
+  task_group(unsigned int max_concurrency= 100000, bool m_enable_task_release= true);
   void set_max_tasks(unsigned int max_concurrent_tasks);
   void execute(task* t);
   void cancel_pending(task *t);