MDEV-16264: Add threadpool library

The library is capable of
- asynchronous execution of tasks (and optionally waiting for them)
- asynchronous file IO
  This is implemented using libaio on Linux and completion ports on
  Windows. Elsewhere, async io is "simulated", which means worker threads
  are performing synchronous IO.
- timers, scheduling work asynchronously in some point of the future.
  Also periodic timers are implemented.

tpool/task.cc

@@ -0,0 +1,68 @@
+/* Copyright(C) 2019 MariaDB Corporation.
+
+This program is free software; you can redistribute itand /or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; version 2 of the License.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02111 - 1301 USA*/
+
+#include <tpool.h>
+#include <queue>
+#include <mutex>
+#include <condition_variable>
+#include <tpool_structs.h>
+
+namespace tpool
+{
+  task::task(callback_func func, void* arg, task_group* group) :
+    m_func(func), m_arg(arg), m_group(group) {}
+
+  void task::execute()
+  {
+    if (m_group)
+    {
+      /* Executing in a group (limiting concurrency).*/
+      m_group->execute(this);
+    }
+    else
+    {
+      /* Execute directly. */
+      m_func(m_arg);
+      release();
+    }
+  }
+
+  /* Task that provide wait() operation. */
+  waitable_task::waitable_task(callback_func func, void* arg, task_group* group) :
+    task(func,arg, group),m_mtx(),m_cv(),m_ref_count(),m_waiter_count(){}
+
+  void waitable_task::add_ref()
+  {
+    std::unique_lock<std::mutex> lk(m_mtx);
+    m_ref_count++;
+  }
+
+  void waitable_task::release()
+  {
+    std::unique_lock<std::mutex> lk(m_mtx);
+    m_ref_count--;
+    if (!m_ref_count && m_waiter_count)
+      m_cv.notify_all();
+  }
+  void waitable_task::wait()
+  {
+    std::unique_lock<std::mutex> lk(m_mtx);
+    m_waiter_count++;
+    while (m_ref_count)
+      m_cv.wait(lk);
+    m_waiter_count--;
+  }
+
+}