MDEV-33840 tpool- switch to longer maintainence timer interval, if pool is idle

Previous solution, that would entirely switch timer off, turned out
to be deadlock prone.

This patch fixed previous attempt to switch between long/short interval
periods in MDEV-24295. Now, initial state of the timer is fixed (it is ON).
Also, avoid switching timer to longer periods if there is any activity in
the pool.

tpool/tpool_generic.cc

@@ -644,7 +644,7 @@ void thread_pool_generic::check_idle(std::chrono::system_clock::time_point now)
   }
 
   /* Switch timer off after 1 minute of idle time */
-  if (now - idle_since > max_idle_time)
+  if (now - idle_since > max_idle_time && m_active_threads.empty())
   {
     idle_since= invalid_timestamp;
     switch_timer(timer_state_t::OFF);
@@ -743,6 +743,12 @@ bool thread_pool_generic::add_thread()
   if (n_threads >= m_max_threads)
     return false;
 
+  /*
+    Deadlock danger exists, so monitor pool health
+    with maintenance timer.
+  */
+  switch_timer(timer_state_t::ON);
+
   if (n_threads >= m_min_threads)
   {
     auto now = std::chrono::system_clock::now();
@@ -753,8 +759,6 @@ bool thread_pool_generic::add_thread()
         Throttle thread creation and wakeup deadlock detection timer,
         if is it off.
       */
-      switch_timer(timer_state_t::ON);
-
       return false;
     }
   }
@@ -837,6 +841,7 @@ thread_pool_generic::thread_pool_generic(int min_threads, int max_threads) :
 
   // start the timer
   m_maintenance_timer.set_time(0, (int)m_timer_interval.count());
+  m_timer_state = timer_state_t::ON;
 }