Fix for MDEV-12730

Assertion `count > 0' failed in rpl_parallel_thread_pool::
get_thread, rpl.rpl_parallel failed in buildbot

The reason for this is that one thread can call
rpl_parallel_resize_pool_if_no_slaves() while
another thread calls at the same time
rpl_parallel_activate_pool(). If rpl_parallel_active_pool() is
called before rpl_parallel_resize_pool_if_no_slaves() has
finished, pool->count will be set to 0 even if there exists
active slave threads.

Added a mutex lock in rpl_parallel_activate_pool() to protect against this scenario, which seams to fix this issue.

sql/rpl_parallel.cc

@@ -1623,10 +1623,19 @@ int rpl_parallel_resize_pool_if_no_slaves(void)
 }
 
 
+/**
+   Resize pool if not active or busy (in which case we may be in
+   resize to 0
+*/
+
 int
 rpl_parallel_activate_pool(rpl_parallel_thread_pool *pool)
 {
-  if (!pool->count)
+  bool resize;
+  mysql_mutex_lock(&pool->LOCK_rpl_thread_pool);
+  resize= !pool->count || pool->busy;
+  mysql_mutex_unlock(&pool->LOCK_rpl_thread_pool);
+  if (resize)
     return rpl_parallel_change_thread_count(pool, opt_slave_parallel_threads,
                                             0);
   return 0;