Fix for bug#22740 Events: Decouple Event_queue from Event_db_repository

This patch implements the idea of the bug report by making Event_queue
unaware of Event_db_repository by making a higher level class - Events,
which is aware of most of all classes, responsible for passing all data
needed for adding/updating/deleting an event to/from the queue.

Introduces few new classes :
 - Event_worker_thread
 - Event_queue_element_for_exec


sql/event_data_objects.cc:
  Introduced a new class Event_queue_element_for_exec
  According to Konstantin it should be named Event_name and hold
  only two LEX_STRINGs but `dropped` is not saved on disk and will
  require additional logic in Event_worker_thread class, after loading
  to compute whether the event should be dropped or not. It's easier
  just to pass this flag around.
  
  Removed Event_queue_element::drop(). This method was a source of a
  race condition. At the place where the event should be dropped we
  call Events::drop_event() which is the only code-flow for dropping.
  In addition, because ::drop_event() holds Events::LOCK_metadata there is
  no source of race conditions. Before this patch dropping from ::drop()
  wasn't under LOCK_metadata and races were possible.
  
  Because Events::open_event_table was removed as a method, provisionally
  events_event_db_repository was exported from events.cc till a solution is
  build where Event_queue_element does not access directly mysql.event.
sql/event_data_objects.h:
  New class Event_queue_element_for_exec added which is returned
  from Event_queue::get_top_if_time() and passed through Event_scheduler
  to Event_worker_thread. There by using the (db)name Event_job_data is
  instanciated and executed.
  
  Dropped Event_queue_element::drop()
  
  thd was moved out of Event_job_data as it is now part of
  Event_queue_element_for_exec
sql/event_queue.cc:
  Removed dependency of Event_queue on Event_db_repository.
  The instantiation of Event_job_data was moved to class Event_worker_thread
  In place is a return of an object of Event_queue_element_for_exec is used
  later for instantiating Event_job_data.
  
  The `dropped` flag of Event_queue_element is passed over
  Event_queue_element_for_exec to the code in Event_worker_thread.
sql/event_queue.h:
  Removed dependency of Event_queue on Event_db_repository
  Removed dependency on Event_scheduler
sql/event_scheduler.cc:
  Added class Event_worker_thread, which is used during
  the execution of an event. It has a static init() method
  to get a pointer to Event_db_repository to be used for
  instantiation of Event_job_data object. This object it then
  executed.
sql/event_scheduler.h:
  Added class Event_worker_thread, which is used during
  the execution of an event.
sql/events.cc:
  Removed Events::open_event_table() because it was a product of
  a bad architecture.
sql/events.h:
  Removed friend definition, unneeded.
  
  Fixed Events::drop_event() to have the previous signature without
  bool only_from_disk
sql/sql_parse.cc:
  Fix call

1 files changed, 27 insertions, 2 deletions

diff --git a/sql/event_scheduler.h b/sql/event_scheduler.h
index 18625ef35f3..2ab21464057 100644
--- a/sql/event_scheduler.h
+++ b/sql/event_scheduler.h
@@ -15,8 +15,11 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
+
 class Event_queue;
 class Event_job_data;
+class Event_db_repository;
+class Events;
 
 void
 pre_init_event_thread(THD* thd);
@@ -27,6 +30,29 @@ post_init_event_thread(THD* thd);
 void
 deinit_event_thread(THD *thd);
 
+
+class Event_worker_thread 
+{
+public:
+  static void
+  init(Events *events, Event_db_repository *db_repo)
+  {
+    db_repository= db_repo;
+    events_facade= events;
+  }
+
+  void
+  run(THD *thd, Event_queue_element_for_exec *event);
+
+private:
+  void
+  print_warnings(THD *thd, Event_job_data *et);
+
+  static Event_db_repository *db_repository;
+  static Events *events_facade;
+};
+
+
 class Event_scheduler
 {
 public:
@@ -71,10 +97,9 @@ private:
   uint
   workers_count();
 
-
   /* helper functions */
   bool
-  execute_top(THD *thd, Event_job_data *job_data);
+  execute_top(THD *thd, Event_queue_element_for_exec *event_name);
 
   /* helper functions for working with mutexes & conditionals */
   void