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, 4 insertions, 12 deletions

diff --git a/sql/events.h b/sql/events.h
index 621ab0ffca5..35ee3c569d0 100644
--- a/sql/events.h
+++ b/sql/events.h
@@ -42,13 +42,6 @@ sortcmp_lex_string(LEX_STRING s, LEX_STRING t, CHARSET_INFO *cs);
 class Events
 {
 public:
-  /*
-    Quite NOT the best practice and will be removed once
-    Event_timed::drop() and Event_timed is fixed not do drop directly
-    or other scheme will be found.
-  */
-  friend class Event_queue_element;
-  
   /* The order should match the order in opt_typelib */
   enum enum_opt_event_scheduler
   {
@@ -92,15 +85,11 @@ public:
   update_event(THD *thd, Event_parse_data *parse_data, sp_name *rename_to);
 
   bool
-  drop_event(THD *thd, LEX_STRING dbname, LEX_STRING name, bool if_exists,
-             bool only_from_disk);
+  drop_event(THD *thd, LEX_STRING dbname, LEX_STRING name, bool if_exists);
 
   void
   drop_schema_events(THD *thd, char *db);
 
-  int
-  open_event_table(THD *thd, enum thr_lock_type lock_type, TABLE **table);
-
   bool
   show_create_event(THD *thd, LEX_STRING dbname, LEX_STRING name);
 
@@ -119,6 +108,9 @@ private:
   bool
   check_system_tables(THD *thd);
 
+  int
+  load_events_from_db(THD *thd);
+
   /* Singleton DP is used */
   Events();
   ~Events(){}