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

sql/event_data_objects.cc

@@ -20,6 +20,8 @@
 #include "event_db_repository.h"
 #include "sp_head.h"
 
+/* That's a provisional solution */
+extern Event_db_repository events_event_db_repository;
 
 #define EVEX_MAX_INTERVAL_VALUE 1000000000L
 
@@ -30,6 +32,47 @@ event_change_security_context(THD *thd, LEX_STRING user, LEX_STRING host,
 static void
 event_restore_security_context(THD *thd, Security_context *backup);
 
+
+/*
+  Initiliazes dbname and name of an Event_queue_element_for_exec
+  object
+
+  SYNOPSIS
+    Event_queue_element_for_exec::init()
+
+  RETURN VALUE
+    FALSE  OK
+    TRUE   Error (OOM)
+*/
+
+bool
+Event_queue_element_for_exec::init(LEX_STRING db, LEX_STRING n)
+{
+  if (!(dbname.str= my_strndup(db.str, dbname.length= db.length, MYF(MY_WME))))
+    return TRUE;
+  if (!(name.str= my_strndup(n.str, name.length= n.length, MYF(MY_WME))))
+  {
+    my_free((gptr) dbname.str, MYF(0));
+    return TRUE;
+  }
+  return FALSE;
+}
+
+
+/*
+  Destructor
+
+  SYNOPSIS
+    Event_queue_element_for_exec::~Event_queue_element_for_exec()
+*/
+
+Event_queue_element_for_exec::~Event_queue_element_for_exec()
+{
+  my_free((gptr) dbname.str, MYF(0));
+  my_free((gptr) name.str, MYF(0));
+}
+
+
 /*
   Returns a new instance
 
@@ -743,7 +786,7 @@ Event_timed::~Event_timed()
 */
 
 Event_job_data::Event_job_data()
-  :thd(NULL), sphead(NULL), sql_mode(0)
+  :sphead(NULL), sql_mode(0)
 {
 }
 
@@ -1239,6 +1282,7 @@ Event_queue_element::compute_next_execution_time()
     DBUG_PRINT("info", ("Dropped: %d", dropped));
     status= Event_queue_element::DISABLED;
     status_changed= TRUE;
+    dropped= TRUE;
 
     goto ret;
   }
@@ -1446,32 +1490,6 @@ Event_queue_element::mark_last_executed(THD *thd)
 }
 
 
-/*
-  Drops the event
-
-  SYNOPSIS
-    Event_queue_element::drop()
-      thd   thread context
-
-  RETURN VALUE
-    0       OK
-   -1       Cannot open mysql.event
-   -2       Cannot find the event in mysql.event (already deleted?)
-
-   others   return code from SE in case deletion of the event row
-            failed.
-*/
-
-int
-Event_queue_element::drop(THD *thd)
-{
-  DBUG_ENTER("Event_queue_element::drop");
-
-  DBUG_RETURN(Events::get_instance()->
-                    drop_event(thd, dbname, name, FALSE, TRUE));
-}
-
-
 /*
   Saves status and last_executed_at to the disk if changed.
 
@@ -1503,13 +1521,13 @@ Event_queue_element::update_timing_fields(THD *thd)
 
   thd->reset_n_backup_open_tables_state(&backup);
 
-  if (Events::get_instance()->open_event_table(thd, TL_WRITE, &table))
+  if (events_event_db_repository.open_event_table(thd, TL_WRITE, &table))
   {
     ret= TRUE;
     goto done;
   }
   fields= table->field;
-  if ((ret= Events::get_instance()->db_repository->
+  if ((ret= events_event_db_repository.
                                  find_named_event(thd, dbname, name, table)))
     goto done;