MDEV-34724: Skipping a row operation from a trigger

Implementation of this task adds ability to raise the signal with
SQLSTATE '02TRG' from a BEFORE INSERT/UPDATE/DELETE trigger and handles
this signal as an indicator meaning 'to throw away the current row'
on processing the INSERT/UPDATE/DELETE statement. The signal with
SQLSTATE '02TRG' has special meaning only in case it is raised inside
BEFORE triggers, for AFTER trigger's this value of SQLSTATE isn't treated
in any special way. In according with SQL standard, the SQLSTATE class '02'
means NO DATA and sql_errno for this class is set to value
ER_SIGNAL_NOT_FOUND by current implementation of MariaDB server.
Implementation of this task assigns the value ER_SIGNAL_SKIP_ROW_FROM_TRIGGER
to sql_errno in Diagnostics_area in case the signal is raised from a trigger
and SQLSTATE has value '02TRG'.

To catch signal with SQLTSATE '02TRG' and handle it in special way, the methods
 Table_triggers_list::process_triggers
 select_insert::store_values
 select_create::store_values
 Rows_log_event::process_triggers
and the overloaded function
 fill_record_n_invoke_before_triggers
were extended with extra out parameter for returning the flag whether
to skip the current values being processed by INSERT/UPDATE/DELETE
statement. This extra parameter is passed as nullptr in case of AFTER trigger
and BEFORE trigger this parameter points to a variable to store a marker
whether to skip the current record or store it by calling write_record().

sql/sql_trigger.cc

@@ -2568,6 +2568,41 @@ end:
 }
 
 
+/**
+  Check that a BEFORE trigger has raised the signal to inform that
+  a current row being processed must be skipped.
+
+  @param      da                  Diagnostics area
+  @param[out] skip_row_indicator  where to store the fact about skipping
+                                  the row
+  @param      time_type           time when trigger is invoked (i.e. before or
+                                  after)
+
+  @return true in case the current row must be skipped, else false
+*/
+
+static inline bool do_skip_row_indicator(Diagnostics_area *da,
+                                         bool *skip_row_indicator,
+                                         trg_action_time_type time_type)
+{
+  if (!skip_row_indicator)
+    return false;
+
+  if (time_type == TRG_ACTION_BEFORE &&
+      /*
+        The '02' class signals a 'no data' condition, the subclass '02TRG'
+        means 'no data in trigger' and this condition shouldn't be treated
+        as an error.
+      */
+      strcmp(da->get_sqlstate(), "02TRG") == 0)
+  {
+    *skip_row_indicator= true;
+    return true;
+  }
+  return false;
+}
+
+
 /**
   Execute trigger for given (event, time) pair.
 
@@ -2578,6 +2613,8 @@ end:
   @param event
   @param time_type
   @param old_row_is_record1
+  @param[out] skip_row_indicator  the flag to tell whether a row must be
+                                  skipped by the INSERT statement
 
   @return Error status.
     @retval FALSE on success.
@@ -2588,6 +2625,7 @@ bool Table_triggers_list::process_triggers(THD *thd,
                                            trg_event_type event,
                                            trg_action_time_type time_type,
                                            bool old_row_is_record1,
+                                           bool *skip_row_indicator,
                                            List<Item> *fields_in_update_stmt)
 {
   bool err_status;
@@ -2595,6 +2633,18 @@ bool Table_triggers_list::process_triggers(THD *thd,
   Trigger *trigger;
   SELECT_LEX *save_current_select;
 
+  /*
+    skip_row_indicator != nullptr for BEFORE INSERT/UPDATE/DELETE triggers
+  */
+  DBUG_ASSERT((time_type == TRG_ACTION_BEFORE && skip_row_indicator) ||
+              (time_type == TRG_ACTION_AFTER && !skip_row_indicator));
+  /*
+    In case skip_indicator points to an out variable, its initial value
+    must be false
+  */
+  DBUG_ASSERT(!skip_row_indicator ||
+              (skip_row_indicator && *skip_row_indicator == false));
+
   if (check_for_broken_triggers())
     return TRUE;
 
@@ -2618,8 +2668,21 @@ bool Table_triggers_list::process_triggers(THD *thd,
   */
   DBUG_ASSERT(trigger_table->pos_in_table_list->trg_event_map & trg2bit(event));
 
-  thd->reset_sub_statement_state(&statement_state, SUB_STMT_TRIGGER);
-
+  if (time_type == TRG_ACTION_AFTER)
+    thd->reset_sub_statement_state(&statement_state, SUB_STMT_TRIGGER);
+  else
+    /*
+      For time type TRG_ACTION_BEFORE, set extra flag SUB_STMT_BEFORE_TRIGGER
+      at sub statement state in addition to SUB_STMT_TRIGGER in order to
+      be able to reset the m_sql_errno to the value
+        ER_SIGNAL_SKIP_ROW_FROM_TRIGGER
+      in case signal is raised with SQLSTATE "02TRG" from within a
+      BEFORE trigger and don't modify m_sql_errno in case the signal is raised
+      from AFTER trigger.
+      @see Sql_state_errno_level::assign_defaults
+    */
+    thd->reset_sub_statement_state(&statement_state,
+                                   SUB_STMT_TRIGGER | SUB_STMT_BEFORE_TRIGGER);
   /*
     Reset current_select before call execute_trigger() and
     restore it after return from one. This way error is set
@@ -2656,6 +2719,18 @@ bool Table_triggers_list::process_triggers(THD *thd,
                                      &trigger_table->s->db,
                                      &trigger_table->s->table_name,
                                      &trigger->subject_table_grants);
+
+    if (err_status &&
+        do_skip_row_indicator(thd->get_stmt_da(), skip_row_indicator,
+                              time_type))
+    {
+      /* Reset DA that is set on handling the statement
+           SIGNAL SSQLSTATE "02TRG"
+         raised from within a trigger */
+      err_status= false;
+      thd->get_stmt_da()->reset_diagnostics_area();
+    }
+
     status_var_increment(thd->status_var.executed_triggers);
   } while (!err_status && (trigger= trigger->next));
   thd->bulk_param= save_bulk_param;