MDEV-25659 trigger name is empty after upgrade to 10.4

Problem:

At some point, we made stored rountines fail at CREATE time
instead of execution time in case of this syntax:

   IF unknown_variable
     ...
   END IF

As a result, a trigger created before this change and contained an unknown
variable worked in a bad way after upgrade:
- It was displayed with an empty trigger name by SHOW CREATE TRIGGER
- It was displayed with an empty trigger name by INFORMATION_SCHEMA.TRIGGERS
- An attempt to DROP this trigger returned errors - nothing happened.
- DROP TABLE did not remove the .TRN file corresponding to this broken trigger.

Underlying code observations:

The old code assumed that the trigger name resides in the current lex:

  if(thd->lex->spname)
    m_trigger_name= &thd->lex->spname->m_name;

This is not always the case. Some SP statements (e.g. IF)
do the following in their beginning:

- create a separate local LEX
- set thd->lex to this new local LEX
- push the new local LEX to the stack in sp_head::m_lex

and the following at the end of the statement:

- pop the previous LEX from the stack sp_head::m_lex
- set thd->lex back to the popped value

So when the parse error happens inside e.g. IF statement, thd->lex->spname
is a NULL pointer, because thd->lex points to the local LEX (without SP name)
rather than the top level LEX (with SP name).

Fix:
- Adding a new method sp_head::find_spname_recursive()
  which walks inside the LEX stack sp_head::m_lex from
  the top (the newest, most local) to the bottom (the oldest),
  and finds the one which contains a non-zero spname pointer.

- Using the new method inside
  Deprecated_trigger_syntax_handler::handle_condition():
  First it still tests thd->lex->spname (like before this change),
  and uses it in case it is not empty.
  Otherwise (if thd->lex->spname is empty), it calls
  sp_head::find_spname_recursive() to find the LEX with a
  non-empty spname inside the LEX stack of the current sphead.

mysql-test/main/trigger-compat.test

@@ -289,3 +289,219 @@ DROP TRIGGER tr11;
 
 DROP TABLE t1;
 DROP TABLE t2;
+
+
+--echo #
+--echo # MDEV-25659 trigger name is empty after upgrade to 10.4
+--echo #
+
+--echo # START: Total triggers 1, broken triggers 1, DROP TABLE
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+
+--write_file $MYSQLD_DATADIR/test/tr1.TRN
+TYPE=TRIGGERNAME
+trigger_table=t1
+EOF
+
+--write_file $MYSQLD_DATADIR/test/t1.TRG
+TYPE=TRIGGERS
+triggers='CREATE DEFINER=`root`@`localhost` TRIGGER tr1 AFTER DELETE ON t1 FOR EACH ROW\nBEGIN\n  IF unknown_variable\n  THEN\n    INSERT INTO t2 VALUES (OLD.a);\n  END IF;\nEND'
+sql_modes=1411383296
+definers='root@localhost'
+client_cs_names='utf8'
+connection_cl_names='utf8_general_ci'
+db_cl_names='latin1_swedish_ci'
+created=164206218647
+EOF
+
+FLUSH TABLES;
+--error ER_PARSE_ERROR
+DELETE FROM t1 WHERE a=1;
+--error ER_PARSE_ERROR
+INSERT INTO t1 VALUES (2);
+
+SET time_zone='+00:00';
+--vertical_results
+SHOW TRIGGERS LIKE 't1';
+SELECT * FROM INFORMATION_SCHEMA.TRIGGERS WHERE TRIGGER_NAME='tr1';
+--horizontal_results
+SET time_zone=DEFAULT;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+DROP TABLE t1;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+--echo # END: Total triggers 1, broken triggers 1, DROP TABLE
+
+
+--echo # START: Total triggers 1, broken triggers 1, DROP TRIGGER
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+
+--write_file $MYSQLD_DATADIR/test/tr1.TRN
+TYPE=TRIGGERNAME
+trigger_table=t1
+EOF
+
+--write_file $MYSQLD_DATADIR/test/t1.TRG
+TYPE=TRIGGERS
+triggers='CREATE DEFINER=`root`@`localhost` TRIGGER tr1 AFTER DELETE ON t1 FOR EACH ROW\nBEGIN\n  IF unknown_variable\n  THEN\n    INSERT INTO t2 VALUES (OLD.a);\n  END IF;\nEND'
+sql_modes=1411383296
+definers='root@localhost'
+client_cs_names='utf8'
+connection_cl_names='utf8_general_ci'
+db_cl_names='latin1_swedish_ci'
+created=164206218647
+EOF
+
+FLUSH TABLES;
+--error ER_PARSE_ERROR
+DELETE FROM t1 WHERE a=1;
+--error ER_PARSE_ERROR
+INSERT INTO t1 VALUES (2);
+
+SET time_zone='+00:00';
+--vertical_results
+SHOW TRIGGERS LIKE 't1';
+SELECT * FROM INFORMATION_SCHEMA.TRIGGERS WHERE TRIGGER_NAME='tr1';
+--horizontal_results
+SET time_zone=DEFAULT;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+DROP TRIGGER tr1;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+DROP TABLE t1;
+
+--echo # END: Total triggers 1, broken triggers 1, DROP TRIGGER
+
+
+--echo # START: Total triggers 2, broken triggers 1, DROP TABLE
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+
+--write_file $MYSQLD_DATADIR/test/tr1.TRN
+TYPE=TRIGGERNAME
+trigger_table=t1
+EOF
+
+--write_file $MYSQLD_DATADIR/test/tr2.TRN
+TYPE=TRIGGERNAME
+trigger_table=t1
+EOF
+
+--write_file $MYSQLD_DATADIR/test/t1.TRG
+TYPE=TRIGGERS
+triggers='CREATE DEFINER=`root`@`localhost` TRIGGER tr2 AFTER INSERT ON t1 FOR EACH ROW INSERT INTO t2 VALUES (NEW.a+100)' 'CREATE DEFINER=`root`@`localhost` TRIGGER tr1 AFTER DELETE ON t1 FOR EACH ROW\nBEGIN\n  IF unknown_variable\n  THEN\n    INSERT INTO t2 VALUES (OLD.a);\n  END IF;\nEND'
+sql_modes=1411383296 1411383296
+definers='root@localhost' 'root@localhost'
+client_cs_names='utf8' 'utf8'
+connection_cl_names='utf8_general_ci' 'utf8_general_ci'
+db_cl_names='latin1_swedish_ci' 'latin1_swedish_ci'
+created=164206810874 164206810873
+EOF
+
+FLUSH TABLES;
+--error ER_PARSE_ERROR
+DELETE FROM t1 WHERE a=1;
+--error ER_PARSE_ERROR
+INSERT INTO t1 VALUES (2);
+
+SET time_zone='+00:00';
+--vertical_results
+SHOW TRIGGERS LIKE 't1';
+SELECT * FROM INFORMATION_SCHEMA.TRIGGERS WHERE TRIGGER_NAME='tr1';
+--horizontal_results
+SET time_zone=DEFAULT;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+DROP TABLE t1;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+--echo # END: Total triggers 2, broken triggers 1, using DROP TABLE
+
+
+--echo # START: Total triggers 2, broken triggers 1, DROP TRIGGER
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+
+--write_file $MYSQLD_DATADIR/test/tr1.TRN
+TYPE=TRIGGERNAME
+trigger_table=t1
+EOF
+
+--write_file $MYSQLD_DATADIR/test/tr2.TRN
+TYPE=TRIGGERNAME
+trigger_table=t1
+EOF
+
+--write_file $MYSQLD_DATADIR/test/t1.TRG
+TYPE=TRIGGERS
+triggers='CREATE DEFINER=`root`@`localhost` TRIGGER tr2 AFTER INSERT ON t1 FOR EACH ROW INSERT INTO t2 VALUES (NEW.a+100)' 'CREATE DEFINER=`root`@`localhost` TRIGGER tr1 AFTER DELETE ON t1 FOR EACH ROW\nBEGIN\n  IF unknown_variable\n  THEN\n    INSERT INTO t2 VALUES (OLD.a);\n  END IF;\nEND'
+sql_modes=1411383296 1411383296
+definers='root@localhost' 'root@localhost'
+client_cs_names='utf8' 'utf8'
+connection_cl_names='utf8_general_ci' 'utf8_general_ci'
+db_cl_names='latin1_swedish_ci' 'latin1_swedish_ci'
+created=164206810874 164206810873
+EOF
+
+FLUSH TABLES;
+--error ER_PARSE_ERROR
+DELETE FROM t1 WHERE a=1;
+--error ER_PARSE_ERROR
+INSERT INTO t1 VALUES (2);
+
+SET time_zone='+00:00';
+--vertical_results
+SHOW TRIGGERS LIKE 't1';
+SELECT * FROM INFORMATION_SCHEMA.TRIGGERS WHERE TRIGGER_NAME='tr1';
+--horizontal_results
+SET time_zone=DEFAULT;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+DROP TRIGGER tr1;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+# Now we dropped the broken trigger. Make sure the good one is fired.
+# If everything goes as expected, it will try to insert into t2,
+# which does not exists, hence the (expected) error.
+--error ER_NO_SUCH_TABLE
+INSERT INTO t1 VALUES (100);
+
+DROP TABLE t1;
+
+--echo # Listing trigger files
+--list_files $MYSQLD_DATADIR/test *.TR?
+--echo # Listing trigger files done
+
+--echo # END: Total triggers 2, broken triggers 1, using DROP TRIGGER