MDEV-18873 Server crashes in Compare_identifiers::operator or in my_strcasecmp_utf8 upon ADD PERIOD IF NOT EXISTS with empty name

empty identifier specified as `` ends up with a NULL LEX_CSTRING::str in lexer.
This is not considered correct in upper layers, for example in Compare_identifiers::operator().
Empty column name is usually avoided by a check_column_name() call while parsing,
and period name matches the column name completely.
Hence, this fix uses the mentioned call for verification, too.

2 files changed, 37 insertions, 0 deletions

diff --git a/mysql-test/suite/period/r/alter.result b/mysql-test/suite/period/r/alter.result
index a6466c8944b..875dab93b68 100644
--- a/mysql-test/suite/period/r/alter.result
+++ b/mysql-test/suite/period/r/alter.result
@@ -190,3 +190,17 @@ alter table t1 add primary key(x, s, e);
 ERROR 23000: Duplicate entry '1-2020-03-01-2020-03-02' for key 'PRIMARY'
 alter table t1 add system versioning;
 drop table t1;
+#
+# MDEV-18873 Server crashes in Compare_identifiers::operator or in 
+# my_strcasecmp_utf8 upon ADD PERIOD IF NOT EXISTS with empty name
+#
+alter table t add period if not exists for `` (s,e);
+ERROR 42000: Incorrect column name ''
+create table t(s DATE, e DATE);
+alter table t add period if not exists for `` (s,e);
+ERROR 42000: Incorrect column name ''
+alter table t add period if not exists for ` ` (s,e);
+ERROR 42000: Incorrect column name ' '
+create table t2 (period for `` (s,e)) select * from t;
+ERROR 42000: Incorrect column name ''
+drop table t;
diff --git a/mysql-test/suite/period/t/alter.test b/mysql-test/suite/period/t/alter.test
index 3fa3c5c87d5..2a82f74b670 100644
--- a/mysql-test/suite/period/t/alter.test
+++ b/mysql-test/suite/period/t/alter.test
@@ -151,3 +151,26 @@ alter table t1 add system versioning;
 
 # cleanup
 drop table t1;
+
+--echo #
+--echo # MDEV-18873 Server crashes in Compare_identifiers::operator or in 
+--echo # my_strcasecmp_utf8 upon ADD PERIOD IF NOT EXISTS with empty name
+--echo #
+
+# When there is no table defined.
+--error ER_WRONG_COLUMN_NAME
+alter table t add period if not exists for `` (s,e);
+
+# When there is an actual table.
+create table t(s DATE, e DATE);
+--error ER_WRONG_COLUMN_NAME
+alter table t add period if not exists for `` (s,e);
+
+# When the last character is space
+--error ER_WRONG_COLUMN_NAME
+alter table t add period if not exists for ` ` (s,e);
+
+# Create table with an empty period name
+--error ER_WRONG_COLUMN_NAME
+create table t2 (period for `` (s,e)) select * from t;
+drop table t;