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.

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;

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;

sql/sql_lex.h

@@ -33,6 +33,7 @@
 #include "sql_tvc.h"
 #include "item.h"
 #include "sql_schema.h"
+#include "table.h"
 
 /* Used for flags of nesting constructs */
 #define SELECT_NESTING_MAP_SIZE 64
@@ -4449,6 +4450,11 @@ public:
 
   int add_period(Lex_ident name, Lex_ident_sys_st start, Lex_ident_sys_st end)
   {
+    if (check_period_name(name.str)) {
+      my_error(ER_WRONG_COLUMN_NAME, MYF(0), name.str);
+      return 1;
+    }
+
     if (lex_string_cmp(system_charset_info, &start, &end) == 0)
     {
       my_error(ER_FIELD_SPECIFIED_TWICE, MYF(0), start.str);

sql/table.cc

@@ -4918,6 +4918,12 @@ bool check_column_name(const char *name)
 }
 
 
+bool check_period_name(const char *name)
+{
+  return check_column_name(name);
+}
+
+
 /**
   Checks whether a table is intact. Should be done *just* after the table has
   been opened.

sql/table.h

@@ -3231,6 +3231,7 @@ void open_table_error(TABLE_SHARE *share, enum open_frm_error error,
 void update_create_info_from_table(HA_CREATE_INFO *info, TABLE *form);
 bool check_db_name(LEX_STRING *db);
 bool check_column_name(const char *name);
+bool check_period_name(const char *name);
 bool check_table_name(const char *name, size_t length, bool check_for_path_chars);
 int rename_file_ext(const char * from,const char * to,const char * ext);
 char *get_field(MEM_ROOT *mem, Field *field);

sql/vers_string.h

@@ -39,6 +39,8 @@ struct Compare_identifiers
 {
   int operator()(const LEX_CSTRING& a, const LEX_CSTRING& b) const
   {
+    DBUG_ASSERT(a.str != NULL);
+    DBUG_ASSERT(b.str != NULL);
     DBUG_ASSERT(a.str[a.length] == 0);
     DBUG_ASSERT(b.str[b.length] == 0);
     return my_strcasecmp(system_charset_info, a.str, b.str);