MDEV-18862 Unfortunate error message upon attempt to drop system versioning

Special case for DROP PERIOD when system fields are implicit.

mysql-test/suite/versioning/r/alter.result

@@ -616,5 +616,18 @@ Warning	1906	The value specified for generated column 'e' in table 't1' has been
 Warning	1906	The value specified for generated column 'e' in table 't1' has been ignored
 alter table t1 force;
 set sql_mode= default;
+#
+# MDEV-18862 Unfortunate error message upon attempt to drop system versioning
+#
+set system_versioning_alter_history= keep;
+create or replace table t1 (x int) with system versioning;
+alter table t1 drop column `row_start`, drop column `row_end`, drop period for system_time, drop system versioning;
+ERROR HY000: No 'PERIOD FOR SYSTEM_TIME' in system-versioned `t1`
+alter table t1 drop period for system_time;
+ERROR HY000: No 'PERIOD FOR SYSTEM_TIME' in system-versioned `t1`
+alter table t1 drop column `row_start`, drop column `row_end`, drop system versioning;
+ERROR 42000: Can't DROP COLUMN `row_start`; check that it exists
+alter table t1 drop column `row_end`;
+ERROR 42000: Can't DROP COLUMN `row_end`; check that it exists
 drop database test;
 create database test;

mysql-test/suite/versioning/t/alter.test

@@ -514,5 +514,19 @@ update t1 set e= 1;
 alter table t1 force;
 set sql_mode= default;
 
+--echo #
+--echo # MDEV-18862 Unfortunate error message upon attempt to drop system versioning
+--echo #
+set system_versioning_alter_history= keep;
+create or replace table t1 (x int) with system versioning;
+--error ER_VERS_NO_PERIOD
+alter table t1 drop column `row_start`, drop column `row_end`, drop period for system_time, drop system versioning;
+--error ER_VERS_NO_PERIOD
+alter table t1 drop period for system_time;
+--error ER_CANT_DROP_FIELD_OR_KEY
+alter table t1 drop column `row_start`, drop column `row_end`, drop system versioning;
+--error ER_CANT_DROP_FIELD_OR_KEY
+alter table t1 drop column `row_end`;
+
 drop database test;
 create database test;

sql/share/errmsg-utf8.txt

@@ -7898,3 +7898,5 @@ ER_KEY_DOESNT_SUPPORT
         eng "%s index %`s does not support this operation"
 ER_ALTER_OPERATION_TABLE_OPTIONS_NEED_REBUILD
 	eng "Changing table options requires the table to be rebuilt"
+ER_VERS_NO_PERIOD
+        eng "No 'PERIOD FOR SYSTEM_TIME' in system-versioned %`s"

sql/sql_table.cc

@@ -7833,6 +7833,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   KEY *key_info=table->key_info;
   bool rc= TRUE;
   bool modified_primary_key= FALSE;
+  bool vers_system_invisible= false;
   Create_field *def;
   Field **f_ptr,*field;
   MY_BITMAP *dropped_fields= NULL; // if it's NULL - no dropped fields
@@ -7941,7 +7942,11 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
       bitmap_set_bit(dropped_fields, field->field_index);
       continue;
     }
-
+    if (field->invisible == INVISIBLE_SYSTEM &&
+        field->flags & VERS_SYSTEM_FIELD)
+    {
+      vers_system_invisible= true;
+    }
     /* invisible versioning column is dropped automatically on DROP SYSTEM VERSIONING */
     if (!drop && field->invisible >= INVISIBLE_SYSTEM &&
         field->flags & VERS_SYSTEM_FIELD &&
@@ -8059,7 +8064,8 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   dropped_sys_vers_fields &= VERS_SYSTEM_FIELD;
   if ((dropped_sys_vers_fields ||
        alter_info->flags & ALTER_DROP_PERIOD) &&
-      dropped_sys_vers_fields != VERS_SYSTEM_FIELD)
+      dropped_sys_vers_fields != VERS_SYSTEM_FIELD &&
+      !vers_system_invisible)
   {
     StringBuffer<NAME_LEN*3> tmp;
     append_drop_column(thd, dropped_sys_vers_fields & VERS_SYS_START_FLAG,
@@ -8069,6 +8075,11 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
     my_error(ER_MISSING, MYF(0), table->s->table_name.str, tmp.c_ptr());
     goto err;
   }
+  else if (alter_info->flags & ALTER_DROP_PERIOD && vers_system_invisible)
+  {
+    my_error(ER_VERS_NO_PERIOD, MYF(0), table->s->table_name.str);
+    goto err;
+  }
   alter_info->flags &= ~(ALTER_DROP_PERIOD | ALTER_ADD_PERIOD);
   def_it.rewind();
   while ((def=def_it++))			// Add new columns