MDEV-31631 Adding auto-increment to table with history online misbehaves

Adding an auto_increment column online leads to an undefined behavior.
Basically any DEFAULTs that depend on a row order in the table, or on
the non-deterministic (in scope of the ALTER TABLE statement) function
is UB.

For example, NOW() is considered generally non-deterministic
(Item_func_now_utc is marked with VCOL_NON_DETERMINISTIC), but it's fixed
in scope of a single statement.

Same for any other function that depends only on the session/status vars
apart from its arguments.

Only two UB cases are known:
* adding new AUTO_INCREMENT column. Modifying the existing column may be
fine under certain circumstances, see MDEV-31058.
* adding new column with DEFAULT(nextval(...)). Modifying the existing
column is possible, since its value will be always present in the online
event, except for the NULL -> NOT NULL modification

mysql-test/main/alter_table_online.result

@@ -224,3 +224,17 @@ alter table s engine=Aria, lock=none;
 ERROR 0A000: LOCK=NONE is not supported. Reason: SEQUENCE. Try LOCK=SHARED
 alter table s engine=Aria;
 drop sequence  s;
+# MDEV-31631 Adding auto-increment column to a table with history online
+# behaves differently from non-online
+create sequence s;
+create table t1(a int, x int NULL default(nextval(s)));
+alter table t1 add b int default (nextval(s)), lock=none;
+ERROR 0A000: LOCK=NONE is not supported. Reason: Function or expression 'NEXTVAL()' cannot be used in the DEFAULT clause of `b`. Try LOCK=SHARED
+alter table t1 add b int primary key auto_increment, lock=none;
+ERROR 0A000: LOCK=NONE is not supported. Reason: ADD COLUMN ... AUTO_INCREMENT. Try LOCK=SHARED
+create table t2(a int, b int NULL default(nextval(s)));
+alter table t2 modify b int not null default (nextval(s)), lock=none;
+ERROR 0A000: LOCK=NONE is not supported. Reason: Function or expression 'NEXTVAL()' cannot be used in the DEFAULT clause of `b`. Try LOCK=SHARED
+drop table t2;
+drop table t1;
+drop sequence s;

mysql-test/main/alter_table_online.test

@@ -228,3 +228,21 @@ create sequence s engine=MyISAM;
 alter table s engine=Aria, lock=none;
 alter table s engine=Aria;
 drop sequence  s;
+
+
+--echo # MDEV-31631 Adding auto-increment column to a table with history online
+--echo # behaves differently from non-online
+create sequence s;
+create table t1(a int, x int NULL default(nextval(s)));
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table t1 add b int default (nextval(s)), lock=none;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table t1 add b int primary key auto_increment, lock=none;
+
+create table t2(a int, b int NULL default(nextval(s)));
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table t2 modify b int not null default (nextval(s)), lock=none;
+
+drop table t2;
+drop table t1;
+drop sequence s;

mysql-test/main/alter_table_online_debug.result

@@ -990,23 +990,6 @@ set debug_sync= reset;
 #
 set @old_dbug=@@debug_dbug;
 set debug_dbug="+d,rpl_report_chosen_key";
-create table t (a int);
-insert into t values (10),(20),(30);
-set debug_sync= 'alter_table_online_downgraded signal downgraded wait_for goforit';
-alter table t add pk int auto_increment primary key, algorithm=copy, lock=none;
-connection con2;
-set debug_sync= 'now wait_for downgraded';
-delete from t where a = 20;
-update t set a = a + 1 where a = 10;
-set debug_sync= 'now signal goforit';
-connection default;
-Warnings:
-Note	1105	Key chosen: -1
-Note	1105	Key chosen: -1
-select * from t;
-a	pk
-11	1
-30	3
 #
 # Add clumsy DEFAULT
 #

mysql-test/main/alter_table_online_debug.test

@@ -1172,21 +1172,23 @@ set debug_sync= reset;
 set @old_dbug=@@debug_dbug;
 set debug_dbug="+d,rpl_report_chosen_key";
 
-create table t (a int);
-insert into t values (10),(20),(30);
-
-set debug_sync= 'alter_table_online_downgraded signal downgraded wait_for goforit';
---send
-alter table t add pk int auto_increment primary key, algorithm=copy, lock=none;
---connection con2
-set debug_sync= 'now wait_for downgraded';
-delete from t where a = 20;
-update t set a = a + 1 where a = 10;
-set debug_sync= 'now signal goforit';
-
---connection default
---reap
-select * from t;
+# UB
+#
+# create table t (a int);
+# insert into t values (10),(20),(30);
+#
+# set debug_sync= 'alter_table_online_downgraded signal downgraded wait_for goforit';
+# --send
+# alter table t add pk int auto_increment primary key, algorithm=copy, lock=none;
+# --connection con2
+# set debug_sync= 'now wait_for downgraded';
+# delete from t where a = 20;
+# update t set a = a + 1 where a = 10;
+# set debug_sync= 'now signal goforit';
+#
+# --connection default
+# --reap
+# select * from t;
 
 --echo #
 --echo # Add clumsy DEFAULT

sql/sql_table.cc

@@ -9953,6 +9953,33 @@ const char *online_alter_check_supported(const THD *thd,
     }
   }
 
+  for (auto &c: alter_info->create_list)
+  {
+    *online= c.field || !(c.flags & AUTO_INCREMENT_FLAG);
+    if (!*online)
+      return "ADD COLUMN ... AUTO_INCREMENT";
+
+    auto *def= c.default_value;
+    *online= !(def && def->flags & VCOL_NEXTVAL
+            // either it's a new field, or a NULL -> NOT NULL change
+             && (!c.field || (!(c.field->flags & NOT_NULL_FLAG)
+                              && (c.flags & NOT_NULL_FLAG))));
+    if (!*online)
+    {
+      if (alter_info->requested_lock != Alter_info::ALTER_TABLE_LOCK_NONE)
+        return NULL; // Avoid heavy string op
+      const char *fmt= ER_THD(thd, ER_GENERATED_COLUMN_FUNCTION_IS_NOT_ALLOWED);
+
+      LEX_CSTRING dflt{STRING_WITH_LEN("DEFAULT")};
+      LEX_CSTRING nxvl{STRING_WITH_LEN("NEXTVAL()")};
+      size_t len= strlen(fmt) + nxvl.length + c.field_name.length + dflt.length;
+      char *resp= (char*)thd->alloc(len);
+      // expression %s cannot be used in the %s clause of %`s
+      my_snprintf(resp, len, fmt, nxvl.str, dflt.str, c.field_name.str);
+      return resp;
+    }
+  }
+
   *online= online_alter_check_autoinc(thd, alter_info, table);
   if (!*online)
     return "CHANGE COLUMN ... AUTO_INCREMENT";