MDEV-12936 upgrade to 10.2.6 failed upon tables with virtual columns

when opening 10.1- table that has virtual columns:

1. don't error out if it has vcols over autoinc columns.
   just issue a warning.
2. set vcol type properly
3. in innodb: use table->s->stored_fields instead of table->s->fields,
   because that's what was stored in innodb data dictionary

mysql-test/suite/vcol/r/upgrade.result

+check table vcol_autoinc for upgrade;
+Table	Op	Msg_type	Msg_text
+test.vcol_autoinc	check	Warning	Function or expression 'AUTO_INCREMENT' cannot be used in the GENERATED ALWAYS AS clause of `pk`
+test.vcol_autoinc	check	status	OK
+show create table vcol_autoinc;
+Table	Create Table
+vcol_autoinc	CREATE TABLE `vcol_autoinc` (
+  `pk` int(11) NOT NULL AUTO_INCREMENT,
+  `v3` int(11) GENERATED ALWAYS AS (`pk`) VIRTUAL,
+  PRIMARY KEY (`pk`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select * from vcol_autoinc;
+pk	v3
+insert vcol_autoinc (pk) values (1);
+select * from vcol_autoinc;
+pk	v3
+1	1
+drop table vcol_autoinc;

mysql-test/suite/vcol/t/upgrade.test

+#
+# MDEV-12936 upgrade to 10.2.6 failed upon tables with virtual columns
+#
+let $datadir=`select @@datadir`;
+copy_file std_data/vcol_autoinc.frm $datadir/test/vcol_autoinc.frm;
+copy_file std_data/vcol_autoinc.MYD $datadir/test/vcol_autoinc.MYD;
+copy_file std_data/vcol_autoinc.MYI $datadir/test/vcol_autoinc.MYI;
+check table vcol_autoinc for upgrade;
+show create table vcol_autoinc;
+select * from vcol_autoinc;
+insert vcol_autoinc (pk) values (1);
+select * from vcol_autoinc;
+drop table vcol_autoinc;

sql/table.cc

@@ -1860,7 +1860,9 @@ int TABLE_SHARE::init_from_binary_frm_image(THD *thd, bool write,
           interval_nr= (uint)vcol_screen_pos[3];
         else if ((uint)vcol_screen_pos[0] != 1)
           goto err;
-        vcol_info->stored_in_db= vcol_screen_pos[2] & 1;
+        bool stored= vcol_screen_pos[2] & 1;
+        vcol_info->stored_in_db= stored;
+        vcol_info->set_vcol_type(stored ? VCOL_GENERATED_STORED : VCOL_GENERATED_VIRTUAL);
         vcol_expr_length= vcol_info_length -
                           (uint)(FRM_VCOL_OLD_HEADER_SIZE(opt_interval_id));
         vcol_info->utf8= 0; // before 10.2.1 the charset was unknown
@@ -2862,8 +2864,10 @@ static bool fix_and_check_vcol_expr(THD *thd, TABLE *table,
       of the statement because the field item does not have a field
       pointer at that time
     */
-    my_error(ER_VIRTUAL_COLUMN_FUNCTION_IS_NOT_ALLOWED, MYF(0),
+    myf warn= table->s->frm_version < FRM_VER_EXPRESSSIONS ? ME_JUST_WARNING : 0;
+    my_error(ER_VIRTUAL_COLUMN_FUNCTION_IS_NOT_ALLOWED, MYF(warn),
              "AUTO_INCREMENT", vcol->get_vcol_type_name(), res.name);
+    if (!warn)
       DBUG_RETURN(1);
   }
   vcol->flags= res.errors;

storage/innobase/handler/ha_innodb.cc

@@ -291,6 +291,11 @@ void set_my_errno(int err)
 	errno = err;
 }
 
+static uint mysql_fields(const TABLE *table)
+{
+	return table->s->frm_version < FRM_VER_EXPRESSSIONS
+		? table->s->stored_fields : table->s->fields;
+}
 
 /** Checks whether the file name belongs to a partition of a table.
 @param[in]	file_name	file name
@@ -6495,16 +6500,17 @@ ha_innobase::open(const char* name, int, uint)
 
 	ib_table = open_dict_table(name, norm_name, is_part, ignore_err);
 
+	uint n_fields = mysql_fields(table);
+
 	if (ib_table != NULL
 	    && ((!DICT_TF2_FLAG_IS_SET(ib_table, DICT_TF2_FTS_HAS_DOC_ID)
-		 && table->s->fields != dict_table_get_n_tot_u_cols(ib_table))
+		 && n_fields != dict_table_get_n_tot_u_cols(ib_table))
 		|| (DICT_TF2_FLAG_IS_SET(ib_table, DICT_TF2_FTS_HAS_DOC_ID)
-		    && (table->s->fields
-			!= dict_table_get_n_tot_u_cols(ib_table) - 1)))) {
+		    && (n_fields != dict_table_get_n_tot_u_cols(ib_table) - 1)))) {
 
 		ib::warn() << "Table " << norm_name << " contains "
-			<< dict_table_get_n_user_cols(ib_table) << " user"
-			" defined columns in InnoDB, but " << table->s->fields
+			<< dict_table_get_n_tot_u_cols(ib_table) << " user"
+			" defined columns in InnoDB, but " << n_fields
 			<< " columns in MariaDB. Please check"
 			" INFORMATION_SCHEMA.INNODB_SYS_COLUMNS and " REFMAN
 			"innodb-troubleshooting.html for how to resolve the"
@@ -8033,7 +8039,7 @@ ha_innobase::build_template(
 	/* Below we check column by column if we need to access
 	the clustered index. */
 
-	n_fields = (ulint) table->s->fields; /* number of columns */
+	n_fields = (ulint) mysql_fields(table);
 
 	if (!m_prebuilt->mysql_template) {
 		m_prebuilt->mysql_template = (mysql_row_templ_t*)
@@ -8894,6 +8900,7 @@ calc_row_difference(
 	trx_t* const	trx = prebuilt->trx;
 	doc_id_t	doc_id = FTS_NULL_DOC_ID;
 	ulint		num_v = 0;
+	uint		n_fields = mysql_fields(table);
 
 	ut_ad(!srv_read_only_mode);
 
@@ -8903,7 +8910,7 @@ calc_row_difference(
 	/* We use upd_buff to convert changed fields */
 	buf = (byte*) upd_buff;
 
-	for (i = 0; i < table->s->fields; i++) {
+	for (i = 0; i < n_fields; i++) {
 		field = table->field[i];
 		bool		is_virtual = innobase_is_v_fld(field);
 		dict_col_t*	col;
@@ -9285,7 +9292,7 @@ wsrep_calc_row_hash(
 	void *ctx = alloca(my_md5_context_size());
 	my_md5_init(ctx);
 
-	n_fields = table->s->fields;
+	n_fields = mysql_fields(table);
 
 	for (i = 0; i < n_fields; i++) {
 		byte null_byte=0;
@@ -11352,7 +11359,9 @@ create_table_check_doc_id_col(
 					ULINT_UNDEFINED if column is of the
 					wrong type/name/size */
 {
-	for (ulint i = 0; i < form->s->fields; i++) {
+	uint		n_fields = mysql_fields(form);
+
+	for (ulint i = 0; i < n_fields; i++) {
 		const Field*	field;
 		ulint		col_type;
 		ulint		col_len;