MDEV-20154 Assertion len <= col->len | ... failed in row_merge_buf_add

len was containing garbage, since vctempl->mysql_col_offset was
containing old value while calling row_mysql_store_col_in_innobase_format
from innobase_get_computed_value().

It was not updated after the first ALTER TABLE call, because it's INPLACE
logic considered there's nothing to update, and exited immediately from
ha_innobase::inplace_alter_table().

However, vcol metadata needs an update, since vcols structure is changed
in mysql record.

The regression was introduced by 12614af. There, refcount==1 condition
was removed, which turned out to be crucial, though racy. The idea was to
update vc_templ after each (sequencing) ALTER TABLE.

We should do the same another way, and there may be a plenty of solutions,
but the simplest one is to add a following condition:
  if vcol structure is changed, drop vc_templ; it will be recreated on next
  ha_innobase::open() call.

in prepare_inplace_alter_table. It is safe, since innodb inplace changes
require at least HA_ALTER_INPLACE_SHARED_LOCK_AFTER_PREPARE, which
guarantee MDL_EXCLUSIVE on this stage.

alter_templ_needs_rebuild() also has to track the columns not indexed, to
keep vc_templ correct.

Note that vc_templ is always kept constructed and available after
ha_innobase::open() call, even on INSERT, though no virtual columns are
evaluated during that statement
inside innodb.

In the test case suplied, it will be recreated on the second ALTER TABLE.

mysql-test/suite/gcol/r/innodb_virtual_index.result

@@ -300,3 +300,17 @@ Table	Op	Msg_type	Msg_text
 test.t1	optimize	note	Table does not support optimize, doing recreate + analyze instead
 test.t1	optimize	status	OK
 DROP TABLE t1;
+#
+# MDEV-20154 Assertion `len <= col->len || ((col->mtype) == 5
+# || (col->mtype) == 14)' failed in row_merge_buf_add
+#
+CREATE TABLE t1 (
+a VARCHAR(2500),
+b VARCHAR(2499) AS (a) VIRTUAL
+) ENGINE=InnoDB;
+INSERT INTO t1 (a) VALUES ('foo');
+ALTER TABLE t1 MODIFY a VARCHAR(2600), ALGORITHM=INPLACE;
+ALTER TABLE t1 ADD KEY (b), ALGORITHM=INPLACE;
+# Cleanup
+DROP TABLE t1;
+# End of 10.2 tests

mysql-test/suite/gcol/t/innodb_virtual_index.test

@@ -314,3 +314,23 @@ CREATE TABLE t1 (id INT PRIMARY KEY, a CHAR(3),
 INSERT INTO t1 (id,a) VALUES (1,'foo');
 OPTIMIZE TABLE t1;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-20154 Assertion `len <= col->len || ((col->mtype) == 5
+--echo # || (col->mtype) == 14)' failed in row_merge_buf_add
+--echo #
+
+CREATE TABLE t1 (
+    a VARCHAR(2500),
+    b VARCHAR(2499) AS (a) VIRTUAL
+) ENGINE=InnoDB;
+INSERT INTO t1 (a) VALUES ('foo');
+
+ALTER TABLE t1 MODIFY a VARCHAR(2600), ALGORITHM=INPLACE;
+ALTER TABLE t1 ADD KEY (b), ALGORITHM=INPLACE;
+
+--echo # Cleanup
+DROP TABLE t1;
+
+--echo # End of 10.2 tests
+

storage/innobase/handler/handler0alter.cc

@@ -5355,6 +5355,10 @@ alter_fill_stored_column(
 	}
 }
 
+static bool alter_templ_needs_rebuild(const TABLE* altered_table,
+                                      const Alter_inplace_info* ha_alter_info,
+                                      const dict_table_t* table);
+
 
 /** Allows InnoDB to update internal structures with concurrent
 writes blocked (provided that check_if_supported_inplace_alter()
@@ -5951,9 +5955,9 @@ ha_innobase::prepare_inplace_alter_table(
 		== Alter_inplace_info::CHANGE_CREATE_OPTION
 		&& !innobase_need_rebuild(ha_alter_info, table))) {
 
+		ha_innobase_inplace_ctx *ctx = NULL;
 		if (heap) {
-			ha_alter_info->handler_ctx
-				= new ha_innobase_inplace_ctx(
+			ctx = new ha_innobase_inplace_ctx(
 					m_prebuilt,
 					drop_index, n_drop_index,
 					rename_index, n_rename_index,
@@ -5962,6 +5966,7 @@ ha_innobase::prepare_inplace_alter_table(
 					ha_alter_info->online,
 					heap, indexed_table,
 					col_names, ULINT_UNDEFINED, 0, 0, 0);
+			ha_alter_info->handler_ctx = ctx;
 		}
 
 		DBUG_ASSERT(m_prebuilt->trx->dict_operation_lock_mode == 0);
@@ -5981,6 +5986,24 @@ ha_innobase::prepare_inplace_alter_table(
 			DBUG_RETURN(true);
 		}
 
+		if (!(ha_alter_info->handler_flags & INNOBASE_ALTER_DATA)
+		    && alter_templ_needs_rebuild(altered_table, ha_alter_info,
+						 ctx->new_table)
+		    && ctx->new_table->n_v_cols > 0) {
+			/* Changing maria record structure may end up here only
+			if virtual columns were altered. In this case, however,
+			vc_templ should be rebuilt. Since we don't actually
+			change any stored data, we can just dispose vc_templ;
+			it will be recreated on next ha_innobase::open(). */
+
+			DBUG_ASSERT(ctx->new_table == ctx->old_table);
+
+			dict_free_vc_templ(ctx->new_table->vc_templ);
+			UT_DELETE(ctx->new_table->vc_templ);
+
+			ctx->new_table->vc_templ = NULL;
+		}
+
 		DBUG_RETURN(false);
 	}
 
@@ -6098,35 +6121,6 @@ ha_innobase::prepare_inplace_alter_table(
 			    add_fts_doc_id_idx));
 }
 
-/** Check that the column is part of a virtual index(index contains
-virtual column) in the table
-@param[in]	table		Table containing column
-@param[in]	col		column to be checked
-@return true if this column is indexed with other virtual columns */
-static
-bool
-dict_col_in_v_indexes(
-	dict_table_t*	table,
-	dict_col_t*	col)
-{
-	for (dict_index_t* index = dict_table_get_next_index(
-		dict_table_get_first_index(table)); index != NULL;
-		index = dict_table_get_next_index(index)) {
-		if (!dict_index_has_virtual(index)) {
-			continue;
-		}
-		for (ulint k = 0; k < index->n_fields; k++) {
-			dict_field_t*   field
-				= dict_index_get_nth_field(index, k);
-			if (field->col->ind == col->ind) {
-				return(true);
-			}
-		}
-	}
-
-	return(false);
-}
-
 /* Check whether a columnn length change alter operation requires
 to rebuild the template.
 @param[in]	altered_table	TABLE object for new version of table.
@@ -6138,9 +6132,9 @@ to rebuild the template.
 static
 bool
 alter_templ_needs_rebuild(
-	TABLE*                  altered_table,
-	Alter_inplace_info*     ha_alter_info,
-	dict_table_t*		table)
+	const TABLE*            altered_table,
+	const Alter_inplace_info*     ha_alter_info,
+	const dict_table_t*		table)
 {
         ulint	i = 0;
         List_iterator_fast<Create_field>  cf_it(
@@ -6152,8 +6146,7 @@ alter_templ_needs_rebuild(
 			for (ulint j=0; j < table->n_cols; j++) {
 				dict_col_t* cols
                                    = dict_table_get_nth_col(table, j);
-				if (cf->length > cols->len
-				    && dict_col_in_v_indexes(table, cols)) {
+				if (cf->length > cols->len) {
 					return(true);
 				}
 			}