MDEV-20154 Assertion `len <= col->len | ...` failed in row_merge_buf_addbb-10.6-MDEV-20154

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.

There may be a plenty of solutions, but the simplest one is:
if vcol structure is changed, drop vc_templ; it will be recreated on next
`open()` call.

3 files changed, 57 insertions, 8 deletions

diff --git a/mysql-test/suite/gcol/r/innodb_virtual_index.result b/mysql-test/suite/gcol/r/innodb_virtual_index.result
index 2e9b762500d..8f8e7946b87 100644
--- a/mysql-test/suite/gcol/r/innodb_virtual_index.result
+++ b/mysql-test/suite/gcol/r/innodb_virtual_index.result
@@ -296,3 +296,16 @@ 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;
diff --git a/mysql-test/suite/gcol/t/innodb_virtual_index.test b/mysql-test/suite/gcol/t/innodb_virtual_index.test
index bb47379f3b2..f7796bd8a86 100644
--- a/mysql-test/suite/gcol/t/innodb_virtual_index.test
+++ b/mysql-test/suite/gcol/t/innodb_virtual_index.test
@@ -321,3 +321,20 @@ 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;
diff --git a/storage/innobase/handler/handler0alter.cc b/storage/innobase/handler/handler0alter.cc
index 62f9d103aa1..9b9d986cf7a 100644
--- a/storage/innobase/handler/handler0alter.cc
+++ b/storage/innobase/handler/handler0alter.cc
@@ -8301,8 +8301,7 @@ 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);
 				}
 			}
@@ -8363,7 +8362,32 @@ ha_innobase::inplace_alter_table(
 
 	DEBUG_SYNC(m_user_thd, "innodb_inplace_alter_table_enter");
 
+	ha_innobase_inplace_ctx* ctx =static_cast<ha_innobase_inplace_ctx*>
+					(ha_alter_info->handler_ctx);
+
+	bool field_len_changed= ctx ?
+				alter_templ_needs_rebuild(altered_table,
+							  ha_alter_info,
+							  ctx->new_table) :
+				0;
+
 	if (!(ha_alter_info->handler_flags & INNOBASE_ALTER_DATA)) {
+		if (field_len_changed && ctx->new_table->n_v_cols > 0) {
+			/* Changing mysql record structure may end up here if
+			only virtual fields 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(). */
+
+			dict_free_vc_templ(ctx->new_table->vc_templ);
+			UT_DELETE(ctx->new_table->vc_templ);
+
+			DBUG_ASSERT(ctx->new_table->vc_templ ==
+					ctx->old_table->vc_templ);
+			ctx->new_table->vc_templ = NULL;
+			ctx->old_table->vc_templ = NULL;
+		}
 ok_exit:
 		DEBUG_SYNC(m_user_thd, "innodb_after_inplace_alter_table");
 		DBUG_RETURN(false);
@@ -8377,10 +8401,6 @@ ok_exit:
 		goto ok_exit;
 	}
 
-	ha_innobase_inplace_ctx*	ctx
-		= static_cast<ha_innobase_inplace_ctx*>
-		(ha_alter_info->handler_ctx);
-
 	DBUG_ASSERT(ctx);
 	DBUG_ASSERT(ctx->trx);
 	DBUG_ASSERT(ctx->prebuilt == m_prebuilt);
@@ -8412,8 +8432,7 @@ ok_exit:
 	     = ctx->need_rebuild()
 	       || ((ha_alter_info->handler_flags
 		& ALTER_COLUMN_TYPE_CHANGE_BY_ENGINE)
-		&& alter_templ_needs_rebuild(
-		   altered_table, ha_alter_info, ctx->new_table));
+		&& field_len_changed);
 
 	if ((ctx->new_table->n_v_cols > 0) && rebuild_templ) {
 		/* Save the templ if isn't NULL so as to restore the