MDEV-29299 SELECT from table with vcol index reports warning

As of now innodb does not store trx_id for each record in secondary index.
The idea behind is following: let us store only per-page max_trx_id, and
delete-mark the records when they are deleted/updated.

If the read starts, it rememders the lowest id of currently active
transaction. Innodb refers to it as trx->read_view->m_up_limit_id.
See also ReadView::open.

When the page is fetched, its max_trx_id is compared to m_up_limit_id.
If the value is lower, then this page is just safe to read as is.
Else, a clustered index could be needed ato access. See page_get_max_trx_id
call in row_search_mvcc, and the correctonding
switch (row_search_idx_cond_check(...)) below.

Virtual columns are required to be updated in case if the record was
delete-marked. The motivation behind it is documented in
Row_sel_get_clust_rec_for_mysql::operator() near
row_sel_sec_rec_is_for_clust_rec call.

This was basically a description why virtual column computation can
normally happen during SELECT, and, generally, a vcol index access.

Sometimes stats tables are updated by innodb. This starts a new
transaction, and it can happen that it didn't finish to the moment of
SELECT execution, forcing virtual columns recomputation. If the result was
a something that normally outputs a warning, like division by zero, then
it could be outputted in a racy manner.

Therefore, the solution is just to suppressthe warnings if the access was
read-only.

4 files changed, 77 insertions, 0 deletions

diff --git a/mysql-test/suite/gcol/r/innodb_virtual_index.result b/mysql-test/suite/gcol/r/innodb_virtual_index.result
index 34a0b0c66e8..d7240db8fa4 100644
--- a/mysql-test/suite/gcol/r/innodb_virtual_index.result
+++ b/mysql-test/suite/gcol/r/innodb_virtual_index.result
@@ -309,4 +309,32 @@ ALTER TABLE t1 MODIFY a VARCHAR(2600), ALGORITHM=INPLACE;
 ALTER TABLE t1 ADD KEY (b), ALGORITHM=INPLACE;
 # Cleanup
 DROP TABLE t1;
+# MDEV-29299 SELECT from table with vcol index reports warning
+connect  con2, localhost, root,,;
+connection default;
+set default_storage_engine= innodb;
+CREATE TABLE t(fld1 INT NOT NULL,
+fld2 INT AS (100/fld1) VIRTUAL,
+KEY(fld1), KEY(fld2));
+CREATE TABLE t_odd(id int);
+INSERT INTO t(fld1) VALUES(1), (2);
+connection con2;
+begin;
+INSERT INTO t_odd VALUES(10000);
+connection default;
+UPDATE IGNORE t SET fld1= 0 WHERE fld1= 2;
+Warnings:
+Warning	1365	Division by 0
+SELECT fld2 FROM t FORCE INDEX(fld2);
+fld2
+NULL
+100
+SELECT fld2 FROM t FORCE INDEX(fld1);
+fld2
+100
+NULL
+Warnings:
+Warning	1365	Division by 0
+disconnect con2;
+DROP TABLE t, t_odd;
 # End of 10.2 tests
diff --git a/mysql-test/suite/gcol/t/innodb_virtual_index.test b/mysql-test/suite/gcol/t/innodb_virtual_index.test
index 4a41623264e..6ff22bd9ff6 100644
--- a/mysql-test/suite/gcol/t/innodb_virtual_index.test
+++ b/mysql-test/suite/gcol/t/innodb_virtual_index.test
@@ -339,5 +339,28 @@ ALTER TABLE t1 ADD KEY (b), ALGORITHM=INPLACE;
 --echo # Cleanup
 DROP TABLE t1;
 
+--echo # MDEV-29299 SELECT from table with vcol index reports warning
+
+--connect (con2, localhost, root,,)
+--connection default
+set default_storage_engine= innodb;
+
+CREATE TABLE t(fld1 INT NOT NULL,
+               fld2 INT AS (100/fld1) VIRTUAL,
+               KEY(fld1), KEY(fld2));
+CREATE TABLE t_odd(id int);
+INSERT INTO t(fld1) VALUES(1), (2);
+--connection con2
+begin;
+INSERT INTO t_odd VALUES(10000);
+--connection default
+UPDATE IGNORE t SET fld1= 0 WHERE fld1= 2;
+SELECT fld2 FROM t FORCE INDEX(fld2);
+SELECT fld2 FROM t FORCE INDEX(fld1);
+
+--disconnect con2
+DROP TABLE t, t_odd;
+
+
 --echo # End of 10.2 tests
 
diff --git a/sql/sql_class.h b/sql/sql_class.h
index 2fd9bff01e2..8c290502205 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -2038,6 +2038,22 @@ public:
 };
 
 
+struct Suppress_warnings_error_handler : public Internal_error_handler
+{
+public:
+  bool handle_condition(THD *thd,
+                        uint sql_errno,
+                        const char *sqlstate,
+                        Sql_condition::enum_warning_level *level,
+                        const char *msg,
+                        Sql_condition **cond_hdl) override
+  {
+    return *level == Sql_condition::WARN_LEVEL_WARN;
+  }
+};
+
+
+
 /**
   Tables that were locked with LOCK TABLES statement.
 
diff --git a/sql/table.cc b/sql/table.cc
index 75ad563de69..b36bf07be8c 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -8923,7 +8923,14 @@ int TABLE::update_virtual_field(Field *vf)
   DBUG_ENTER("TABLE::update_virtual_field");
   Query_arena backup_arena;
   Counting_error_handler count_errors;
+  Suppress_warnings_error_handler warnings_handler;
+  bool ignore_warnings= pos_in_table_list
+                        && pos_in_table_list->lock_type < TL_FIRST_WRITE;
+
   in_use->push_internal_handler(&count_errors);
+  if (ignore_warnings)
+    in_use->push_internal_handler(&warnings_handler);
+
   /*
     TODO: this may impose memory leak until table flush.
           See comment in
@@ -8936,6 +8943,9 @@ int TABLE::update_virtual_field(Field *vf)
   vf->vcol_info->expr->save_in_field(vf, 0);
   DBUG_RESTORE_WRITE_SET(vf);
   in_use->restore_active_arena(expr_arena, &backup_arena);
+
+  if (ignore_warnings)
+    in_use->pop_internal_handler();
   in_use->pop_internal_handler();
   DBUG_RETURN(count_errors.errors);
 }