MDEV-23547 InnoDB: Failing assertion: *len in row_upd_ext_fetch

This bug was originally repeated on 10.4 after defining a UNIQUE KEY
on a TEXT column, which is implemented by MDEV-371 by creating the
index on a hidden virtual column.

While row_vers_vc_matches_cluster() is executing in a purge thread
to find out if an index entry may be removed in a secondary index
that comprises a virtual column, another purge thread may process
the undo log record that this check is interested in, and write
a null BLOB pointer in that record. This would trip the assertion.

To prevent this from occurring, we must propagate the 'missing BLOB'
error up the call stack.

row_upd_ext_fetch(): Return NULL when the error occurs.

row_upd_index_replace_new_col_val(): Return whether the previous
version was built successfully.

row_upd_index_replace_new_col_vals_index_pos(): Check the error
result. Yes, we would intentionally crash on this error if it
occurs outside the purge thread.

row_upd_index_replace_new_col_vals(): Check for the error condition,
and simplify the logic.

trx_undo_prev_version_build(): Check for the error condition.

1 files changed, 5 insertions, 1 deletions

diff --git a/storage/innobase/trx/trx0rec.cc b/storage/innobase/trx/trx0rec.cc
index ee022b4f1fd..623d8990381 100644
--- a/storage/innobase/trx/trx0rec.cc
+++ b/storage/innobase/trx/trx0rec.cc
@@ -2446,7 +2446,11 @@ trx_undo_prev_version_build(
 		/* The page containing the clustered index record
 		corresponding to entry is latched in mtr.  Thus the
 		following call is safe. */
-		row_upd_index_replace_new_col_vals(entry, index, update, heap);
+		if (!row_upd_index_replace_new_col_vals(entry, *index, update,
+							heap)) {
+			ut_a(v_status & TRX_UNDO_PREV_IN_PURGE);
+			return false;
+		}
 
 		/* Get number of externally stored columns in updated record */
 		const ulint n_ext = dtuple_get_n_ext(entry);