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MDEV-31808 crash upon altering table with NEXTVAL for default under exclusive lock #5125

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MDEV-31808 crash upon altering table with NEXTVAL for default under exclusive lock #5125

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9 changes: 9 additions & 0 deletions mysql-test/main/alter_table_combinations.result
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Original file line number Diff line number Diff line change
Expand Up @@ -321,4 +321,13 @@ drop table t1;
#
# End of 10.5 tests
#
#
# MDEV-31808 Server crash upon altering table with NEXTVAL for default under exclusive lock
#
CREATE SEQUENCE s ENGINE=Aria;
CREATE TABLE t (a INT DEFAULT(NEXTVAL(s)), b INT);
ALTER TABLE t FORCE, ALGORITHM=COPY, LOCK=EXCLUSIVE;
DROP TABLE t;
DROP SEQUENCE s;
# End of 10.6 tests
set @@default_storage_engine= @save_default_engine;
14 changes: 14 additions & 0 deletions mysql-test/main/alter_table_combinations.test
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Expand Up @@ -260,4 +260,18 @@ drop table t1;
--echo # End of 10.5 tests
--echo #

--echo #
--echo # MDEV-31808 Server crash upon altering table with NEXTVAL for default under exclusive lock
--echo #

CREATE SEQUENCE s ENGINE=Aria;
CREATE TABLE t (a INT DEFAULT(NEXTVAL(s)), b INT);
ALTER TABLE t FORCE, ALGORITHM=COPY, LOCK=EXCLUSIVE;

# Cleanup
DROP TABLE t;
DROP SEQUENCE s;

--echo # End of 10.6 tests

set @@default_storage_engine= @save_default_engine;
6 changes: 5 additions & 1 deletion sql/sql_base.cc
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Expand Up @@ -2085,7 +2085,7 @@ bool open_table(THD *thd, TABLE_LIST *table_list, Open_table_context *ot_ctx)
goto retry_share;
}

if (thd->open_tables && thd->open_tables->s->tdc->flushed)
if (!table_list->sequence && thd->open_tables && thd->open_tables->s->tdc->flushed)
{
/*
If the version changes while we're opening the tables,
Expand Down Expand Up @@ -5006,6 +5006,10 @@ bool open_and_lock_internal_tables(TABLE *table, bool lock_table)
MYSQL_LOCK_USE_MALLOC))
goto err;

/* no existing lock to merge with */
if (save_lock == nullptr)
DBUG_RETURN(0);

if (!(new_lock= mysql_lock_merge(save_lock, thd->lock)))
Comment on lines +5009 to 5013
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@gemini-code-assist gemini-code-assist Bot May 26, 2026

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medium

Instead of returning early with DBUG_RETURN(0) when save_lock is nullptr, we can conditionally merge the locks using a ternary operator. Returning early from open_and_lock_internal_tables is risky because it bypasses any potential cleanup, monitoring, or other logic that might exist or be added to the end of the function.

By using a ternary operator, we can safely handle the case where there is no existing lock to merge, while allowing the function to proceed to its natural exit point.

    /* Merge with the existing lock if there is one, otherwise keep the new lock. */
    new_lock= save_lock ? mysql_lock_merge(save_lock, thd->lock) : thd->lock;
    if (!new_lock)

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@grooverdan grooverdan May 26, 2026

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There's no cleanup required, and an assignment of thd->lock back to itself a bit of an odd logic flow.

{
thd->lock= save_lock;
Expand Down