** Description changed:
+ [Impact]
+ This is a regression in MySQL 5.7.30 (does not affect 8.0 which is in focal
and development release, nor 5.7.29). The CONCAT function has inconsistent
behavior depending on whether or not the result is assigned to a variable,
which is leading to a crash in slurmdbd.
+
+ [Test Case]
+ Run the following two queries:
+
+ - SELECT CONCAT('');
+ - SET @var = ""; SELECT @var := CONCAT('');
+ They should give the same output, but the second one returns NULL instead of
an empty string when using 5.7.30.
+
+ [Regression Potential]
+ The patch changes fairly low-level code, which can always carry some risk.
The patch will also need to be removed if next upstream release (5.7.31) is
picked, since it will have its own fix for this issue.
+
+
+ --- ORIGINAL DESCRIPTION ---
Hi,
Yesterday we upgraded from 5.7.29-0ubuntu0.18.04.1 to
5.7.30-0ubuntu0.18.04.1 (via unattended-upgrade) on Ubuntu 18.04 and
slurmdbd (https://packages.ubuntu.com/bionic/admin/slurmdbd) started
segfaulting.
Upon inspection, the slurmdbd crash is caused by a change of behaviour
of the SQL CONCAT function. The new behaviour seems wrong.
Calling
SELECT CONCAT('');
will result in an empty string as expected however if the result is assigned
to a variable with:
SET @var = ""; SELECT @var := CONCAT('');
the variable will be NULL instead of an empty string.
It seems that when the output of CONCAT is assigned to a variable; if
CONCAT should have returned an empty string, it will set the variable to
NULL instead.
SLURM itself crashes because it uses a stored procedure which relies on
the output of CONCAT to be an empty string rather than a NULL variable
and tries to dereference the pointer.
I wasn’t able to get hold of 5.7.29-0ubuntu0.18.04.1 via APT so I tested
on 5.7.21-1ubuntu1. On 5.7.21, the variable was correctly set to an
empty string. I have also tried on 20.04 and the behaviour is correct as
well. So the problem seems to be present only in the version
5.7.30-0ubuntu0.18.04.1.
I think that the behaviour was introduced in the 5.7 branch by this commit:
https://github.com/mysql/mysql-server/commit/addb2aab601d98e685eccae545f79d120561ad9b
but I am not entirely sure.
This bug is a bit annoying because it makes SLURM with accounting
enabled completely unusable. However downgrading MySQL to
5.7.29-0ubuntu0.18.04.1 temporarily fixed the problem.
I have attached a log file with examples to reproduce the bug.
Thank you very much.
** Description changed:
[Impact]
This is a regression in MySQL 5.7.30 (does not affect 8.0 which is in focal
and development release, nor 5.7.29). The CONCAT function has inconsistent
behavior depending on whether or not the result is assigned to a variable,
which is leading to a crash in slurmdbd.
+ Since it's limited to 5.7.30, the bug only affects stable releases
+ Bionic and Xenial.
+
[Test Case]
- Run the following two queries:
-
- - SELECT CONCAT('');
- - SET @var = ""; SELECT @var := CONCAT('');
+ - Start up MySQL Server 5.7.30
+ - Connect to it with the mysql cli (with default install of the server, "sudo
mysql" will connect to the MySQL root user)
+ - Run the following two queries:
+ * SELECT CONCAT('');
+ * SET @var = ""; SELECT @var := CONCAT('');
They should give the same output, but the second one returns NULL instead of
an empty string when using 5.7.30.
[Regression Potential]
- The patch changes fairly low-level code, which can always carry some risk.
The patch will also need to be removed if next upstream release (5.7.31) is
picked, since it will have its own fix for this issue.
+ The patch changes fairly low-level code, which can always carry some risk.
+ [Other info]
+ This bug is fixed in the next upstream release of MySQL (5.7.31), so the
patch would only be needed until that is picked.
--- ORIGINAL DESCRIPTION ---
Hi,
Yesterday we upgraded from 5.7.29-0ubuntu0.18.04.1 to
5.7.30-0ubuntu0.18.04.1 (via unattended-upgrade) on Ubuntu 18.04 and
slurmdbd (https://packages.ubuntu.com/bionic/admin/slurmdbd) started
segfaulting.
Upon inspection, the slurmdbd crash is caused by a change of behaviour
of the SQL CONCAT function. The new behaviour seems wrong.
Calling
SELECT CONCAT('');
will result in an empty string as expected however if the result is assigned
to a variable with:
SET @var = ""; SELECT @var := CONCAT('');
the variable will be NULL instead of an empty string.
It seems that when the output of CONCAT is assigned to a variable; if
CONCAT should have returned an empty string, it will set the variable to
NULL instead.
SLURM itself crashes because it uses a stored procedure which relies on
the output of CONCAT to be an empty string rather than a NULL variable
and tries to dereference the pointer.
I wasn’t able to get hold of 5.7.29-0ubuntu0.18.04.1 via APT so I tested
on 5.7.21-1ubuntu1. On 5.7.21, the variable was correctly set to an
empty string. I have also tried on 20.04 and the behaviour is correct as
well. So the problem seems to be present only in the version
5.7.30-0ubuntu0.18.04.1.
I think that the behaviour was introduced in the 5.7 branch by this commit:
https://github.com/mysql/mysql-server/commit/addb2aab601d98e685eccae545f79d120561ad9b
but I am not entirely sure.
This bug is a bit annoying because it makes SLURM with accounting
enabled completely unusable. However downgrading MySQL to
5.7.29-0ubuntu0.18.04.1 temporarily fixed the problem.
I have attached a log file with examples to reproduce the bug.
Thank you very much.
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https://bugs.launchpad.net/bugs/1877442
Title:
Broken SQL CONCAT function behaviour in 5.7.30-0ubuntu0.18.04.1
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