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[Lena Voytek <1877442@xxxxxxxxxxxxxxxxxx>]

** Changed in: mysql-5.7 (Ubuntu Bionic)
       Status: Triaged => Fix Released

** Changed in: mysql-8.0 (Ubuntu Focal)
       Status: Triaged => Fix Released

** Changed in: mysql-5.7 (Ubuntu)
       Status: Triaged => Invalid

** Changed in: mysql-8.0 (Ubuntu)
       Status: Triaged => Invalid

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https://bugs.launchpad.net/bugs/1877442

Title:
  Broken SQL CONCAT function behaviour in 5.7.30-0ubuntu0.18.04.1

Status in mysql-5.7 package in Ubuntu:
  Invalid
Status in mysql-8.0 package in Ubuntu:
  Invalid
Status in mysql-5.7 source package in Xenial:
  Won't Fix
Status in mysql-8.0 source package in Xenial:
  Invalid
Status in mysql-5.7 source package in Bionic:
  Fix Released
Status in mysql-8.0 source package in Bionic:
  Invalid
Status in mysql-5.7 source package in Eoan:
  Invalid
Status in mysql-8.0 source package in Eoan:
  Won't Fix
Status in mysql-5.7 source package in Focal:
  Invalid
Status in mysql-8.0 source package in Focal:
  Fix Released

Bug description:
  [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]
  - 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. 

  [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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