[Bug 1889472] [NEW] mysql-server-5.7 postinst fails when in read-only mode

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You have been subscribed to a public bug by Simon Déziel (sdeziel):

[Impact]

Updates of the mysql-server-5.7 package fail to install (error during postinst) if operating in (super) read-only mode.
A read-only replica is common in redundant/HA setups.


[Test Case]

1) Setup a container (bionic or xenial would do)
$ lxc launch images:ubuntu/bionic sql1
$ lxc shell sql1
# apt-get update && apt-get install -y --no-install-recommends mysql-server

2) Configure read-only mode
# cat << EOF >> /etc/mysql/my.cnf

[mysqld]
super_read_only = ON
read_only = ON
EOF
# service mysql restart
# mysql -e "SELECT @@global.read_only, @@global.super_read_only;"
+--------------------+--------------------------+
| @@global.read_only | @@global.super_read_only |
+--------------------+--------------------------+
| 1 | 1 |
+--------------------+--------------------------+

3) Trigger the postinst code
# apt-get install --reinstall mysql-server-5.7
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
Need to get 0 B/2,931 kB of archives.
After this operation, 0 B of additional disk space will be used.
Preconfiguring packages ...
(Reading database ... 16169 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.7_5.7.31-0ubuntu0.18.04.1_amd64.deb ...
Unpacking mysql-server-5.7 (5.7.31-0ubuntu0.18.04.1) over (5.7.31-0ubuntu0.18.04.1) ...
Setting up mysql-server-5.7 (5.7.31-0ubuntu0.18.04.1) ...
Checking if update is needed.
Checking server version.
Running queries to upgrade MySQL server.
mysql_upgrade: [ERROR] 1290: The MySQL server is running with the --super-read-only option so it cannot execute this statement
mysql_upgrade failed with exit status 5
dpkg: error processing package mysql-server-5.7 (--configure):
 installed mysql-server-5.7 package post-installation script subprocess returned error exit status 1
Processing triggers for systemd (237-3ubuntu10.41) ...
Errors were encountered while processing:
 mysql-server-5.7
E: Sub-process /usr/bin/dpkg returned an error code (1)

Step 3) should not cause a dpkg error.


[Regression Potential]

This patch runs a MySQL query to check if the service is in (super) read-only mode before calling mysql_upgrade. The SQL query might
fail thus preventing the proper detection of the read-only mode. The patch assumes read-write mode by default so that it would still
call mysql_upgrade. Fortunately, mysql_upgrade is known to safely error out when in read-only as this is what the bug is about.

Another possibility is to wrongly detect read-only mode which would skip running mysql_upgrade when it should have been. To mitigate
this, the patch informs the user ("mysql_upgrade skipped due to (super) read-only mode") who could then run mysql_upgrade manually.

** Affects: mysql-5.7 (Ubuntu)
     Importance: Undecided
         Status: Invalid

** Affects: mysql-5.7 (Ubuntu Xenial)
     Importance: Low
         Status: Triaged

** Affects: mysql-5.7 (Ubuntu Bionic)
     Importance: Low
         Status: Triaged


** Tags: patch
-- 
mysql-server-5.7 postinst fails when in read-only mode
https://bugs.launchpad.net/bugs/1889472
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