Debian – Repairing a failed Debian upgrade

First of all, despite the last line start with E: (which indicates an error), apt didn't fail entirely; it's downloaded most of the updated package lists, it only skipped those for Opera and the Google Talk plugin. So apt upgrade should still offer to upgrade all the other packages.

The warnings give you some indication of what went wrong:

W: gpgv:/var/lib/apt/lists/dl.google.com_linux_talkplugin_deb_dists_stable_Release.gpg: The repository is insufficiently signed by key 4CCA1EAF950CEE4AB83976DCA040830F7FAC5991 (weak digest)
W: gpgv:/var/lib/apt/lists/deb.opera.com_opera_dists_stable_InRelease: The repository is insufficiently signed by key 419D0ACF314E8E993F7F92E563F7D4AFF6D61D45 (weak digest)
W: Failed to fetch http://dl.google.com/linux/talkplugin/deb/dists/stable/Release  No Hash entry in Release file /var/lib/apt/lists/dl.google.com_linux_talkplugin_deb_dists_stable_Release, which is considered strong enough for security purposes

The first two mean that the repository descriptors were signed with an old digest algorithm, which apt now complains about. The third is caused by apt's recent switch to SHA-256 or SHA-512 hashes only; the Talk plugin repository only provides MD5 and SHA-1 hashes, which are now ignored by apt.

To fix this, you can either remove the repositories for the time being, or wait for Opera and Google to fix them...

In the tutorial you should install mariadb-server in place of mysql-server. I've got the same problem on DEBIAN 9 install and these command worked for me :

sudo apt purge mysql*
sudo apt purge mariadb*
sudo apt autoremove
sudo apt autoclean

Check if still something in the system :

dpkg -l | grep -e mysql -e mariadb 

if the list is empty then install mariadb server :

sudo apt install mariadb-server

Now try to check status on stop and start the database :

Status after install :

sudo systemctl status mysql.service
● mariadb.service - MariaDB database server
   Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-03-21 16:08:49 CET; 2min 1s ago
 Main PID: 15699 (mysqld)
   Status: "Taking your SQL requests now..."
   CGroup: /system.slice/mariadb.service
           └─15699 /usr/sbin/mysqld

.../..

Check the PID :

sudo more /var/run/mysqld/mysqld.pid 
15699

Stop the database :

sudo systemctl stop mysql.service
sudo systemctl status mysql.service
● mariadb.service - MariaDB database server
   Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Wed 2018-03-21 16:14:35 CET; 1s ago
 Main PID: 15699 (code=exited, status=0/SUCCESS)
   Status: "MariaDB server is down"

.../...

You could check the PID file is not present as well

sudo more /var/run/mysqld/mysqld.pid 
more: stat on /var/run/mysqld/mysqld.pid: No file or folder

Start the database :

sudo systemctl start mysql.service
sudo systemctl status mysql.service
● mariadb.service - MariaDB database server
   Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-03-21 16:14:54 CET; 3s ago
  Process: 15911 ExecStartPost=/bin/sh -c systemctl unset-environment _WSREP_START_POSITION (code=exited, status=0/SUCCESS)
  Process: 15908 ExecStartPost=/etc/mysql/debian-start (code=exited, status=0/SUCCESS)
  Process: 15791 ExecStartPre=/bin/sh -c [ ! -e /usr/bin/galera_recovery ] && VAR= ||   VAR=`/usr/bin/galera_recovery`; [ $? -eq 0 ]   && systemctl set-environment _WSREP_S
  Process: 15787 ExecStartPre=/bin/sh -c systemctl unset-environment _WSREP_START_POSITION (code=exited, status=0/SUCCESS)
  Process: 15784 ExecStartPre=/usr/bin/install -m 755 -o mysql -g root -d /var/run/mysqld (code=exited, status=0/SUCCESS)
 Main PID: 15881 (mysqld)
   Status: "Taking your SQL requests now..."
    Tasks: 26 (limit: 4915)
   CGroup: /system.slice/mariadb.service
           └─15881 /usr/sbin/mysqld

You could check the PID file it should be present. May help on a empty database. If there is something in the database you should do a backup first.