MySQLのエラーでDjangoアプリがローカルで実行できなくなったときの話

久しぶりにDjangoのアプリケーションを立ち上げようと以下のコマンドを打ったところ
python.manage.py runserver
こんなエラーが。前は動いてたはずなのに何故
django.db.utils.OperationalError: (2003, "Can't connect to MySQL server on '127.0.0.1' ([Errno 111] Connection refused)")

MySQLの起動確認

$ mysql.server start
Starting MySQL
.... * The server quit without updating PID file (/home/linuxbrew/.linuxbrew/var/mysql/***.pid).

PIDファイルとは，Linux上であるサービスが起動したときに作成されるファイルで，プロセス番号が割り振られているらしい．サービスが動いているかどうかの判断基準になる．
***は端末名

空のPIDファイルを作ってみる

$ touch /home/linuxbrew/.linuxbrew/var/mysql/***.pid

$ mysql.server restart
 * MySQL server PID file could not be found!
Starting MySQL
.rm: cannot remove '/tmp/mysql.sock': Operation not permitted
2020-03-20T18:09:18.611443Z mysqld_safe Fatal error: Can't remove the socket file:
/tmp/mysql.sock.
Please remove the file manually and start /home/linuxbrew/.linuxbrew/Cellar/mysql/8.0.18_1/bin/mysqld_safe again;
mysqld daemon not started
 * The server quit without updating PID file (/home/linuxbrew/.linuxbrew/var/mysql/***.pid).

/tmp/mysql.sock削除＆再チャレンジ

$ sudo rm /tmp/mysql.sock
$ touch /home/linuxbrew/.linuxbrew/var/mysql/***.pid
$ mysql.server restart
 * MySQL server PID file could not be found!
Starting MySQL
... *
$ mysql -u root
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 8
Server version: 8.0.18 Homebrew

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通った！