MySQL 5.7.5からはmysql.sock.lockというのを作るようになった

A server could have its socket file taken over by a second server listening on different TCP/IP port but the same socket file. The socket file also would be unlinked by normal shutdown of the second server. To avoid this, the server now uses a lock file with the same name as the socket file and a .lock suffix. (For example, /tmp/mysql.sock has a lock file of /tmp/mysql.sock.lock.) The lock file contains the process ID of the server process that created the socket file. (Bug #17286856)

MySQL :: MySQL 5.7 Release Notes :: Changes in MySQL 5.7.5 (2014-09-25, Milestone 15)

やっぱり誰しも、--socketの設定間違えてUNIXソケットつぶしたことありますよねわかりますん！ orz

$ bin/mysqld --no-defaults --initialize --datadir=/tmp/mysql1
2015-05-10T17:04:32.148319Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2015-05-10T17:04:32.526383Z 0 [Warning] InnoDB: New log files created, LSN=45790
2015-05-10T17:04:32.562985Z 0 [Warning] InnoDB: Creating foreign key constraint system tables.
2015-05-10T17:04:32.622323Z 0 [Warning] Failed to setup SSL
2015-05-10T17:04:32.622341Z 0 [Warning] SSL error: SSL context is not usable without certificate and private key
2015-05-10T17:04:32.622849Z 1 [Warning] A temporary password is generated for root@localhost: gOxQq0_u2l)B

$ bin/mysqld --no-defaults --socket=/tmp/mysql.sock --port=3306 --datadir=/tmp/mysql1
2015-05-10T17:04:54.881779Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2015-05-10T17:04:54.881843Z 0 [Warning] Insecure configuration for --secure-file-priv: Current value does not restrict location of generated files. Consider setting it to a valid, non-empty path.
2015-05-10T17:04:54.881879Z 0 [Note] bin/mysqld (mysqld 5.7.7-rc) starting as process 22478 ...
..
Version: '5.7.7-rc'  socket: '/tmp/mysql.sock'  port: 3306  Source distribution

$ ll /tmp/mysql.sock*
srwxrwxrwx 1 yoku0825 yoku0825 0 May 11 02:04 /tmp/mysql.sock
-rw------- 1 yoku0825 yoku0825 6 May 11 02:04 /tmp/mysql.sock.lock

$ cat /tmp/mysql.sock.lock
22478

こうして起動したmysqldと
(ちなみに--no-defaultsで--log-timestamps=SYSTEMが打ち消されているので、ログの時間(UTF)とファイルシステムの時間(JST)が盛大にズレて見えている)
See also, 日々の覚書: MySQL 5.7で絶対に必要になると思うlog_timestampsの設定

$ bin/mysqld --no-defaults --initialize --datadir=/tmp/mysql2
2015-05-10T17:06:46.667942Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2015-05-10T17:06:47.268897Z 0 [Warning] InnoDB: New log files created, LSN=45790
2015-05-10T17:06:47.307634Z 0 [Warning] InnoDB: Creating foreign key constraint system tables.
2015-05-10T17:06:47.367505Z 0 [Warning] Failed to setup SSL
2015-05-10T17:06:47.367526Z 0 [Warning] SSL error: SSL context is not usable without certificate and private key
2015-05-10T17:06:47.368373Z 1 [Warning] A temporary password is generated for root@localhost: au*pq3)kf6oR

$ bin/mysqld --no-defaults --socket=/tmp/mysql.sock --port=13306 --datadir=/tmp/mysql2
2015-05-10T17:07:18.521509Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2015-05-10T17:07:18.521566Z 0 [Warning] Insecure configuration for --secure-file-priv: Current value does not restrict location of generated files. Consider setting it to a valid, non-empty path.
2015-05-10T17:07:18.521601Z 0 [Note] bin/mysqld (mysqld 5.7.7-rc) starting as process 22603 ...
..
2015-05-10T17:07:18.648282Z 0 [ERROR] Another process with pid 22478 is using unix socket file.
2015-05-10T17:07:18.648287Z 0 [ERROR] Unable to setup unix socket lock file.
2015-05-10T17:07:18.648301Z 0 [ERROR] Aborting

$ ll /tmp/mysql.sock*
srwxrwxrwx 1 yoku0825 yoku0825 0 May 11 02:04 /tmp/mysql.sock
-rw------- 1 yoku0825 yoku0825 6 May 11 02:04 /tmp/mysql.sock.lock

$ cat /tmp/mysql.sock.lock
22478

datadir, portをずらしてsocketだけ一致させたmysqldを起動しようとすると、後から起動しようとした方はスタートアップ処理でmysql.sock.lockの存在を検知してAbortしてくれる。
なんか導入直後の5.7.5で試した時に転けるはいいけどmysql.sockはやっぱり消しちゃうみたいなダメな動作だった気がしていたんだけど、少なくとも5.7.7では正しく動作している。