分析Macosx下使用docker/mysql的问题

在MacOSX下使用docker/mysql时，如果指定本地目录替换/var/lib/mysql，就会出现文件权限的错误，从而导致mysqld无法正常工作。

具体表现为：

```
Creating mysql_test-mysql_1
Attaching to mysql_test-mysql_1
test-mysql_1 | Initializing database
test-mysql_1 | 2016-03-23T04:32:37.437789Z 0 [Warning] Setting lower_case_table_names=2 because filesystem for /var/lib/mysql/ is case insensitive
test-mysql_1 | 2016-03-23T04:32:37.466955Z 0 [ERROR] InnoDB: Operating system error number 13 in a file operation.
test-mysql_1 | 2016-03-23T04:32:37.467828Z 0 [ERROR] InnoDB: The error means mysqld does not have the access rights to the directory.
test-mysql_1 | 2016-03-23T04:32:37.468824Z 0 [ERROR] InnoDB: Operating system error number 13 in a file operation.
test-mysql_1 | 2016-03-23T04:32:37.468912Z 0 [ERROR] InnoDB: The error means mysqld does not have the access rights to the directory.
test-mysql_1 | 2016-03-23T04:32:37.470280Z 0 [ERROR] InnoDB: Cannot open data file './ibdata1'
test-mysql_1 | 2016-03-23T04:32:37.470309Z 0 [ERROR] InnoDB: Could not open or create the system tablespace. If you tried to add new data files to the system tablespace, and it failed here, you should now edit innodb_data_file_path in my.cnf back to what it was, and remove the new ibdata files InnoDB created in this failed attempt. InnoDB only wrote those files full of zeros, but did not yet use them in any way. But be careful: do not remove old data files which contain your precious data!
test-mysql_1 | 2016-03-23T04:32:37.470317Z 0 [ERROR] InnoDB: InnoDB Database creation was aborted with error Cannot open a file. You may need to delete the ibdata1 file before trying to start up again.
test-mysql_1 | 2016-03-23T04:32:38.073222Z 0 [ERROR] Plugin 'InnoDB' init function returned error.
test-mysql_1 | 2016-03-23T04:32:38.073268Z 0 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
test-mysql_1 | 2016-03-23T04:32:38.073283Z 0 [ERROR] Failed to initialize plugins.
test-mysql_1 | 2016-03-23T04:32:38.073289Z 0 [ERROR] Aborting
test-mysql_1 |
mysql_test-mysql_1 exited with code 1
```

解决这个问题的方法是创建mysqld启动脚本run-mysqld.sh，然后将这个脚本指定为docker-compose.yml的容器入口entrypoint。

run-mysqld.sh

```
#!/bin/bash
# From https://github.com/docker-library/mysql/issues/99
set -e # fail on any error

echo '* Working around permission errors in Docker on Mac locally by making sure that "mysql" uses the same uid and gid as the host volume'
TARGET_UID=$(stat -c "%u" /var/lib/mysql)
echo '-- Setting mysql user to use uid '$TARGET_UID
usermod -o -u $TARGET_UID mysql || true
TARGET_GID=$(stat -c "%g" /var/lib/mysql)
echo '-- Setting mysql group to use gid '$TARGET_GID
groupmod -o -g $TARGET_GID mysql || true
echo

echo '* Starting MySQL'
chown -R mysql:root /var/run/mysqld/
/entrypoint.sh mysqld --user=mysql --console
```

docker-compose.yml

```
test-mysql:
image: mysql
ports:
- "3306:3306"
volumes:
- /localhost/mysql/data:/var/lib/mysql
- ./scripts/run-mysqld.sh:/run-mysqld.sh
environment:
MYSQL_DATABASE: play
MYSQL_USER: play
MYSQL_PASSWORD: play
MYSQL_ROOT_PASSWORD: 123456
entrypoint: /run-mysqld.sh
```

通过以上步骤，就可以在MacOSX下使用docker/mysql了。

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