[Rails] mysql2 で bundle installが失敗するときの対処法

最近手伝っていたプロジェクトでローカルPCにMySQLを入れなきゃいけないことがあったんですが、`bundle install`時にエラーが起こり解決にけっこう時間を取られたので、メモとして残しておきます。

## バージョン
- ruby 2.5.3
- Rails 5.2.3
- MySQL 5.7

## エラー内容
gemfileに`gem 'mysql2', '>= 0.4.4', '< 0.6.0'`と記述がある状態で`bundle install`を行ったときに下記のようなエラーが発生しました。

```
Don't know how to set rpath on your system, if MySQL libraries are not in path mysql2
may not load
-----
-----
Setting libpath to /usr/local/opt/mysql@5.7/lib
-----
creating Makefile

current directory:
/Users/gunji/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR=" clean

current directory:
/Users/gunji/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR="
compiling client.c
compiling infile.c
compiling mysql2_ext.c
compiling result.c
compiling statement.c
linking shared-object mysql2/mysql2.bundle
ld: library not found for -lssl
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [mysql2.bundle] Error 1

make failed, exit code 2

Gem files will remain installed in
/Users/gunji/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mysql2-0.5.3 for
inspection.
Results logged to
/Users/gunji/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/extensions/x86_64-darwin-19/2.5.0/mysql2-0.5.3/gem_make.out

An error occurred while installing mysql2 (0.5.3), and Bundler cannot
continue.
Make sure that `gem install mysql2 -v '0.5.3' --source 'https://rubygems.org/'`
succeeds before bundling.

In Gemfile:
  mysql2
```

エラーログにあるように`gem install mysql2 -v '0.5.3' --source 'https://rubygems.org/'`を試してもやはり通らず。

```
Building native extensions. This could take a while...
ERROR:  Error installing mysql2:
	ERROR: Failed to build gem native extension.

    current directory: /Users/gunji/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mysql2-0.5.3/ext/mysql2
/Users/gunji/.rbenv/versions/2.5.3/bin/ruby -r ./siteconf20200229-60710-12bhnvq.rb extconf.rb
checking for rb_absint_size()... yes
checking for rb_absint_singlebit_p()... yes
checking for rb_wait_for_single_fd()... yes
-----
Using mysql_config at /usr/local/opt/mysql@5.7/bin/mysql_config
-----
checking for mysql.h... yes
checking for errmsg.h... yes
checking for SSL_MODE_DISABLED in mysql.h... yes
checking for SSL_MODE_PREFERRED in mysql.h... yes
checking for SSL_MODE_REQUIRED in mysql.h... yes
checking for SSL_MODE_VERIFY_CA in mysql.h... yes
checking for SSL_MODE_VERIFY_IDENTITY in mysql.h... yes
checking for MYSQL.net.vio in mysql.h... yes
checking for MYSQL.net.pvio in mysql.h... no
checking for MYSQL_ENABLE_CLEARTEXT_PLUGIN in mysql.h... yes
checking for SERVER_QUERY_NO_GOOD_INDEX_USED in mysql.h... yes
checking for SERVER_QUERY_NO_INDEX_USED in mysql.h... yes
checking for SERVER_QUERY_WAS_SLOW in mysql.h... yes
checking for MYSQL_OPTION_MULTI_STATEMENTS_ON in mysql.h... yes
checking for MYSQL_OPTION_MULTI_STATEMENTS_OFF in mysql.h... yes
checking for my_bool in mysql.h... yes
-----
Don't know how to set rpath on your system, if MySQL libraries are not in path mysql2 may not load
-----
-----
Setting libpath to /usr/local/opt/mysql@5.7/lib
-----
creating Makefile

current directory: /Users/gunji/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR=" clean

current directory: /Users/gunji/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/mysql2-0.5.3/ext/mysql2
make "DESTDIR="
compiling client.c
compiling infile.c
compiling mysql2_ext.c
compiling result.c
compiling statement.c
linking shared-object mysql2/mysql2.bundle
ld: library not found for -lssl
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [mysql2.bundle] Error 1

make failed, exit code 2
```

どうやらSSL周りでエラーが発生しているようで、ぐぐっていたら[StackOverflow](https://stackoverflow.com/questions/30834421/error-when-trying-to-install-app-with-mysql2-gem)に以下のような内容がありました。

> Apple has deprecated use of OpenSSL in favor of its own TLS and crypto libraries

> Generally there are no consequences of this for you. If you build your own > software and it requires this formula, you'll need to add to your build variables:

> LDFLAGS: -L/usr/local/opt/openssl/lib
> CPPFLAGS: -I/usr/local/opt/openssl/include
> PKG_CONFIG_PATH: /usr/local/opt/openssl/lib/pkgconfig


## 解決策

記事を参考にしつつ、最終的には以下のようにすることで解決できました。brewで`openssl@1.1`が入っている前提です。

```sh
$ bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl@1.1/lib"
```
上記のように設定をすると、`.bundle/config` に設定値が書かれます。

```
---
BUNDLE_BUILD__MYSQL2: "--with-ldflags=-L/usr/local/opt/openssl@1.1/lib"
```

これで`bundle install`が無事に通るようになりました。

## まとめ
ローカル環境に色々インストールして開発してると変なハマりがよくあります。
プロジェクトにDockerが導入されていたら起こらなかったエラーかもしれません。