mysql2 が原因でbundle installにてエラーを吐く(Mac OS X)

問題点

Mac OS X(El Capitan)で、人様のgemをgit cloneしてbundle installしようとしたらエラーが出てコケたので、メモ。 & 発生したエラーメッセージでググって到達した方へ向けて。

実行ログ

$ bundle install
...
...
...
Installing mysql2 0.4.5 with native extensions

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/ym/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/mysql2-0.4.5/ext/mysql2
/Users/ym/.rbenv/versions/2.3.1/bin/ruby -r ./siteconf20161204-22899-sy18n5.rb extconf.rb
checking for rb_absint_size()... yes
checking for rb_absint_singlebit_p()... yes
checking for ruby/thread.h... yes
checking for rb_thread_call_without_gvl() in ruby/thread.h... yes
checking for rb_thread_blocking_region()... no
checking for rb_wait_for_single_fd()... yes
checking for rb_hash_dup()... yes
checking for rb_intern3()... yes
checking for rb_big_cmp()... yes
checking for mysql_query() in -lmysqlclient... no
-----
mysql client is missing. You may need to 'brew install mysql' or 'port install mysql', and try again.
-----
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/Users/ym/.rbenv/versions/2.3.1/bin/$(RUBY_BASE_NAME)
	--with-mysql-dir
	--without-mysql-dir
	--with-mysql-include
	--without-mysql-include=${mysql-dir}/include
	--with-mysql-lib
	--without-mysql-lib=${mysql-dir}/lib
	--with-mysql-config
	--without-mysql-config
	--with-mysql-dir
	--without-mysql-dir
	--with-mysql-include
	--without-mysql-include=${mysql-dir}/include
	--with-mysql-lib
	--without-mysql-lib=${mysql-dir}/lib
	--with-mysqlclientlib
	--without-mysqlclientlib

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/ym/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/mysql2-0.4.5/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/ym/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/mysql2-0.4.5 for inspection.
Results logged to /Users/ym/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0-static/mysql2-0.4.5/gem_make.out
...
...
...
An error occurred while installing mysql2 (0.4.5), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.4.5'` succeeds before bundling.

原因

mysqlがOSにインストールされていないから。
と、エラーメッセージをよくよく読んだら、親切に書いてあるという。

-----
mysql client is missing. You may need to 'brew install mysql' or 'port install mysql', and try again.
-----

というわけで、発生するエラー原因は Mac OS X に限ったものではないですね。

解決方法

brew install mysql を bundle install する前に打つだけ。
エラーメッセージはちゃんと読みましょうね、という教訓。