Q. またnokogiriですか
A. はい

いつものようにRackアプリケーションを動かそうとbundle installしたら
以下のエラーが出ました。nokogiriですね。

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

    /home/user/.rbenv/versions/2.1.5/bin/ruby extconf.rb 
checking if the C compiler accepts ... yes
Building nokogiri using packaged libraries.
checking for gzdopen() in -lz... yes
checking for iconv... yes
************************************************************************
IMPORTANT NOTICE:

Building Nokogiri with a packaged version of libxml2-2.9.2
with the following patches applied:
  - 0001-Revert-Missing-initialization-for-the-catalog-module.patch
  - 0002-Fix-missing-entities-after-CVE-2014-3660-fix.patch

Team Nokogiri will keep on doing their best to provide security
updates in a timely manner, but if this is a concern for you and want
to use the system library instead; abort this installation process and
reinstall nokogiri as follows:

    gem install nokogiri -- --use-system-libraries
        [--with-xml2-config=/path/to/xml2-config]
        [--with-xslt-config=/path/to/xslt-config]

If you are using Bundler, tell it to use the option:

    bundle config build.nokogiri --use-system-libraries
    bundle install

Note, however, that nokogiri is not fully compatible with arbitrary
versions of libxml2 provided by OS/package vendors.
************************************************************************
Extracting libxml2-2.9.2.tar.gz into tmp/x86_64-unknown-linux-gnu/ports/libxml2/2.9.2... OK
Running patch with /service/application/.bundle/gems/ruby/2.1.0/gems/nokogiri-1.6.6.2/ports/patches/libxml2/0001-Revert-Missing-initialization-for-the-catalog-module.patch...
Running 'patch' for libxml2 2.9.2... ERROR, review '/service/application/.bundle/gems/ruby/2.1.0/gems/nokogiri-1.6.6.2/ext/nokogiri/tmp/x86_64-unknown-linux-gnu/ports/libxml2/2.9.2/patch.log' to see what happened.
*** 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=/home/user/.rbenv/versions/2.1.5/bin/ruby
  --help
  --clean
  --use-system-libraries
  --enable-static
  --disable-static
  --with-zlib-dir
  --without-zlib-dir
  --with-zlib-include
  --without-zlib-include=${zlib-dir}/include
  --with-zlib-lib
  --without-zlib-lib=${zlib-dir}/lib
  --enable-cross-build
  --disable-cross-build
/service/application/.bundle/gems/ruby/2.1.0/gems/mini_portile-0.6.2/lib/mini_portile.rb:279:in `block in execute': Failed to complete patch task (RuntimeError)
  from /service/application/.bundle/gems/ruby/2.1.0/gems/mini_portile-0.6.2/lib/mini_portile.rb:271:in `chdir'
  from /service/application/.bundle/gems/ruby/2.1.0/gems/mini_portile-0.6.2/lib/mini_portile.rb:271:in `execute'
  from extconf.rb:311:in `block in patch'
  from extconf.rb:308:in `each'
  from extconf.rb:308:in `patch'
  from /service/application/.bundle/gems/ruby/2.1.0/gems/mini_portile-0.6.2/lib/mini_portile.rb:108:in `cook'
  from extconf.rb:278:in `block in process_recipe'
  from extconf.rb:177:in `tap'
  from extconf.rb:177:in `process_recipe'
  from extconf.rb:475:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /service/application/.bundle/gems/ruby/2.1.0/gems/nokogiri-1.6.6.2 for inspection.
Results logged to /service/application/.bundle/gems/ruby/2.1.0/extensions/x86_64-linux/2.1.0-static/nokogiri-1.6.6.2/gem_make.out

記載のエラーログを見てみたら以下の通り。

$ cat /service/application/.bundle/gems/ruby/2.1.0/gems/nokogiri-1.6.6.2/ext/nokogiri/tmp/x86_64-unknown-linux-gnu/ports/libxml2/2.9.2/patch.log
sh: patch: command not found

なるほど、patchコマンドがなかったのね。
というわけでinstall。

$ yum -y install patch

からの bundle install。

$ bundle
(中略)
Installing nokogiri 1.6.6.2

無事nokogiriが入りました。
おのれnokogiri。