RailsアプリをHerokuにアップしたら`/favicon.ico`へのパスが無いって怒られたけど犯人は`CarrierWave`だった	

```terminal
$ heroku logs
heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/favicon.ico" host=mapachannel.herokuapp.com request_id=1466aab7-3935-4f00-9fef-f993a4bb1a1f fwd="211.2.161.143" dyno= connect= service= status=503 bytes= protocol=https
```

`favicon.ico`へのパスが無いって感じに見えるので`favicon.ico`を追加してみる。

```ruby
# views/layouts/application.html.erb
    <%= favicon_link_tag('favicon.ico') %>
```

こうやって書くと、`app/assets/`の下にあるファイルを探し行く。
実際は、こんな感じで展開されます！

```html
<link rel="shortcut icon" type="image/x-icon" href="/assets/favicon-5bbd3af2bf083a71c2171a1acc0c01f1368e7960c1209dc62ea88db0dd8fcfad.ico">
```

で、もっかいチャレンジ！

```terminal
heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=mapachannel.herokuapp.com request_id=41420f2d-b1d3-42a8-a81e-7e04237f04d9 fwd="211.2.161.143" dyno= connect= service= status=503 bytes= protocol=https

heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/favicon.ico" host=mapachannel.herokuapp.com request_id=ff2a1697-90ee-4643-a54f-fe71b9575e29 fwd="211.2.161.143" dyno= connect= service= status=503 bytes= protocol=https
```

あれっエラーが増えたし、favicon関係無いのか？
ということで、もう少し詳細なログを。

```ruby
$ heroku run rails c
Running rails c on ⬢ mapachannel... up, run.7405 (Free)
/app/app/uploaders/picture_uploader.rb:1:in `<top (required)>': uninitialized constant CarrierWave (NameError)
	from /app/vendor/bundle/ruby/2.3.0/gems/activesupport-5.0.6/lib/active_support/dependencies/interlock.rb:12:in `block in loading'
	from /app/vendor/bundle/ruby/2.3.0/gems/activesupport-5.0.6/lib/active_support/concurrency/share_lock.rb:150:in `exclusive'
	from /app/vendor/bundle/ruby/2.3.0/gems/activesupport-5.0.6/lib/active_support/dependencies/interlock.rb:11:in `loading'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/engine.rb:476:in `block (2 levels) in eager_load!'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/engine.rb:475:in `each'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/engine.rb:475:in `block in eager_load!'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/engine.rb:473:in `each'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/engine.rb:473:in `eager_load!'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/engine.rb:354:in `eager_load!'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/application/finisher.rb:59:in `each'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/application/finisher.rb:59:in `block in <module:Finisher>'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/initializable.rb:30:in `instance_exec'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/initializable.rb:30:in `run'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/initializable.rb:55:in `block in run_initializers'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:228:in `block in tsort_each'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:431:in `each_strongly_connected_component_from'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:349:in `block in each_strongly_connected_component'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:347:in `each'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:347:in `call'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:347:in `each_strongly_connected_component'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:226:in `tsort_each'
	from /app/vendor/ruby-2.3.4/lib/ruby/2.3.0/tsort.rb:205:in `tsort_each'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/initializable.rb:54:in `run_initializers'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/application.rb:352:in `initialize!'
	from /app/config/environment.rb:5:in `<top (required)>'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/application.rb:328:in `require'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/application.rb:328:in `require_environment!'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/commands/commands_tasks.rb:157:in `require_application_and_environment!'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/commands/commands_tasks.rb:77:in `console'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/commands/commands_tasks.rb:49:in `run_command!'
	from /app/vendor/bundle/ruby/2.3.0/gems/railties-5.0.6/lib/rails/commands.rb:18:in `<top (required)>'
	from /app/bin/rails:9:in `require'
	from /app/bin/rails:9:in `<main>'
```

全然エラーの内容が違うじゃ無いか。
`CarrierWave`が何かおかしいと。

ここでピンと来る！

最初は`CarrierWave`を使ってファイルのアップロードとかしようと思っていたのだが、ファイルをそのまま保存するのも嫌だし、s3へアップロードするまでも無いと思って使わないように（コメントアウト）してた。

```ruby
  # mount_uploader :picture, PictureUploader
```

が、`CarrierWave`を使うためのファイル`PictureUploader`はそのまま残ったままだったのでHerokuはこれを多分見に行ってるのだけど使わないようになってるからエラーになってるっぽい。

なので、`app/uploaders`フォルダを削除したところ解決！

めでたしめでたし