routes: add a new 404 page.

* Fix a bug where non-GET 404 requests weren't handled.
* Fix a bug where non-HTML 404 requests weren't handled.
* Show a random image from a specified pool on the 404 page.

app/controllers/application_controller.rb

@@ -124,17 +124,17 @@ class ApplicationController < ActionController::Base
     end
   end
 
-  def render_error_page(status, exception, message: exception.message, template: "static/error", format: request.format.symbol)
+  def render_error_page(status, exception = nil, message: exception.message, template: "static/error", format: request.format.symbol)
     @exception = exception
     @expected = status < 500
     @message = message.encode("utf-8", invalid: :replace, undef: :replace)
-    @backtrace = Rails.backtrace_cleaner.clean(@exception.backtrace)
+    @backtrace = Rails.backtrace_cleaner.clean(@exception.backtrace) if @exception
     format = :html unless format.in?(%i[html json xml js atom])
 
     # if InvalidAuthenticityToken was raised, CurrentUser isn't set so we have to use the blank layout.
     layout = CurrentUser.user.present? ? "default" : "blank"
 
-    DanbooruLogger.log(@exception, expected: @expected)
+    DanbooruLogger.log(@exception, expected: @expected) if @exception
     render template, layout: layout, status: status, formats: format
   rescue ActionView::MissingTemplate
     render "static/error", layout: layout, status: status, formats: format

app/controllers/static_controller.rb

@@ -1,4 +1,6 @@
 class StaticController < ApplicationController
+  respond_to :html, :json, :xml
+
   def privacy_policy
   end
 
@@ -6,7 +8,11 @@ class StaticController < ApplicationController
   end
 
   def not_found
-    render plain: "not found", status: :not_found
+    @pool = Pool.find(Danbooru.config.page_not_found_pool_id) if Danbooru.config.page_not_found_pool_id.present?
+    @post = @pool.posts.sample if @pool.present?
+    @artist = @post.tags.select(&:artist?).first if @post.present?
+
+    render_error_page(404, nil, template: "static/not_found", message: "Page not found")
   end
 
   def error
@@ -38,7 +44,7 @@ class StaticController < ApplicationController
       @search = { is_deleted: "false" }
     when "posts"
       @relation = Post.order(id: :asc)
-      @serach = {}
+      @search = {}
     when "tags"
       @relation = Tag.nonempty
       @search = {}

app/views/static/not_found.html.erb

@@ -0,0 +1,26 @@
+<% page_title "Page not found" %>
+
+<div id="c-static">
+  <div id="a-not-found">
+    <div style="text-align: center">
+      <h1>Page not found</h1>
+
+      <% if @post.present? && @artist.present? %>
+        <p>
+          <%= link_to @post do %>
+            <%= tag.img src: @post.large_file_url %>
+          <% end %>
+
+          <p class="fineprint">
+            <%= link_to "post ##{@post.id}", @post %>
+            by <%= link_to @artist.name, posts_path(tags: @artist.name), class: tag_class(@artist) %>
+          </p>
+        </p>
+      <% else %>
+        <p>Nobody here but us chickens!</p>
+      <% end %>
+
+      <p><%= link_to "Return to previous page", :back %></p>
+    </div>
+  </div>
+</div>