application controller: clean up exception handling.

* Simplify code.
* Show backtraces for all users, not just builders.
* Show backtraces only for unexpected server errors (status 5xx), not
  for normal client errors (status 4xx).
* Log expected errors at info level (reduce noise in production logs).

app/controllers/application_controller.rb

@@ -73,21 +73,19 @@ class ApplicationController < ActionController::Base
   end
 
   def rescue_exception(exception)
-    @exception = exception
-
     case exception
     when ActiveRecord::QueryCanceled
       render_error_page(500, exception, message: "The database timed out running your query.")
     when ActionController::BadRequest
-      render_error_page(400, exception, expected: true)
+      render_error_page(400, exception)
     when ActiveRecord::RecordNotFound
-      render_error_page(404, exception, message: "That record was not found", expected: true)
+      render_error_page(404, exception, message: "That record was not found.")
     when ActionController::RoutingError
-      render_error_page(405, exception, expected: true)
+      render_error_page(405, exception)
     when ActionController::UnknownFormat, ActionView::MissingTemplate
-      render_error_page(406, exception, message: "#{request.format.to_s} is not a supported format for this page", html: true, expected: true)
+      render_error_page(406, exception, message: "#{request.format.to_s} is not a supported format for this page", format: :html)
     when Danbooru::Paginator::PaginationError
-      render_error_page(410, exception, expected: true)
+      render_error_page(410, exception)
     when NotImplementedError
       render_error_page(501, exception, message: "This feature isn't available: #{exception.message}")
     when PG::ConnectionBad
@@ -97,16 +95,15 @@ class ApplicationController < ActionController::Base
     end
   end
 
-  def render_error_page(status, exception, message: exception.message, html: false, expected: false)
-    @error_message = message
+  def render_error_page(status, exception, message: exception.message, 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)
+    format = :html unless format.in?(%i[html json xml js atom])
 
-    DanbooruLogger.log(exception, expected: expected)
-
-    if html || !request.format.symbol.in?(%i[html json xml js atom])
-      render template: "static/error.html", status: status
-    else
-      render template: "static/error", status: status
-    end
+    DanbooruLogger.log(@exception, expected: @expected)
+    render "static/error", status: status, formats: format
   end
 
   def authentication_failed

app/logical/danbooru_logger.rb

@@ -1,6 +1,8 @@
 class DanbooruLogger
   def self.log(exception, expected: false, **params)
-    if !expected
+    if expected
+      Rails.logger.info("#{exception.class}: #{exception.message}")
+    else
       backtrace = Rails.backtrace_cleaner.clean(exception.backtrace).join("\n")
       Rails.logger.error("#{exception.class}: #{exception.message}\n#{backtrace}")
     end

app/views/static/_backtrace.html.erb

@@ -1,6 +1,10 @@
 <%# backtrace %>
 
 <ul class="backtrace">
+  <% if exception.present? %>
+    <li><%= exception.class.to_s %> exception raised</li>
+  <% end %>
+
   <% Rails.backtrace_cleaner.clean(backtrace).each do |b| %>
     <li class="backtrace-line"><%= b %></li>
   <% end %>

app/views/static/error.html.erb

@@ -1,10 +1,7 @@
-<% if CurrentUser.user.try(:is_builder?) && @exception.present? %>
-    <h1><%= @exception.class.to_s %> exception raised</h1>
+<h1>Error</h1>
+<p><%= @message %></p>
 
-    <p><%= @exception.message.dup.force_encoding("utf-8") %></p>
-    <%= render "static/backtrace", backtrace: @exception.backtrace %>
-<% elsif @error_message %>
-  <p><%= @error_message %></p>
-<% else %>
-  <p><%= @exception.message.dup.force_encoding("utf-8") %></p>
+<% unless @expected %>
+  <h6>Details</h6>
+  <%= render "static/backtrace", exception: @exception, backtrace: @backtrace %>
 <% end %>

app/views/static/error.js.erb

@@ -1,2 +1,2 @@
-var message = <%= raw @error_message.try(:to_json) || @exception.message.to_json %>;
+var message = <%= raw @message.to_json %>;
 Danbooru.Utility.error(message);

app/views/static/error.json.erb

@@ -1,9 +1,5 @@
-<% if @error_message %>
-{"success": false, "message": <%= raw @error_message.encode("utf-8", {:invalid => :replace, :undef => :replace, :replace => "?"}).to_json %>}
-<% else %>
-  {
-    "success": false,
-    "message": <%= raw @exception.to_s.encode("utf-8", {:invalid => :replace, :undef => :replace, :replace => "?"}).to_json %>,
-    "backtrace": <%= raw Rails.backtrace_cleaner.clean(@exception.backtrace).to_json %>
-  }
-<% end %>
+{
+  "success": false,
+  "message": <%= raw @message.to_json %>,
+  "backtrace": <%= raw @backtrace.to_json %>
+}

app/views/static/error.xml.erb

@@ -1,6 +1,2 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<% if @error_message %>
-<result success="false"><%= @error_message %></result>
-<% else %>
-<result success="false"><%= @exception.to_s %></result>
-<% end %>
+<result success="false"><%= @message %></result>