BUGFIX: 500 error on some invalid uploads

app/controllers/uploads_controller.rb

@@ -16,8 +16,10 @@ class UploadsController < ApplicationController
   end
 
   def show
+    return render_404 if !RailsMultisite::ConnectionManagement.has_db?(params[:site])
+
     RailsMultisite::ConnectionManagement.with_connection(params[:site]) do |db|
-      return render nothing: true, status: 404 unless Discourse.store.internal?
+      return render_404 unless Discourse.store.internal?
 
       id = params[:id].to_i
       url = request.fullpath
@@ -26,9 +28,15 @@ class UploadsController < ApplicationController
       if upload = Upload.find_by(id: id, url: url)
         send_file(Discourse.store.path_for(upload), filename: upload.original_filename)
       else
-        render nothing: true, status: 404
+        render_404
       end
     end
   end
 
+  protected
+
+  def render_404
+    render nothing: true, status: 404
+  end
+
 end

vendor/gems/rails_multisite/lib/rails_multisite/connection_management.rb

@@ -1,13 +1,19 @@
 module RailsMultisite
   class ConnectionManagement
     CONFIG_FILE = 'config/multisite.yml'
+    DEFAULT = 'default'.freeze
+
+    def self.has_db?(db)
+      return true if db == DEFAULT
+      (defined? @@db_spec_cache) && @@db_spec_cache && @@db_spec_cache[db]
+    end
 
     def self.rails4?
       !!(Rails.version =~ /^4/)
     end
 
     def self.establish_connection(opts)
-      if opts[:db] == "default" && (!defined?(@@default_spec) || !@@default_spec)
+      if opts[:db] == DEFAULT && (!defined?(@@default_spec) || !@@default_spec)
         # don't do anything .. handled implicitly
       else
         spec = connection_spec(opts) || @@default_spec
@@ -119,33 +125,10 @@ module RailsMultisite
 
       @@default_connection_handler = ActiveRecord::Base.connection_handler
 
-      # inject our connection_handler pool
-      # WARNING MONKEY PATCH
-      #
-      # see: https://github.com/rails/rails/issues/8344#issuecomment-10800848
-      if ActiveRecord::VERSION::MAJOR == 3
-        ActiveRecord::Base.send :include, NewConnectionHandler
-        ActiveRecord::Base.connection_handler = @@default_connection_handler
-      end
-
       @@connection_handlers = {}
       @@established_default = false
     end
 
-    module NewConnectionHandler
-      def self.included(klass)
-        klass.class_eval do
-          define_singleton_method :connection_handler do
-            Thread.current[:connection_handler] || @connection_handler
-          end
-          define_singleton_method :connection_handler= do |handler|
-            @connection_handler ||= handler
-            Thread.current[:connection_handler] = handler
-          end
-        end
-      end
-    end
-
 
     def initialize(app, config = nil)
       @app = app