Add Rack::ErrorHandler for generic error handling.

lib/rack.rb

@@ -25,6 +25,7 @@ module Rack
   autoload :ContentLength, "rack/content_length"
   autoload :ContentType, "rack/content_type"
   autoload :ETag, "rack/etag"
+  autoload :ErrorHandler, "rack/error_handler"
   autoload :Events, "rack/events"
   autoload :File, "rack/file"
   autoload :Files, "rack/files"

lib/rack/error_handler.rb

@@ -0,0 +1,40 @@
+require_relative 'constants'
+
+module Rack
+  class ErrorHandler
+    def initialize(app)
+      @app = app
+    end
+
+    def log_error(env, error)
+      if error_stream = env[RACK_ERRORS]
+        # Exception#full_message was introduced in Ruby 2.5+. If it's not
+        # available, we fall back to the old behavior of just printing the
+        # exception message.
+        if error.respond_to?(:full_message)
+          error_stream.puts(error.full_message)
+        else
+          error_stream.puts(error.message)
+        end
+      end
+    end
+
+    def bad_request(error)
+      [400, {}, ["Bad Request: #{error.class}"]]
+    end
+
+    def internal_server_error(error)
+      [500, {}, ["Internal Server Error: #{error.class}"]]
+    end
+
+    def call(env)
+      @app.call(env)
+    rescue BadRequest => error
+      log_error(env, error)
+      return bad_request(error)
+    rescue ScriptError, StandardError => error
+      log_error(env, error)
+      return internal_server_error(error)
+    end
+  end
+end

test/spec_error_handler.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+require_relative 'helper'
+require 'tempfile'
+require 'fileutils'
+
+separate_testing do
+  require_relative '../lib/rack/error_handler'
+  require_relative '../lib/rack/lint'
+  require_relative '../lib/rack/mock_request'
+end
+
+describe Rack::ErrorHandler do
+  it "catches bad requests as 400 Bad Request" do
+    error = Class.new(StandardError)
+    error.include(Rack::BadRequest)
+
+    app = lambda do |env|
+      raise error, "b00m"
+    end
+
+    middleware = Rack::Lint.new(Rack::ErrorHandler.new(app))
+    response = middleware.call(Rack::MockRequest.env_for("/"))
+
+    response[0].must_equal 400
+    response[2].to_ary.must_equal ["Bad Request: #{error}"]
+  end
+
+  it "catches standard errors as 500 Internal Server Error" do
+    error = Class.new(StandardError)
+
+    app = lambda do |env|
+      raise error, "b00m"
+    end
+
+    middleware = Rack::Lint.new(Rack::ErrorHandler.new(app))
+    response = middleware.call(Rack::MockRequest.env_for("/"))
+
+    response[0].must_equal 500
+    response[2].to_ary.must_equal ["Internal Server Error: #{error}"]
+  end
+
+  it "logs errors to the rack.errors stream" do
+    app = lambda do |env|
+      raise "b00m"
+    end
+
+    middleware = Rack::Lint.new(Rack::ErrorHandler.new(app))
+    errors = StringIO.new
+    response = middleware.call(Rack::MockRequest.env_for("/", "rack.errors" => errors))
+
+    errors.string.must_match(/RuntimeError/)
+    errors.string.must_match(/b00m/)
+
+    response[0].must_equal 500
+    response[2].to_ary.must_equal ["Internal Server Error: RuntimeError"]
+  end
+end