Add bootchecking for rails applications

Adds "RailsBootcheck" to the Rails buildpack.

RailsBootcheck attempts to load your rails application and will fail the
the build of your application if it is unable to load.

This can help prevent scenarios where you can accidentally ship a
version of your application that cannot boot properly. Which will lead
to errors as the application is released and traffic is routed to it.

You can opt-in to this feature via the `HEROKU_RAILS_BOOTCHECK_ENABLE`
variable, new rails applications will have this variable set by default.

You can opt-out of this feature via the `HEROKU_RAILS_BOOTCHECK_DISABLE`
variable.

Closes heroku#1120

lib/language_pack/helpers/rails_bootcheck.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+class LanguagePack::Helpers::RailsBootcheck
+  include LanguagePack::ShellHelpers
+
+  def initialize(timeout = 65)
+    @timeout = timeout
+  end
+
+  def call
+    return unless opted_in? && !opted_out?
+
+    topic("Bootchecking rails application")
+
+    process = ProcessSpawn.new(
+      "rails runner 'puts Rails.env'",
+      user_env: true,
+      timeout:  @timeout,
+      file:     "./.heroku/ruby/compile/rails_bootcheck.txt"
+    )
+
+    if process.timeout?
+      failure("timeout", process.output)
+    elsif !process.success?
+      failure("failure", process.output)
+    end
+  end
+
+  private
+
+  def failure(type, output)
+    message = String.new("Bootchecking rails application #{type}\n")
+    message << "set HEROKU_RAILS_BOOTCHECK_DISABLE=1 to disable this feature\n"
+
+    if !output.empty?
+      message << "\n"
+      message << output
+    end
+
+    error(message)
+  end
+
+  def opted_in?
+    env("HEROKU_RAILS_BOOTCHECK_ENABLE")
+  end
+
+  def opted_out?
+    env("HEROKU_RAILS_BOOTCHECK_DISABLE")
+  end
+end

lib/language_pack/rails2.rb

@@ -1,6 +1,7 @@
 require "fileutils"
 require "language_pack"
 require "language_pack/rack"
+require "language_pack/helpers/rails_bootcheck"
 
 # Rails 2 Language Pack. This is for any Rails 2.x apps.
 class LanguagePack::Rails2 < LanguagePack::Ruby
@@ -19,6 +20,7 @@ def self.use?
   def initialize(*args)
     super(*args)
     @rails_runner = LanguagePack::Helpers::RailsRunner.new
+    @bootcheck = LanguagePack::Helpers::RailsBootcheck.new
   end
 
   def name
@@ -29,7 +31,7 @@ def default_env_vars
     {
       "RAILS_ENV" => "production",
       "RACK_ENV" => "production"
-    }
+    }.merge(bootcheck_env_vars)
   end
 
   def default_config_vars
@@ -60,6 +62,7 @@ def compile
     instrument "rails2.compile" do
       install_plugins
       super
+      @bootcheck.call
     end
   end
 
@@ -106,4 +109,11 @@ def setup_profiled(*args)
     end
   end
 
+  def bootcheck_env_vars
+    value = env("HEROKU_RAILS_BOOTCHECK_ENABLE")
+
+    return {} unless value || new_app?
+
+    { "HEROKU_RAILS_BOOTCHECK_ENABLE" => value || "1" }
+  end
 end