Fix backwards compatibility with Rails 3 apps.

When we removed script/rails and introduced bin/rails, we accidentally
introduced a regression. If you install Rails 4 as a gem, then try to do
something in a Rails 3 application:

    $ rails g

This will throw the 'please type rails new foo' message rather than the
proper generator documentation message. This is because older apps don't
have bin/rails.

Therefore, we now *prefer* bin/rails, but still search for script/rails,
and exec the one we find.

2 files changed, 16 insertions, 11 deletions

diff --git a/railties/lib/rails/app_rails_loader.rb b/railties/lib/rails/app_rails_loader.rb
index 8937e10db3..44f4d3dabc 100644
--- a/railties/lib/rails/app_rails_loader.rb
+++ b/railties/lib/rails/app_rails_loader.rb
@@ -3,12 +3,16 @@ require 'pathname'
 module Rails
   module AppRailsLoader
     RUBY = File.join(*RbConfig::CONFIG.values_at("bindir", "ruby_install_name")) + RbConfig::CONFIG["EXEEXT"]
-    EXECUTABLE = 'bin/rails'
+    EXECUTABLES = ['bin/rails', 'script/rails']
 
     def self.exec_app_rails
-      cwd = Dir.pwd
-      return unless in_rails_application_or_engine? || in_rails_application_or_engine_subdirectory?
-      exec RUBY, EXECUTABLE, *ARGV if in_rails_application_or_engine?
+      cwd   = Dir.pwd
+
+      exe   = find_executable
+      exe ||= find_executable_in_parent_path
+      return unless exe
+
+      exec RUBY, exe, *ARGV if find_executable
       Dir.chdir("..") do
         # Recurse in a chdir block: if the search fails we want to be sure
         # the application is generated in the original working directory.
@@ -18,12 +22,16 @@ module Rails
       # could not chdir, no problem just return
     end
 
-    def self.in_rails_application_or_engine?
-      File.exists?(EXECUTABLE) && File.read(EXECUTABLE) =~ /(APP|ENGINE)_PATH/
+    def self.find_executable
+      EXECUTABLES.find do |exe|
+        File.exists?(exe) && File.read(exe) =~ /(APP|ENGINE)_PATH/
+      end
     end
 
-    def self.in_rails_application_or_engine_subdirectory?(path = Pathname.new(Dir.pwd))
-      File.exists?(File.join(path, EXECUTABLE)) || !path.root? && in_rails_application_or_engine_subdirectory?(path.parent)
+    def self.find_executable_in_parent_path(path = Pathname.new(Dir.pwd))
+      EXECUTABLES.find do |exe|
+        File.exists?(File.join(path, exe)) || !path.root? && find_executable_in_parent_path(path.parent)
+      end
     end
   end
 end
diff --git a/railties/lib/rails/cli.rb b/railties/lib/rails/cli.rb
index b717b026de..e5341ac436 100644
--- a/railties/lib/rails/cli.rb
+++ b/railties/lib/rails/cli.rb
@@ -3,9 +3,6 @@ require 'rails/app_rails_loader'
 
 # If we are inside a Rails application this method performs an exec and thus
 # the rest of this script is not run.
-#
-# TODO: when we hit this, advise adding ./bin to $PATH instead. Then the
-# app's `rails` executable is run immediately.
 Rails::AppRailsLoader.exec_app_rails
 
 require 'rails/ruby_version_check'