support `-` as an argument to `rails runner`

in Rails 4.0, you could use `/dev/stdin` on both Linux and Mac, but with
the switch to Kernel.load in Rails 4.1, this broke on Linux (you get
a LoadError). Instead, explicitly detect `-` as meaning stdin, then
read from stdin explicitly, instead of performing file gymnastics. This
should now work on any platform uniformly.

Passing a script via stdin is useful when you're sshing to a server,
and the script you want to run is stored locally. You could theoretically
pass the entire script on the command line, but in reality you'll run
into problems with the command being too long.

railties/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Support `-` as a platform-agnostic way to run a script from stdin with
+    `rails runner`
+
+    *Cody Cutrer*
+
 *   Add `bootsnap` to default `Gemfile`.
 
     *Burke Libbey*

railties/lib/rails/commands/runner/USAGE

@@ -8,6 +8,9 @@ Run the Ruby file located at `path/to/filename.rb` after loading the app:
 
   <%= executable %> path/to/filename.rb
 
+Run the Ruby script read from stdin after loading the app:
+  <%= executable %> -
+
 <% unless Gem.win_platform? %>
 You can also use the runner command as a shebang line for your executables:
 

railties/lib/rails/commands/runner/runner_command.rb

@@ -13,7 +13,7 @@ module Rails
       end
 
       def self.banner(*)
-        "#{super} [<'Some.ruby(code)'> | <filename.rb>]"
+        "#{super} [<'Some.ruby(code)'> | <filename.rb> | -]"
       end
 
       def perform(code_or_file = nil, *command_argv)
@@ -29,7 +29,9 @@ module Rails
 
         ARGV.replace(command_argv)
 
-        if File.exist?(code_or_file)
+        if code_or_file == "-"
+          eval($stdin.read, binding, "stdin")
+        elsif File.exist?(code_or_file)
           $0 = code_or_file
           Kernel.load code_or_file
         else

railties/test/application/runner_test.rb

@@ -84,6 +84,14 @@ module ApplicationTests
       assert_match %w( a b ).to_s, Dir.chdir(app_path) { `bin/rails runner "bin/program_name.rb" a b` }
     end
 
+    def test_should_run_stdin
+      app_file "bin/count_users.rb", <<-SCRIPT
+      puts User.count
+      SCRIPT
+
+      assert_match "42", Dir.chdir(app_path) { `cat bin/count_users.rb | bin/rails runner -` }
+    end
+
     def test_with_hook
       add_to_config <<-RUBY
         runner do |app|