Warn if ruby/rails/bundle binstub shebangs are bad

lib/language_pack/ruby.rb

@@ -88,13 +88,28 @@ def best_practice_warnings
     end
   end
 
+  def warn_bad_binstubs
+    Dir["bin/{rake,bundle,rails}"].select do |binstub|
+      begin
+        if File.file?(binstub)
+          shebang = File.open(binstub, &:readline)
+          if !shebang.match %r{\A#!/usr/bin/env ruby(.exe)?\z}
+            warn("Binstub #{binstub} contains shebang #{shebang}. This may cause issues if the program specified is unavailable.", inline: true)
+          end
+        end
+      rescue EOFError
+      end
+    end
+  end
+
   def compile
     instrument 'ruby.compile' do
       # check for new app at the beginning of the compile
       new_app?
       Dir.chdir(build_path)
       remove_vendor_bundle
       warn_bundler_upgrade
+      warn_bad_binstubs
       install_ruby
       install_jvm
       setup_language_pack_environment

spec/installers/heroku_ruby_installer_spec.rb

@@ -48,4 +48,5 @@
     end
 
   end
+
 end

spec/ruby_spec.rb

@@ -2,18 +2,18 @@
 
 describe LanguagePack::Ruby do
   describe "#install_binary" do
-    context "installing yarn" do
-      before do
-        @old_path = ENV['PATH']
-        @old_stack = ENV['STACK']
-        ENV['STACK'] = 'cedar-14'
-      end
+    before do
+      @old_path = ENV['PATH']
+      @old_stack = ENV['STACK']
+      ENV['STACK'] = 'cedar-14'
+    end
 
-      after do
-        ENV['PATH'] = @old_path
-        ENV['STACK'] = @old_stack
-      end
+    after do
+      ENV['PATH'] = @old_path
+      ENV['STACK'] = @old_stack
+    end
 
+    context "installing yarn" do
       it "sets up PATH" do
         Dir.mktmpdir do |build_path|
           Dir.mktmpdir do |cache_path|
@@ -27,5 +27,45 @@
       end
 
     end
+  describe "#warn_bad_binstubs" do
+    it "warns about malformed shebangs" do
+      Dir.mktmpdir do |build_path|
+        Dir.mktmpdir do |cache_path|
+          Dir.chdir(build_path) do
+            Dir.mkdir("bin")
+            File.open("bin/rake", 'w') { |f| f.write("#!/usr/bin/env ruby") }
+            File.open("bin/rails", 'w') { |f| f.write("#!/usr/bin/env ruby.exe") }
+            File.open("bin/bundle", 'w') { |f| f.write("#!/usr/bin/env ruby1.7") }
+
+            ruby = LanguagePack::Ruby.new(build_path, cache_path)
+            output = capture(:stdout) { ruby.send(:warn_bad_binstubs) }
+
+            expect(output).to include(<<-WARNING)
+###### WARNING:
+       Binstub bin/bundle contains shebang #!/usr/bin/env ruby1.7. This may cause issues if the program specified is unavailable.
+            WARNING
+          end
+        end
+      end
+    end
+
+    def capture(stream)
+      stream = stream.to_s
+      captured_stream = Tempfile.new(stream)
+      stream_io = eval("$#{stream}")
+      origin_stream = stream_io.dup
+      stream_io.reopen(captured_stream)
+
+      yield
+
+      stream_io.rewind
+      return captured_stream.read
+    ensure
+      captured_stream.close
+      captured_stream.unlink
+      stream_io.reopen(origin_stream)
+    end
   end
+  end
+
 end