Generate a `.keep` file in `tmp` folder

A lot of scripts assumes the existence of this folder and most would fail if it
is absent.

One example of this is `rake restart` (before the previous commit) – it tries to
`touch tmp/restart.txt`, which would fail if `tmp` does not exist, which was the
case for a freshly-cloned project as `tmp` is `.gitignored` by default.

See #20299.

[Yoong Kang Lim, Sunny Juneja]

railties/CHANGELOG.md

+*   Generator a `.keep` file in the `tmp` folder by default as many scripts
+    assume the existence of this folder and most would fail if it is absent.
+
+    See #20299.
+
+    *Yoong Kang Lim*, *Sunny Juneja*
+
 *   `config.static_index` configures directory `index.html` filename
 
     Set `config.static_index` to serve a static directory index file not named

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -141,6 +141,7 @@ def test
     end
 
     def tmp
+      empty_directory_with_keep_file "tmp"
       empty_directory "tmp/cache"
       empty_directory "tmp/cache/assets"
     end

railties/lib/rails/generators/rails/app/templates/gitignore

@@ -15,7 +15,8 @@
 <% end -%>
 # Ignore all logfiles and tempfiles.
 /log/*
+/tmp/*
 <% if keeps? -%>
 !/log/.keep
+!/tmp/.keep
 <% end -%>
-/tmp

railties/test/generators/app_generator_test.rb

@@ -44,6 +44,7 @@
   vendor/assets
   vendor/assets/stylesheets
   vendor/assets/javascripts
+  tmp
   tmp/cache
   tmp/cache/assets
 )
@@ -606,6 +607,32 @@ def test_gitignore_when_non_sqlite3_db
     end
   end
 
+  def test_create_keeps
+    run_generator
+    folders_with_keep = %w(
+      app/assets/images
+      app/mailers
+      app/models
+      app/controllers/concerns
+      app/models/concerns
+      lib/tasks
+      lib/assets
+      log
+      test/fixtures
+      test/fixtures/files
+      test/controllers
+      test/mailers
+      test/models
+      test/helpers
+      test/integration
+      tmp
+      vendor/assets/stylesheets
+    )
+    folders_with_keep.each do |folder|
+      assert_file("#{folder}/.keep")
+    end
+  end
+
   def test_psych_gem
     run_generator
     gem_regex = /gem 'psych',\s+'~> 2.0',\s+platforms: :rbx/