Add testing snippets

Snippet is a self-contained example that defines the configuration,
Rails project code and specs to run.

Snippets:
 - allow for clean separation between different snippets
 - work quite fast
 - reuse the already installed gems that specs and Cukes use
 - do not litter
 - do not depend on other parts of the build

In theory snippets retain the ability to use generators and arbitrary
commands, but it makes the case under test less evident.

Co-authored-by: Benoit Tigeot <[email protected]>

BUILD_DETAIL.md

@@ -66,6 +66,21 @@ $ bundle exec cucumber
 $ bin/cucumber
 ```
 
+## Snippets
+
+RSpec Rails uses snippets, self-contained examples that are used to cover
+cases and regressions that don't need a full-blown example application to
+reproduce.
+
+Snippets reuse the already installed gems, and don't attempt to install gem
+versions that are not on the system already to prevent version mismatches.
+
+Run with:
+
+```
+$ script/run_snippets.sh
+```
+
 ## YARD documentation
 
 RSpec uses [YARD](https://yardoc.org/) for API documentation on the [rspec.info site](https://rspec.info/).

Changelog.md

@@ -16,6 +16,7 @@ Enhancements:
   (Andrew W. Lee, #2372)
 * Improve request spec "controller" scafold when no action is specified.
   (Thomas Hareau, #2399)
+* Introduce testing snippets concept (Phil Pirozhkov, Benoit Tigeot, #2423)
 
 Bug Fixes:
 

Gemfile

@@ -1,6 +1,4 @@
 source "https://rubygems.org"
-version_file = File.expand_path('.rails-version', __dir__)
-RAILS_VERSION = ENV['RAILS_VERSION'] || (File.exist?(version_file) && File.read(version_file).chomp) || ""
 
 gemspec
 
@@ -22,28 +20,11 @@ end
 
 gem 'capybara'
 
-MAJOR =
-  case RAILS_VERSION
-  when /5-2-stable/
-    5
-  when /master/, /stable/, nil, false, ''
-    6
-  else
-    /(\d+)[\.|-]\d+/.match(RAILS_VERSION).captures.first.to_i
-  end
-
-if MAJOR >= 6
-  # sqlite3 is an optional, unspecified, dependency and Rails 6.0 only supports `~> 1.4`
-  gem 'sqlite3', '~> 1.4', platforms: [:ruby]
-else
-  # Similarly, Rails 5.0 only supports '~> 1.3.6'. Rails 5.1-5.2 support '~> 1.3', '>= 1.3.6'
-  gem 'sqlite3', '~> 1.3.6', platforms: [:ruby]
-end
-
 # Until 1.13.2 is released due to Rubygems usage
 gem 'ffi', '~> 1.12.0'
 
 custom_gemfile = File.expand_path('Gemfile-custom', __dir__)
 eval_gemfile custom_gemfile if File.exist?(custom_gemfile)
 
+eval_gemfile 'Gemfile-sqlite-dependencies'
 eval_gemfile 'Gemfile-rails-dependencies'

Gemfile-sqlite-dependencies

@@ -0,0 +1,20 @@
+version_file = File.expand_path('.rails-version', __dir__)
+RAILS_VERSION = ENV['RAILS_VERSION'] || (File.exist?(version_file) && File.read(version_file).chomp) || ""
+
+MAJOR =
+  case RAILS_VERSION
+  when /5-2-stable/
+    5
+  when /master/, /stable/, nil, false, ''
+    6
+  else
+    /(\d+)[\.|-]\d+/.match(RAILS_VERSION).captures.first.to_i
+  end
+
+if MAJOR >= 6
+# sqlite3 is an optional, unspecified, dependency and Rails 6.0 only supports `~> 1.4`
+  gem 'sqlite3', '~> 1.4', platforms: [:ruby]
+else
+# Similarly, Rails 5.0 only supports '~> 1.3.6'. Rails 5.1-5.2 support '~> 1.3', '>= 1.3.6'
+  gem 'sqlite3', '~> 1.3.6', platforms: [:ruby]
+end

script/run_build

@@ -20,6 +20,8 @@ fi
 
 fold "cukes" run_cukes
 
+fold "snippets" script/run_snippets.sh
+
 if documentation_enforced; then
   fold "doc check" check_documentation_coverage
 fi

script/run_snippets.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+set -e
+
+(
+  cd snippets
+  # This is required to load `bundle/inline`
+  unset RUBYOPT
+  for snippet in *.rb;
+  do
+    echo Running $snippet
+    ruby $snippet
+  done
+)

snippets/use_active_record_false.rb

@@ -0,0 +1,61 @@
+if __FILE__ =~ /^snippets/
+  fail "Snippets are supposed to be run from their own directory to avoid side " \
+       "effects as e.g. the root `Gemfile`, or `spec/spec_helpers.rb` to be " \
+       "loaded by the root `.rspec`."
+end
+
+# We opt-out from using RubyGems, but `bundler/inline` requires it
+require 'rubygems'
+
+require "bundler/inline"
+
+# We pass `false` to `gemfile` to skip the installation of gems,
+# because it may install versions that would conflict with versions
+# from the main `Gemfile.lock`.
+gemfile(false) do
+  source "https://rubygems.org"
+
+  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
+
+  # Those Gemfiles carefully pick the right versions depending on
+  # settings in the ENV, `.rails-version` and `maintenance-branch`.
+  Dir.chdir('..') do
+    eval_gemfile 'Gemfile-sqlite-dependencies'
+    # This Gemfile expects `maintenance-branch` file to be present
+    # in the current directory.
+    eval_gemfile 'Gemfile-rspec-dependencies'
+    # This Gemfile expects `.rails-version` file
+    eval_gemfile 'Gemfile-rails-dependencies'
+  end
+
+  gem "rspec-rails", path: "../"
+end
+
+# Run specs at exit
+require "rspec/autorun"
+
+# This snippet describes the case when ActiveRecord is loaded, but
+# `use_active_record` is set to `false` in RSpec configuration.
+
+# Initialization
+require "active_record/railtie"
+require "rspec/rails"
+
+# This connection will do for database-independent bug reports
+ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
+
+# RSpec configuration
+RSpec.configure do |config|
+  config.use_active_record = false
+end
+
+# Rails project code
+class Foo
+end
+
+# Rails project specs
+RSpec.describe Foo do
+  it 'does not not break' do
+    Foo
+  end
+end