[rubygems/rubygems] Introduce the Gem::CIDetector

This is based on the list in Gem::UpdateSuggestion and Bundler::Fetcher;
these have similar purposes (determining whether/what CI we're executing
in), and can benefit from being combined and updated (they're both
slightly out of date).

Noteable changes:
* We'll consider ourselves to be on a CI in more cases - if CI_NAME or
  TASKCLUSTER_ROOT_URL are set specifically, since those represent two
  cases that were either overlooked or are no longer covered by the
  existing implementation. (Or possibly TaskCluster still does provide
  RUN_ID, but failed to document it)
* We will notice/track a few additional services in ci_strings (cirrus,
  dsari, taskcluster), stop tracking 'snap' (they went under in 2017),
  and update buildbox to buildkite (they've been called that for 8
  years, and the BUILDBOX envs have been deprecated for 3).
* We'll also sort/uniq/downcase the values (all of which only matter
  because of the special case of CI_NAME).

https://github.com/rubygems/rubygems/commit/60652b942f

lib/rubygems.rb

@@ -1290,6 +1290,7 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
   MARSHAL_SPEC_DIR = "quick/Marshal.#{Gem.marshal_version}/".freeze
 
   autoload :ConfigFile,         File.expand_path("rubygems/config_file", __dir__)
+  autoload :CIDetector,         File.expand_path("rubygems/ci_detector", __dir__)
   autoload :Dependency,         File.expand_path("rubygems/dependency", __dir__)
   autoload :DependencyList,     File.expand_path("rubygems/dependency_list", __dir__)
   autoload :Installer,          File.expand_path("rubygems/installer", __dir__)

lib/rubygems/ci_detector.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+module Gem
+  module CIDetector
+    # NOTE: Any changes made here will need to be made to both lib/rubygems/ci_detector.rb and
+    # bundler/lib/bundler/ci_detector.rb (which are enforced duplicates).
+    # TODO: Drop that duplication once bundler drops support for RubyGems 3.4
+    #
+    # ## Recognized CI providers, their signifiers, and the relevant docs ##
+    #
+    # Travis CI   - CI, TRAVIS            https://docs.travis-ci.com/user/environment-variables/#default-environment-variables
+    # Cirrus CI   - CI, CIRRUS_CI         https://cirrus-ci.org/guide/writing-tasks/#environment-variables
+    # Circle CI   - CI, CIRCLECI          https://circleci.com/docs/variables/#built-in-environment-variables
+    # Gitlab CI   - CI, GITLAB_CI         https://docs.gitlab.com/ee/ci/variables/
+    # AppVeyor    - CI, APPVEYOR          https://www.appveyor.com/docs/environment-variables/
+    # CodeShip    - CI_NAME               https://docs.cloudbees.com/docs/cloudbees-codeship/latest/pro-builds-and-configuration/environment-variables#_default_environment_variables
+    # dsari       - CI, DSARI             https://github.com/rfinnie/dsari#running
+    # Jenkins     - BUILD_NUMBER          https://www.jenkins.io/doc/book/pipeline/jenkinsfile/#using-environment-variables
+    # TeamCity    - TEAMCITY_VERSION      https://www.jetbrains.com/help/teamcity/predefined-build-parameters.html#Predefined+Server+Build+Parameters
+    # Appflow     - CI_BUILD_ID           https://ionic.io/docs/appflow/automation/environments#predefined-environments
+    # TaskCluster - TASKCLUSTER_ROOT_URL  https://docs.taskcluster.net/docs/manual/design/env-vars
+    # Semaphore   - CI, SEMAPHORE         https://docs.semaphoreci.com/ci-cd-environment/environment-variables/
+    # BuildKite   - CI, BUILDKITE         https://buildkite.com/docs/pipelines/environment-variables
+    # GoCD        - GO_SERVER_URL         https://docs.gocd.org/current/faq/dev_use_current_revision_in_build.html
+    # GH Actions  - CI, GITHUB_ACTIONS    https://docs.github.com/en/actions/learn-github-actions/variables#default-environment-variables
+    #
+    # ### Some "standard" ENVs that multiple providers may set ###
+    #
+    # * CI - this is set by _most_ (but not all) CI providers now; it's approaching a standard.
+    # * CI_NAME - Not as frequently used, but some providers set this to specify their own name
+
+    # Any of these being set is a reasonably reliable indicator that we are
+    # executing in a CI environment.
+    ENV_INDICATORS = [
+      "CI",
+      "CI_NAME",
+      "CONTINUOUS_INTEGRATION",
+      "BUILD_NUMBER",
+      "CI_APP_ID",
+      "CI_BUILD_ID",
+      "CI_BUILD_NUMBER",
+      "RUN_ID",
+      "TASKCLUSTER_ROOT_URL",
+    ].freeze
+
+    # For each CI, this env suffices to indicate that we're on _that_ CI's
+    # containers. (A few of them only supply a CI_NAME variable, which is also
+    # nice). And if they set "CI" but we can't tell which one they are, we also
+    # want to know that - a bare "ci" without another token tells us as much.
+    ENV_DESCRIPTORS = {
+      "TRAVIS" => "travis",
+      "CIRCLECI" => "circle",
+      "CIRRUS_CI" => "cirrus",
+      "DSARI" => "dsari",
+      "SEMAPHORE" => "semaphore",
+      "JENKINS_URL" => "jenkins",
+      "BUILDKITE" => "buildkite",
+      "GO_SERVER_URL" => "go",
+      "GITLAB_CI" => "gitlab",
+      "GITHUB_ACTIONS" => "github",
+      "TASKCLUSTER_ROOT_URL" => "taskcluster",
+      "CI" => "ci",
+    }.freeze
+
+    def self.ci?
+      ENV_INDICATORS.any? {|var| ENV.include?(var) }
+    end
+
+    def self.ci_strings
+      matching_names = ENV_DESCRIPTORS.select {|env, _| ENV[env] }.values
+      matching_names << ENV["CI_NAME"].downcase if ENV["CI_NAME"]
+      matching_names.reject(&:empty?).sort.uniq
+    end
+  end
+end

test/rubygems/test_gem_ci_detector.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+require_relative "helper"
+require "rubygems"
+
+class TestCiDetector < Test::Unit::TestCase
+  def test_ci?
+    with_env("FOO" => "bar") { assert_equal(false, Gem::CIDetector.ci?) }
+    with_env("CI" => "true") { assert_equal(true, Gem::CIDetector.ci?) }
+    with_env("CONTINUOUS_INTEGRATION" => "1") { assert_equal(true, Gem::CIDetector.ci?) }
+    with_env("RUN_ID" => "0", "TASKCLUSTER_ROOT_URL" => "2") do
+      assert_equal(true, Gem::CIDetector.ci?)
+    end
+  end
+
+  def test_ci_strings
+    with_env("FOO" => "bar") { assert_empty(Gem::CIDetector.ci_strings) }
+    with_env("TRAVIS" => "true") { assert_equal(["travis"], Gem::CIDetector.ci_strings) }
+    with_env("CI" => "true", "CIRCLECI" => "true", "GITHUB_ACTIONS" => "true") do
+      assert_equal(["ci", "circle", "github"], Gem::CIDetector.ci_strings)
+    end
+    with_env("CI" => "true", "CI_NAME" => "MYCI") do
+      assert_equal(["ci", "myci"], Gem::CIDetector.ci_strings)
+    end
+    with_env("GITHUB_ACTIONS" => "true", "CI_NAME" => "github") do
+      assert_equal(["github"], Gem::CIDetector.ci_strings)
+    end
+    with_env("TASKCLUSTER_ROOT_URL" => "https://foo.bar", "DSARI" => "1", "CI_NAME" => "") do
+      assert_equal(["dsari", "taskcluster"], Gem::CIDetector.ci_strings)
+    end
+  end
+
+  private
+
+  def with_env(overrides, &block)
+    @orig_env = ENV.to_h
+    ENV.replace(overrides)
+    begin
+      block.call
+    ensure
+      ENV.replace(@orig_env)
+    end
+  end
+end