Print the run seed and let tests read it

.github/workflows/ruby.yml

@@ -33,7 +33,7 @@ jobs:
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically
           bundler-args: --without development
       - name: Quickdraw tests
-        run: bundle exec qt -t 1
+        run: bundle exec qt -p 1
   rubocop:
     runs-on: ubuntu-latest
     strategy:

lib/quickdraw/basic_test.rb

@@ -27,6 +27,10 @@ def run
 		})
 	end
 
+	def seed
+		@runner.seed
+	end
+
 	def match
 		yield
 		success!

lib/quickdraw/cli.rb

@@ -78,6 +78,12 @@ def seed=(value)
 		@seed = value
 	end
 
+	def seed_from_env
+		return unless ENV["SEED"]
+
+		Integer(ENV["SEED"], exception: false) || fail("SEED must be an integer, got #{ENV['SEED'].inspect}")
+	end
+
 	def efficiency!
 		if Quickdraw::Platform.m_series?
 			fail "You cannot use the efficiency flag with the processes flag." if @processes
@@ -124,7 +130,7 @@ def run_once
 			backtrace: @backtrace,
 			processes: @processes || Quickdraw::Config.processes,
 			files: Dir.glob(@files),
-			seed: @seed || Random.new_seed,
+			seed: @seed || seed_from_env || (Random.new_seed % (1 << 32)),
 		)
 
 		$quickdraw_runner.call

lib/quickdraw/runner.rb

@@ -30,7 +30,12 @@ def initialize(backtrace: false, processes:, files:, seed:)
 		@successes = Quickdraw::ConcurrentInteger.new
 	end
 
+	attr_reader :seed
+
 	def call
+		puts "Seed: #{@seed}"
+
+		Kernel.srand(@seed)
 		load_tests
 
 		if @processes > 1
@@ -75,7 +80,7 @@ def call
 
 		puts
 
-		puts "Passed: #{@successes.value} | Failed: #{@failures.size} | Errors: #{@errors.size}"
+		puts "Passed: #{@successes.value} | Failed: #{@failures.size} | Errors: #{@errors.size} | Seed: #{@seed}"
 
 		exit(1) if @failures.any? || @errors.any?
 	end

test/fixtures/seed/recorder.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class SeedRecorderTest < Quickdraw::Test
+	20.times do |i|
+		test "test #{i}" do
+			File.open(ENV.fetch("SEED_LOG"), "a") do |log|
+				log.puts [Process.pid, i, seed, rand(1_000_000)].join(" ")
+			end
+
+			assert true
+		end
+	end
+end

test/seed.test.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+require "open3"
+require "tmpdir"
+
+class SeedTest < Quickdraw::Test
+	QT = File.expand_path("../exe/qt", __dir__)
+	RECORDER = File.expand_path("fixtures/seed/recorder.rb", __dir__)
+
+	def run_recorder(*args, env: {})
+		Dir.mktmpdir do |dir|
+			log = File.join(dir, "seed.log")
+			output, status = Open3.capture2e(env.merge("SEED_LOG" => log), RbConfig.ruby, QT, *args, RECORDER, chdir: dir)
+			assert(status.success?) { output }
+
+			lines = File.readlines(log).map { |it| it.split.map(&:to_i) }
+			{ output:, workers: lines.group_by(&:first).values.map { |it| it.map { |_, *rest| rest } } }
+		end
+	end
+
+	test "seed is readable from inside a test" do
+		assert_equal seed, $quickdraw_runner.seed
+	end
+
+	test "seed is printed at the start and in the summary" do
+		output = run_recorder("-p", "1", "--seed", "1234")[:output]
+
+		assert output.start_with?("Seed: 1234\n")
+		assert_includes output, "| Seed: 1234"
+	end
+
+	test "--seed sets the seed seen by tests" do
+		workers = run_recorder("-p", "1", "--seed", "42")[:workers]
+
+		assert_equal workers.flatten(1).map { |it| it[1] }.uniq, [42]
+	end
+
+	test "SEED is used when --seed is not given" do
+		assert_includes run_recorder("-p", "1", env: { "SEED" => "77" })[:output], "Seed: 77"
+	end
+
+	test "--seed takes precedence over SEED" do
+		assert_includes run_recorder("-p", "1", "-s", "5", env: { "SEED" => "77" })[:output], "Seed: 5"
+	end
+
+	test "a seed is generated when none is given" do
+		output = run_recorder("-p", "1", env: { "SEED" => nil })[:output]
+
+		match { output => /\ASeed: \d+$/ }
+	end
+
+	test "a SEED that is not an integer is refused" do
+		output, status = Open3.capture2e({ "SEED" => "abc" }, RbConfig.ruby, QT, RECORDER, chdir: Dir.tmpdir)
+
+		refute status.success?
+		assert_includes output, "SEED must be an integer"
+	end
+
+	test "same seed gives the same order and the same rand values" do
+		first = run_recorder("-p", "1", "--seed", "1234")[:workers]
+		second = run_recorder("-p", "1", "--seed", "1234")[:workers]
+		other = run_recorder("-p", "1", "--seed", "4321")[:workers]
+
+		assert_equal first, second
+		refute_equal first, other # with 20 tests a collision is vanishingly unlikely
+	end
+end