Add resolv-replace compatibility test

While `dalli` doesn't depend on `resolv-replace`, we want to ensure it
still works if it is required by the host application.

This regression test ensures we don't break compatibility, and adds a
framework allowing us to test compatibility with other gems in the
future.

Gemfile

@@ -14,6 +14,9 @@ group :development, :test do
   gem 'rubocop-performance'
   gem 'rubocop-rake'
   gem 'simplecov'
+
+  # For compatibility testing
+  gem 'resolv-replace', require: false
 end
 
 group :test do

scripts/gems_smoke_test.rb

@@ -0,0 +1,44 @@
+#! /usr/bin/env ruby
+
+# frozen_string_literal: true
+
+# This file is meant to be run in a subprocess from a test file, to test compatibility with other gems in isolation.
+# If the test fails, it will exit with a non-zero status code and print the marshaled error to stdout.
+
+require 'minitest'
+require 'dalli'
+
+def assert_round_trip(message)
+  key = "random-key-#{Time.now.to_f}"
+  expected = Time.now.to_f.to_s
+  ttl = 10 # seconds
+
+  client = Dalli::Client.new(nil)
+  client.set(key, expected, ttl)
+
+  actual = client.get(key)
+
+  # assert_equal is not available in this context, so we implement it ourselves.
+  raise Minitest::Assertion.new, <<~MESSAGE unless expected == actual
+    #{message}.
+    Expected: #{expected.inspect}
+      Actual: #{actual.inspect}
+  MESSAGE
+end
+
+begin
+  gems = ARGV
+  gem_list = gems.map(&:inspect).join(', ')
+  raise 'No gems specified to require' if gems.empty?
+
+  assert_round_trip("Failed to round-trip key before requiring #{gem_list}")
+
+  gems.each { |gem_name| require gem_name }
+
+  assert_round_trip("Failed to round-trip key after requiring #{gem_list}")
+rescue Exception => e # rubocop:disable Lint/RescueException
+  # Marshal the exception to stdout so the parent process can raise it.
+  # Don't use stderr, as it might also contain other output, such as warnings.
+  puts Marshal.dump(e)
+  abort
+end

test/test_gem_compatibility.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require_relative 'helper'
+require 'open3'
+
+describe 'gem compatibility' do
+  [
+    'resolv-replace'
+  ].each do |gem_name|
+    it "passes smoke test with #{gem_name.inspect} gem required" do
+      # We use a separate Ruby process so we can require the gem in isolation without affecting the rest of the tests.
+      stdout, stderr, status = Open3.capture3(File.join(__dir__, '../scripts/gems_smoke_test.rb'), gem_name)
+      return pass if status.success?
+
+      error = begin
+        # Marshal.load is fine here because we're loading what we just dumped in the subprocess.
+        Marshal.load(stdout) # rubocop:disable Security/MarshalLoad
+      rescue StandardError => e
+        raise <<~MESSAGE
+          Failed to unmarshal error from subprocess!
+          #{e.class}: #{e}
+          stdout: #{stdout}
+          stderr: #{stderr}
+        MESSAGE
+      end
+
+      pass if error.is_a?(Minitest::Assertion) # Increment assertion count
+      raise error
+    end
+  end
+end