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

test/test_gem_compatibility.rb

@@ -0,0 +1,87 @@
+# frozen_string_literal: true
+
+# JRuby does not support forking, and it doesn't seem worth the effort to make it work.
+return unless Process.respond_to?(:fork)
+
+require_relative 'helper'
+
+describe 'gem compatibility' do
+  %w[
+    resolv-replace
+  ].each do |gem_name|
+    it "passes smoke test with #{gem_name.inspect} gem required" do
+      memcached(:binary, rand(21_397..21_896)) do |_, port|
+        in_isolation(timeout: 10) do
+          before_client = Dalli::Client.new("127.0.0.1:#{port}")
+
+          assert_round_trip(before_client, "Failed to round-trip key before requiring #{gem_name.inspect}")
+
+          require gem_name
+
+          after_client = Dalli::Client.new("127.0.0.1:#{port}")
+
+          assert_round_trip(after_client, "Failed to round-trip key after requiring #{gem_name.inspect}")
+        end
+      end
+    end
+  end
+
+  private
+
+  def assert_round_trip(client, message)
+    expected = SecureRandom.hex(4)
+    key = "round-trip-#{expected}"
+    ttl = 10 # seconds
+
+    client.set(key, expected, ttl)
+
+    assert_equal(expected, client.get(key), message)
+  end
+
+  def in_isolation(timeout:) # rubocop:disable Metrics
+    r, w = IO.pipe
+
+    pid = fork do
+      yield
+      exit!(0)
+    # We rescue Exception so we can catch everything, including MiniTest::Assertion.
+    rescue Exception => e # rubocop:disable Lint/RescueException
+      w.write(Marshal.dump(e))
+    ensure
+      w.close
+      exit!
+    end
+
+    begin
+      Timeout.timeout(timeout) do
+        _, status = Process.wait2(pid)
+        w.close
+        marshaled_exception = r.read
+        r.close
+
+        unless marshaled_exception.empty?
+          raise begin
+            Marshal.load(marshaled_exception) # rubocop:disable Security/MarshalLoad
+          rescue StandardError => e
+            raise <<~MESSAGE
+              Failed to unmarshal error from fork with exit status #{status.exitstatus}!
+              #{e.class}: #{e}
+              ---MARSHALED_EXCEPTION---
+              #{marshaled_exception}
+              -------------------------
+            MESSAGE
+          end
+        end
+
+        unless status.success?
+          raise "Child process exited with non-zero status #{status.exitstatus} despite piping no exception"
+        end
+
+        pass
+      end
+    rescue Timeout::Error
+      Process.kill('KILL', pid)
+      raise "Child process killed after exceeding #{timeout}s timeout"
+    end
+  end
+end