add fork future primitive

app/lib/fork_future.rb

@@ -0,0 +1,38 @@
+class ForkFuture
+  def initialize(&block)
+    read, write = ::IO.pipe
+    @read = read
+    pid = ::Process.fork do
+      start = Time.now
+      read.close
+      result = block.call
+      duration = Time.now - start
+      ::Marshal.dump({
+        duration: duration,
+        result: result,
+      }, write)
+      ::Process.exit!(true)
+    end
+    write.close
+  end
+
+  def join
+    wait!
+    @result[:result]
+  end
+
+  def duration
+    wait!
+    @result[:duration]
+  end
+
+  private
+
+  def wait!
+    @result ||= begin
+        result_buffer = @read.read
+        @read.close
+        ::Marshal.load(result_buffer)
+      end
+  end
+end

test/lib/fork_future_test.rb

@@ -0,0 +1,18 @@
+require "test_helper"
+
+class ForkFutureTest < ActiveSupport::TestCase
+  def test_works
+    captured = "foo"
+    future1 = ForkFuture.new do
+      captured + "bar"
+    end
+    future2 = ForkFuture.new do
+      captured + "baz"
+    end
+
+    assert_equal "foobar", future1.join
+    assert_equal "foobaz", future2.join
+    assert 0.0 < future1.duration
+    assert 0.0 < future2.duration
+  end
+end