Process#spawn should call #to_io on non-IO file descriptor objects

spec/ruby/core/process/spawn_spec.rb

@@ -477,6 +477,52 @@ def child_pids(pid)
 
   # redirection
 
+  context "redirects STDOUT to the IO returned by obj.to_io if out: obj" do
+    # When an instance of @wrapped_io_class is passed as a value for Process#spawn
+    # redirection (e.g. as the value for out: or err:), Process#spawn is
+    # expected to call #to_io to get the wrapped IO object.
+    #
+    @wrapped_io_class = Class.new do
+      def initialize(io)
+        @io = io
+        @to_io_called = false
+      end
+
+      def to_io
+        @to_io_called = true
+        @io
+      end
+
+      def to_io_called?
+        @to_io_called
+      end
+    end
+
+    it 'should not raise an error' do
+      out = @wrapped_io_class.new(STDOUT)
+      # Since 'exit' is a shell builtin, the quotes around 0 are necessary to
+      # force spawn to use a shell
+      -> { Process.wait Process.spawn('exit "0"', out: out) }.should_not raise_error
+    end
+
+    it 'should call #to_io to get the wrapped IO object' do
+      out = @wrapped_io_class.new(STDOUT)
+      # Since 'exit' is a shell builtin, the quotes around 0 are necessary to
+      # force spawn to use a shell
+      Process.wait Process.spawn('exit "0"', out: out)
+      out.to_io_called?.should == true
+    end
+
+    it 'should redirect stdout of the subprocess to the wrapped IO object' do
+      File.open(@name, 'w') do |file|
+        -> do
+          out = @wrapped_io_class.new(file)
+          Process.wait Process.spawn('echo "Hello World"', out: out)
+        end.should output_to_fd("Hello World\n", file)
+      end
+    end
+  end
+
   it "redirects STDOUT to the given file descriptor if out: Integer" do
     File.open(@name, 'w') do |file|
       -> do

src/main/ruby/truffleruby/core/truffle/process_operations.rb

@@ -377,7 +377,11 @@ def convert_to_fd(obj, target)
             open_file_for_child(obj[0], convert_file_mode(obj[1]), obj[2])
           end
         else
-          raise ArgumentError, "wrong exec redirect: #{obj.inspect}"
+          if obj.respond_to?(:to_io)
+            obj.to_io.fileno
+          else
+            raise ArgumentError, "wrong exec redirect: #{obj.inspect}"
+          end
         end
       end