Honor to_io-only redirection wrappers

lib/childprocess/process_spawn_process.rb

@@ -51,8 +51,8 @@ def launch_process
 
       options = {}
 
-      options[:out] = io.stdout ? io.stdout.fileno : File::NULL
-      options[:err] = io.stderr ? io.stderr.fileno : File::NULL
+      options[:out] = io.stdout ? fileno_for(io.stdout) : File::NULL
+      options[:err] = io.stderr ? fileno_for(io.stderr) : File::NULL
 
       if duplex?
         reader, writer = ::IO.pipe
@@ -100,6 +100,10 @@ def set_exit_code(status)
       @exit_code = status.exitstatus || status.termsig
     end
 
+    def fileno_for(io)
+      io.respond_to?(:fileno) ? io.fileno : io.to_io.fileno
+    end
+
     def send_term
       send_signal 'TERM'
     end

spec/io_spec.rb

@@ -60,6 +60,23 @@
     end
   end
 
+  it "can redirect stdout through a to_io wrapper" do
+    process = echo
+    out = Tempfile.new("to-io-wrapper-spec")
+    wrapper = Object.new
+    wrapper.define_singleton_method(:to_io) { out.to_io }
+
+    begin
+      process.io.stdout = wrapper
+      process.start
+      process.wait
+
+      expect(rewind_and_read(out)).to eq "hello\n"
+    ensure
+      out.close
+    end
+  end
+
   it "pumps all output" do
     process = echo