Modify Runner to use Open3.capture3

[`Open3.capture3`][docs] captures `STDOUT` and `STDERR`, allowing the
`Runner` to manually redirect their output.

This commit modified `Runner` to **only** output errors to `STDERR`
(instead of to both `STDOUT` and `STDERR`).

[docs]: http://ruby-doc.org/stdlib-1.9.3/libdoc/open3/rdoc/Open3.html#method-c-capture3

CHANGELOG.md

@@ -1,8 +1,11 @@
 master
 ------
 
-* Remove dependency on `tee`. Fixes bug [#417][#417].
+* Only write errors to `STDERR`. [#421]
+* Remove dependency on `tee`. Fixes bug [#417][#417]. [#420]
 
+[#421]: https://github.com/thoughtbot/ember-cli-rails/issues/421
+[#420]: https://github.com/thoughtbot/ember-cli-rails/issues/420
 [#417]: https://github.com/thoughtbot/ember-cli-rails/issues/417
 
 0.7.2

lib/ember_cli/runner.rb

@@ -10,27 +10,20 @@ def initialize(out:, err:, env: {}, options: {})
     end
 
     def run(command)
-      output, status = Open3.capture2e(env, command, options)
+      output, error, status = Open3.capture3(env, command, options)
 
-      write_to_out(output)
+      write(output, streams: out_streams)
+      write(error, streams: err_streams)
 
-      [output, status]
+      status
     end
 
     def run!(command)
-      output, status = run(command)
-
-      unless status.success?
-        write_to_err <<-MSG.strip_heredoc
-          ERROR: Failed command: `#{command}`
-          OUTPUT:
-            #{output}
-        MSG
-
-        exit status.exitstatus
+      run(command).tap do |status|
+        unless status.success?
+          exit status.exitstatus
+        end
       end
-
-      true
     end
 
     protected
@@ -39,15 +32,7 @@ def run!(command)
 
     private
 
-    def write_to_out(output)
-      write(out_streams, output)
-    end
-
-    def write_to_err(output)
-      write(out_streams + err_streams, output)
-    end
-
-    def write(streams, output)
+    def write(output, streams:)
       streams.each do |stream|
         stream.write(output)
       end

spec/lib/ember_cli/runner_spec.rb

@@ -3,20 +3,37 @@
 describe EmberCli::Runner do
   describe "#run!" do
     context "when the command fails" do
-      it "writes to all `err` and `out` streams" do
-        output_streams = Array.new(4) { StringIO.new }
+      it "writes output to `out` streams" do
+        output_streams = Array.new(2) { StringIO.new }
         runner = EmberCli::Runner.new(
-          err: output_streams.first(2),
-          out: output_streams.last(2),
+          err: [],
+          out: output_streams,
         )
 
         expect { runner.run!("echo 'out'; echo 'err' > /dev/stderr; exit 1") }.
           to raise_error(SystemExit)
 
-        output_streams.each(&:rewind)
+        ouput_strings = output_streams.each(&:rewind).map(&:read)
 
-        output_streams.each do |stream|
-          expect(stream.read).to match(/out\nerr/)
+        ouput_strings.each do |output|
+          expect(output).to eq("out\n")
+        end
+      end
+
+      it "writes errors to `err` streams" do
+        error_streams = Array.new(2) { StringIO.new }
+        runner = EmberCli::Runner.new(
+          err: error_streams,
+          out: [],
+        )
+
+        expect { runner.run!("echo 'out'; echo 'err' > /dev/stderr; exit 1") }.
+          to raise_error(SystemExit)
+
+        error_strings = error_streams.each(&:rewind).map(&:read)
+
+        error_strings.each do |error|
+          expect(error).to eq("err\n")
         end
       end
     end
@@ -26,10 +43,11 @@
       err = StringIO.new
       runner = EmberCli::Runner.new(err: [err], out: [out])
 
-      runner.run!("echo 'out'")
+      status = runner.run!("echo 'out'")
 
       [err, out].each(&:rewind)
 
+      expect(status).to be_success
       expect(err.read).to be_empty
       expect(out.read).to eq("out\n")
     end