Forward standard error stream from remote execution

spec/producer/core/actions/shell_command_spec.rb

@@ -19,6 +19,15 @@ module Producer::Core
           sh.apply
           expect(output).to eq "#{command_args}\n"
         end
+
+        context 'when content is written to standard error' do
+          let(:command) { "echo #{command_args} >&2" }
+
+          it 'writes errors to given error stream' do
+            sh.apply
+            expect(error_output).to eq "#{command_args}\n"
+          end
+        end
       end
     end
   end

spec/producer/core/cli_spec.rb

@@ -12,7 +12,9 @@ module Producer::Core
     let(:stdout)      { StringIO.new }
     let(:stderr)      { StringIO.new }
 
-    subject(:cli)     { CLI.new(arguments, stdin: stdin, stdout: stdout) }
+    subject(:cli)     { described_class.new(
+                          arguments,
+                          stdin: stdin, stdout: stdout, stderr: stderr) }
 
     describe '.run!' do
       let(:cli) { double('cli').as_null_object }
@@ -118,6 +120,10 @@ module Producer::Core
       it 'assigns CLI stdout as the env output' do
         expect(cli.env.output).to be stdout
       end
+
+      it 'assigns CLI stderr as the env error output' do
+        expect(cli.env.error_output).to be stderr
+      end
     end
 
     describe '#parse_arguments!' do

spec/producer/core/env_spec.rb

@@ -10,6 +10,10 @@ module Producer::Core
         expect(env.input).to be $stdin
       end
 
+      it 'assigns $stderr as the default error output' do
+        expect(env.error_output).to be $stderr
+      end
+
       it 'assigns no default target' do
         expect(env.target).not_to be
       end
@@ -51,6 +55,15 @@ module Producer::Core
         end
       end
 
+      context 'when error output is given as argument' do
+        let(:error_output)  { StringIO.new }
+        subject(:env)       { described_class.new(error_output: error_output) }
+
+        it 'assigns the given error output' do
+          expect(env.error_output).to be error_output
+        end
+      end
+
       context 'when remote is given as argument' do
         let(:remote)  { double 'remote' }
         subject(:env) { described_class.new(remote: remote) }

spec/producer/core/remote_spec.rb

@@ -106,6 +106,16 @@ module Producer::Core
         expect(output.string).to eq arguments
       end
 
+      it 'writes command error output to provided error output' do
+        error_output = StringIO.new
+        story_with_new_channel do |ch|
+          ch.sends_exec command
+          ch.gets_extended_data arguments
+        end
+        remote.execute command, output, error_output
+        expect(error_output.string).to eq arguments
+      end
+
       context 'when command execution fails' do
         before do
           story_with_new_channel do |ch|

spec/support/shared_action.rb

@@ -29,6 +29,12 @@ module Producer::Core
       end
     end
 
+    describe '#error_output' do
+      it 'returns env error output' do
+        expect(action.error_output).to be env.error_output
+      end
+    end
+
     describe '#remote' do
       it 'returns env remote' do
         expect(action.remote).to be env.remote

spec/support/test_env_helpers.rb

@@ -9,6 +9,10 @@ module TestEnvHelpers
     env.output.string
   end
 
+  def error_output
+    env.error_output.string
+  end
+
   def remote_fs
     env.remote.fs
   end
@@ -17,14 +21,18 @@ module TestEnvHelpers
     opts = { expected_from: caller.first }
     RSpec::Mocks
       .expect_message(env.remote, :execute, opts)
-      .with(command, env.output)
+      .with(command, env.output, env.error_output)
   end
 
 
   private
 
   def build_env
-    Producer::Core::Env.new(output: StringIO.new, remote: build_remote)
+    Producer::Core::Env.new(
+      output:       StringIO.new,
+      error_output: StringIO.new,
+      remote:       build_remote
+    )
   end
 
   def build_remote