Feature: combine_output to control stdout/stderr

This enables an often-requested feature to separate stderr
from stdout within function responses. New flag combine_output is on
by default to match existing behaviour. When combine_output is set
to false it redirects stderr to the container logs rather than
combining it into the function response.

Tested with unit tests for default behaviour and new behaviour.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

watchdog/requesthandler_test.go

@@ -7,8 +7,10 @@ import (
 	"bytes"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"strings"
 	"testing"
 	"time"
@@ -101,6 +103,96 @@ func TestHandler_HasCustomHeaderInFunction_WithCgiMode_AndBody(t *testing.T) {
 	}
 }
 
+func TestHandler_StderrWritesToStderr_CombinedOutput_False(t *testing.T) {
+	rr := httptest.NewRecorder()
+
+	b := bytes.NewBuffer([]byte{})
+	log.SetOutput(b)
+
+	body := ""
+	req, err := http.NewRequest("POST", "/", bytes.NewBufferString(body))
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	config := WatchdogConfig{
+		faasProcess:   "man badtopic",
+		cgiHeaders:    true,
+		combineOutput: false,
+	}
+
+	handler := makeRequestHandler(&config)
+	handler(rr, req)
+
+	required := http.StatusInternalServerError
+
+	if status := rr.Code; status != required {
+		t.Errorf("handler returned wrong status code - got: %v, want: %v",
+			status, required)
+	}
+
+	log.SetOutput(os.Stderr)
+
+	stderrBytes, _ := ioutil.ReadAll(b)
+	stderrVal := string(stderrBytes)
+
+	if strings.Contains(stderrVal, "No manual entry for") == false {
+		t.Logf("Stderr should have contained error from function \"No manual entry for\", but was: %s", stderrVal)
+		t.Fail()
+	}
+}
+
+func TestHandler_StderrWritesToResponse_CombinedOutput_True(t *testing.T) {
+	rr := httptest.NewRecorder()
+
+	b := bytes.NewBuffer([]byte{})
+	log.SetOutput(b)
+
+	body := ""
+	req, err := http.NewRequest("POST", "/", bytes.NewBufferString(body))
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	config := WatchdogConfig{
+		faasProcess:   "man badtopic",
+		cgiHeaders:    true,
+		combineOutput: true,
+	}
+
+	handler := makeRequestHandler(&config)
+	handler(rr, req)
+
+	required := http.StatusInternalServerError
+
+	if status := rr.Code; status != required {
+		t.Errorf("handler returned wrong status code - got: %v, want: %v",
+			status, required)
+	}
+
+	log.SetOutput(os.Stderr)
+
+	stderrBytes, _ := ioutil.ReadAll(b)
+	stderrVal := string(stderrBytes)
+
+	if strings.Contains(stderrVal, "No manual entry for") {
+		t.Logf("stderr should have not included any function errors, but did")
+		t.Fail()
+	}
+
+	bodyBytes, _ := ioutil.ReadAll(rr.Body)
+	bodyStr := string(bodyBytes)
+	want := `exit status 1
+No manual entry for badtopic`
+	if strings.Contains(bodyStr, want) == false {
+		t.Logf("response want: %s, got: %s", want, bodyStr)
+		t.Fail()
+	}
+
+}
+
 func TestHandler_DoesntHaveCustomHeaderInFunction_WithoutCgi_Mode(t *testing.T) {
 	rr := httptest.NewRecorder()