fix: Send log messages to STDERR (#583)

And only useful outputs from helm commands like ones from `helm template` to STDOUT.

Fixes #551

helmexec/runner.go

@@ -1,6 +1,10 @@
 package helmexec
 
 import (
+	"bytes"
+	"errors"
+	"fmt"
+	"go.uber.org/zap"
 	"os"
 	"os/exec"
 	"strings"
@@ -19,6 +23,8 @@ type Runner interface {
 // ShellRunner implemention for shell commands
 type ShellRunner struct {
 	Dir string
+
+	logger *zap.SugaredLogger
 }
 
 // Execute a shell command
@@ -26,7 +32,39 @@ func (shell ShellRunner) Execute(cmd string, args []string, env map[string]strin
 	preparedCmd := exec.Command(cmd, args...)
 	preparedCmd.Dir = shell.Dir
 	preparedCmd.Env = mergeEnv(os.Environ(), env)
-	return preparedCmd.CombinedOutput()
+	return combinedOutput(preparedCmd, shell.logger)
+}
+
+func combinedOutput(c *exec.Cmd, logger *zap.SugaredLogger) ([]byte, error) {
+	if c.Stdout != nil {
+		return nil, errors.New("exec: Stdout already set")
+	}
+	if c.Stderr != nil {
+		return nil, errors.New("exec: Stderr already set")
+	}
+	var stdout bytes.Buffer
+	var stderr bytes.Buffer
+	c.Stdout = &stdout
+	c.Stderr = &stderr
+	err := c.Run()
+
+	o := stdout.Bytes()
+	e := stderr.Bytes()
+
+	if len(e) > 0 {
+		logger.Debugf("%s\n", e)
+	}
+
+	if err != nil {
+		// TrimSpace is necessary, because otherwise helmfile prints the redundant new-lines after each error like:
+		//
+		//   err: release "envoy2" in "helmfile.yaml" failed: exit status 1: Error: could not find a ready tiller pod
+		//   <redundant new line!>
+		//   err: release "envoy" in "helmfile.yaml" failed: exit status 1: Error: could not find a ready tiller pod
+		err = fmt.Errorf("%v: %s", err, strings.TrimSpace(string(e)))
+	}
+
+	return o, err
 }
 
 func mergeEnv(orig []string, new map[string]string) []string {