helpers: Handle error on output buffer write fail

If an error occurred when running an external program, the user should
receive a full report of that program's output. If mixer is unable to
write that output, the user should get and error indicating the problem
instead of an incomplete output, making it easier to debug the problem.

Signed-off-by: Rodrigo Chiossi <rodrigo.chiossi@intel.com>

helpers/helpers.go

@@ -27,6 +27,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"log"
 	"math/big"
 	"net/http"
 	"os"
@@ -50,8 +51,7 @@ func CreateCertTemplate() *x509.Certificate {
 	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
 	serialnumber, err := rand.Int(rand.Reader, serialNumberLimit)
 	if err != nil {
-		fmt.Println("ERROR: Failed to generate serial number")
-		PrintError(err)
+		log.Println(errors.Wrap(err, "ERROR: Failed to generate serial number"))
 	}
 
 	template := x509.Certificate{
@@ -73,8 +73,7 @@ func CreateCertTemplate() *x509.Certificate {
 func CreateKeyPair() (*rsa.PrivateKey, error) {
 	rootKey, err := rsa.GenerateKey(rand.Reader, 4096)
 	if err != nil {
-		fmt.Println("ERROR: Failed to generate random key")
-		PrintError(err)
+		log.Println(errors.Wrap(err, "ERROR: Failed to generate random key"))
 	}
 	return rootKey, nil
 }
@@ -321,22 +320,23 @@ func RunCommandOutputEnv(cmdname string, args []string, envs []string) (*bytes.B
 	cmd.Stdout = &outBuf
 	cmd.Stderr = &errBuf
 	cmd.Env = append(os.Environ(), envs...)
-	err := cmd.Run()
+	runError := cmd.Run()
 
-	if err != nil {
+	if runError != nil {
 		var buf bytes.Buffer
-		fmt.Fprintf(&buf, "failed to execute %s", strings.Join(cmd.Args, " "))
+		logger := log.New(&buf, "", log.Ldate|log.Ltime)
+		logger.Printf("failed to execute %s", strings.Join(cmd.Args, " "))
 		if outBuf.Len() > 0 {
-			fmt.Fprintf(&buf, "\nSTDOUT:\n%s", outBuf.Bytes())
+			logger.Printf("\nSTDOUT:\n%s", outBuf.Bytes())
 		}
 		if errBuf.Len() > 0 {
-			fmt.Fprintf(&buf, "\nSTDERR:\n%s", errBuf.Bytes())
+			logger.Printf("\nSTDERR:\n%s", errBuf.Bytes())
 		}
 		if outBuf.Len() > 0 || errBuf.Len() > 0 {
 			// Finish without a newline to wrap well with the err.
-			fmt.Fprintf(&buf, "failed to execute")
+			logger.Printf("failed to execute")
 		}
-		return &outBuf, errors.Wrap(err, buf.String())
+		return &outBuf, errors.Wrap(runError, buf.String())
 	}
 	return &outBuf, nil
 }