Improve stderr logging for PostCSS and simlilar

Fixes #7584

common/loggers/loggers.go

@@ -39,6 +39,23 @@ func init() {
 	jww.SetLogListeners(jww.LogCounter(GlobalErrorCounter, jww.LevelError))
 }
 
+func LoggerToWriterWithPrefix(logger *log.Logger, prefix string) io.Writer {
+	return prefixWriter{
+		logger: logger,
+		prefix: prefix,
+	}
+}
+
+type prefixWriter struct {
+	logger *log.Logger
+	prefix string
+}
+
+func (w prefixWriter) Write(p []byte) (n int, err error) {
+	w.logger.Printf("%s: %s", w.prefix, p)
+	return len(p), nil
+}
+
 // Logger wraps a *loggers.Logger and some other related logging state.
 type Logger struct {
 	*jww.Notepad
@@ -93,17 +110,32 @@ func NewLogger(stdoutThreshold, logThreshold jww.Threshold, outHandle, logHandle
 
 // NewDebugLogger is a convenience function to create a debug logger.
 func NewDebugLogger() *Logger {
-	return newBasicLogger(jww.LevelDebug)
+	return NewBasicLogger(jww.LevelDebug)
 }
 
 // NewWarningLogger is a convenience function to create a warning logger.
 func NewWarningLogger() *Logger {
-	return newBasicLogger(jww.LevelWarn)
+	return NewBasicLogger(jww.LevelWarn)
+}
+
+// NewInfoLogger is a convenience function to create a info logger.
+func NewInfoLogger() *Logger {
+	return NewBasicLogger(jww.LevelInfo)
 }
 
 // NewErrorLogger is a convenience function to create an error logger.
 func NewErrorLogger() *Logger {
-	return newBasicLogger(jww.LevelError)
+	return NewBasicLogger(jww.LevelError)
+}
+
+// NewBasicLogger creates a new basic logger writing to Stdout.
+func NewBasicLogger(t jww.Threshold) *Logger {
+	return newLogger(t, jww.LevelError, os.Stdout, ioutil.Discard, false)
+}
+
+// NewBasicLoggerForWriter creates a new basic logger writing to w.
+func NewBasicLoggerForWriter(t jww.Threshold, w io.Writer) *Logger {
+	return newLogger(t, jww.LevelError, w, ioutil.Discard, false)
 }
 
 var (
@@ -215,7 +247,3 @@ func newLogger(stdoutThreshold, logThreshold jww.Threshold, outHandle, logHandle
 		errors:       errorBuff,
 	}
 }
-
-func newBasicLogger(t jww.Threshold) *Logger {
-	return newLogger(t, jww.LevelError, os.Stdout, ioutil.Discard, false)
-}

common/loggers/loggers_test.go

@@ -14,6 +14,9 @@
 package loggers
 
 import (
+	"bytes"
+	"fmt"
+	"log"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
@@ -30,3 +33,17 @@ func TestLogger(t *testing.T) {
 	c.Assert(l.ErrorCounter.Count(), qt.Equals, uint64(2))
 
 }
+
+func TestLoggerToWriterWithPrefix(t *testing.T) {
+	c := qt.New(t)
+
+	var b bytes.Buffer
+
+	logger := log.New(&b, "", 0)
+
+	w := LoggerToWriterWithPrefix(logger, "myprefix")
+
+	fmt.Fprint(w, "Hello Hugo!")
+
+	c.Assert(b.String(), qt.Equals, "myprefix: Hello Hugo!\n")
+}