Add acccess log formatter (#3458)

xavivars · web-flow · commit ec2a21e1d7e7 · 2025-03-28T20:01:50.000+01:00
This PR adds the ability to use a custom access log formatter. Currently, accessLogs can be configured in three ways: - If `AccessLogJSONEnabled` is defined, you can either pass a JSONFormatter, or have Skipper initialize one. - If `AccessLogJSONEnabled` is not defined, it uses a basic text formatter. There are cases (for example, when configuring NewRelic logrus integration, [see here](https://github.com/newrelic/go-agent/tree/master/v3/integrations/logcontext-v2/nrlogrus), that a custom formatter is needed, but current logger can't be configured. My initial approach was to change type on `AccessLogJsonFormatter` (typed parameter in Skipper options) to be able to pass any logrus.Formatter (interface), as it wouldn't break anything. But it would be weird to have something called `JSONFormatter` that may not be a JSON formatter. Signed-off-by: Xavi Ivars <xavi.ivars@gmail.com>
diff --git a/logging/log.go b/logging/log.go
@@ -44,7 +44,11 @@ type Options struct {
 
 	// AccessLogJsonFormatter, when set and JSON logging is enabled, is passed along to to the underlying
 	// Logrus logger for access logs. To enable structured logging, use AccessLogJSONEnabled.
+	// Deprecated: use [AccessLogFormatter].
 	AccessLogJsonFormatter *logrus.JSONFormatter
+
+	// AccessLogFormatter, when set is passed along to the underlying Logrus logger for access logs.
+	AccessLogFormatter logrus.Formatter
 }
 
 func (f *prefixFormatter) Format(e *logrus.Entry) ([]byte, error) {
@@ -74,7 +78,9 @@ func initApplicationLog(o Options) {
 
 func initAccessLog(o Options) {
 	l := logrus.New()
-	if o.AccessLogJSONEnabled {
+	if o.AccessLogFormatter != nil {
+		l.Formatter = o.AccessLogFormatter
+	} else if o.AccessLogJSONEnabled {
 		if o.AccessLogJsonFormatter != nil {
 			l.Formatter = o.AccessLogJsonFormatter
 		} else {
diff --git a/logging/log_test.go b/logging/log_test.go
@@ -121,3 +121,28 @@ func TestApplicationLogJSONWithCustomFormatter(t *testing.T) {
 		t.Errorf("unexpected field count")
 	}
 }
+
+type customFormatter struct {
+	innerFormatter *log.JSONFormatter
+}
+
+func (f *customFormatter) Format(entry *log.Entry) ([]byte, error) {
+	originalBytes, err := f.innerFormatter.Format(entry)
+	if err != nil {
+		return nil, err
+	}
+	newBytes := bytes.NewBuffer(originalBytes)
+	newBytes.WriteString(" - Custom Suffix")
+	return newBytes.Bytes(), nil
+}
+
+func TestAccessLogFormatterTakesPrecedence(t *testing.T) {
+	var buf bytes.Buffer
+	f := &customFormatter{innerFormatter: &log.JSONFormatter{}}
+	Init(Options{AccessLogOutput: &buf, AccessLogFormatter: f})
+	LogAccess(&AccessEntry{StatusCode: http.StatusTeapot}, nil)
+	s := buf.String()
+	if !strings.Contains(s, " - Custom Suffix") {
+		t.Error("failed to use custom access log output")
+	}
+}
diff --git a/skipper.go b/skipper.go
@@ -619,10 +619,14 @@ type Options struct {
 	// from the request URI in the access logs.
 	AccessLogStripQuery bool
 
-	// AccessLogJsonFormatter, when set and JSON logging is enabled, is passed along to to the underlying
+	// AccessLogJsonFormatter, when set and JSON logging is enabled, is passed along to the underlying
 	// Logrus logger for access logs. To enable structured logging, use AccessLogJSONEnabled.
+	// Deprecated: use [AccessLogFormatter].
 	AccessLogJsonFormatter *log.JSONFormatter
 
+	// AccessLogFormatter, when set is passed along to the underlying Logrus logger for access logs.
+	AccessLogFormatter log.Formatter
+
 	DebugListener string
 
 	// Path of certificate(s) when using TLS, multiple may be given comma separated
@@ -1177,6 +1181,7 @@ func initLog(o Options) error {
 		AccessLogJSONEnabled:        o.AccessLogJSONEnabled,
 		AccessLogStripQuery:         o.AccessLogStripQuery,
 		AccessLogJsonFormatter:      o.AccessLogJsonFormatter,
+		AccessLogFormatter:          o.AccessLogFormatter,
 	})
 
 	return nil
