logging: avoid '%!F(MISSING)' in logs (#6555)

When only passing a single string into a Printf-style function, any %-format
specs in it will be filled accordingly; but there will be no arguments to use.

So now, we'll avoid calling the `{Debug,Info,Warn,Error}f` functions when we
lack arguments.

Signed-off-by: Stephan Renatus <stephan@styra.com>

logging/logging.go

@@ -126,21 +126,37 @@ func (l *StandardLogger) GetLevel() Level {
 
 // Debug logs at debug level
 func (l *StandardLogger) Debug(fmt string, a ...interface{}) {
+	if len(a) == 0 {
+		l.logger.WithFields(l.getFields()).Debug(fmt)
+		return
+	}
 	l.logger.WithFields(l.getFields()).Debugf(fmt, a...)
 }
 
 // Info logs at info level
 func (l *StandardLogger) Info(fmt string, a ...interface{}) {
+	if len(a) == 0 {
+		l.logger.WithFields(l.getFields()).Info(fmt)
+		return
+	}
 	l.logger.WithFields(l.getFields()).Infof(fmt, a...)
 }
 
 // Error logs at error level
 func (l *StandardLogger) Error(fmt string, a ...interface{}) {
+	if len(a) == 0 {
+		l.logger.WithFields(l.getFields()).Error(fmt)
+		return
+	}
 	l.logger.WithFields(l.getFields()).Errorf(fmt, a...)
 }
 
 // Warn logs at warn level
 func (l *StandardLogger) Warn(fmt string, a ...interface{}) {
+	if len(a) == 0 {
+		l.logger.WithFields(l.getFields()).Warn(fmt)
+		return
+	}
 	l.logger.WithFields(l.getFields()).Warnf(fmt, a...)
 }
 

logging/logging_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/rand"
+	"net/url"
 	"strings"
 	"testing"
 
@@ -45,6 +46,38 @@ func TestCaptureWarningWithErrorSet(t *testing.T) {
 	}
 }
 
+func TestNoFormattingForSingleString(t *testing.T) {
+	buf := bytes.Buffer{}
+	logger := New()
+	logger.SetOutput(&buf)
+	logger.SetLevel(Debug)
+
+	// NOTE(sr): This construction is somewhat realistic: If we fed logger.Error()
+	// a format string but no args, the golang linters would yell. The indirection
+	// taken here is enough to not trigger linters.
+	x := url.PathEscape("/foo/bar/bar")
+	logger.Debug(x)
+	logger.Info(x)
+	logger.Warn(x)
+	logger.Error(x)
+
+	exp := `"%2Ffoo%2Fbar%2Fbar"`
+	expected := []string{
+		`level=error msg=` + exp,
+		`level=warning msg=` + exp,
+		`level=info msg=` + exp,
+		`level=debug msg=` + exp,
+	}
+	for _, exp := range expected {
+		if !strings.Contains(buf.String(), exp) {
+			t.Errorf("expected string %q not found in logs", exp)
+		}
+	}
+	if t.Failed() {
+		t.Logf("actual output:\n%s", buf.String())
+	}
+}
+
 func TestWithFieldsOverrides(t *testing.T) {
 	logger := New().
 		WithFields(map[string]interface{}{"context": "contextvalue"}).