Add logg.ErrStopLogEntry

As a way to stop a log entry from being passed on in the log handler chain

handlers/multi/multi_test.go

@@ -51,3 +51,26 @@ func TestMultiModifyEntry(t *testing.T) {
 	qt.Assert(t, b.Entries[0].Fields[0].Name, qt.Equals, "initial")
 	qt.Assert(t, b.Entries[0].Fields[1].Name, qt.Equals, "added")
 }
+
+func TestStopEntry(t *testing.T) {
+	var a logg.HandlerFunc = func(e *logg.Entry) error {
+		if e.Fields[0].Value == "v2" {
+			return logg.ErrStopLogEntry
+		}
+		return nil
+	}
+
+	b := memory.New()
+
+	l := logg.New(
+		logg.Options{
+			Level:   logg.LevelInfo,
+			Handler: multi.New(a, b),
+		})
+
+	l.WithLevel(logg.LevelInfo).WithField("v", "v1").Log(logg.String("text"))
+	l.WithLevel(logg.LevelInfo).WithField("v", "v2").Log(logg.String("text"))
+	l.WithLevel(logg.LevelInfo).WithField("v", "v3").Log(logg.String("text"))
+
+	qt.Assert(t, b.Entries, qt.HasLen, 2)
+}

logger.go

@@ -137,6 +137,10 @@ func (l *logger) WithError(err error) *Entry {
 	return NewEntry(l).WithError(err)
 }
 
+// ErrStopLogEntry is a sentinel error that can be returned from a
+// handler to stop the entry from being passed to the next handler.
+var ErrStopLogEntry = fmt.Errorf("stop log entry")
+
 // log the message, invoking the handler.
 func (l *logger) log(e *Entry, s fmt.Stringer) {
 	if e.Level < l.Level {
@@ -148,6 +152,8 @@ func (l *logger) log(e *Entry, s fmt.Stringer) {
 	e.finalize(finalized, s.String())
 
 	if err := l.Handler.HandleLog(finalized); err != nil {
-		stdlog.Printf("error logging: %s", err)
+		if err != ErrStopLogEntry {
+			stdlog.Printf("error logging: %s", err)
+		}
 	}
 }