Implement TextUnmarshaller interface for Level type

Since the implementation of the TextMarshaller interface
we could not unmarshal previously json marshalled Level value.
Fixes #873

level_test.go

@@ -0,0 +1,62 @@
+package logrus_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+
+	"github.com/sirupsen/logrus"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLevelJsonEncoding(t *testing.T) {
+	type X struct {
+		Level logrus.Level
+	}
+
+	var x X
+	x.Level = logrus.WarnLevel
+	var buf bytes.Buffer
+	enc := json.NewEncoder(&buf)
+	require.NoError(t, enc.Encode(x))
+	dec := json.NewDecoder(&buf)
+	var y X
+	require.NoError(t, dec.Decode(&y))
+}
+
+func TestLevelUnmarshalText(t *testing.T) {
+	var u logrus.Level
+	for _, level := range logrus.AllLevels {
+		t.Run(level.String(), func(t *testing.T) {
+			require.NoError(t, u.UnmarshalText([]byte(level.String())))
+			require.Equal(t, level, u)
+		})
+	}
+	t.Run("invalid", func(t *testing.T) {
+		require.Error(t, u.UnmarshalText([]byte("invalid")))
+	})
+}
+
+func TestLevelMarshalText(t *testing.T) {
+	levelStrings := []string{
+		"panic",
+		"fatal",
+		"error",
+		"warning",
+		"info",
+		"debug",
+		"trace",
+	}
+	for idx, val := range logrus.AllLevels {
+		level := val
+		t.Run(level.String(), func(t *testing.T) {
+			var cmp logrus.Level
+			b, err := level.MarshalText()
+			require.NoError(t, err)
+			require.Equal(t, levelStrings[idx], string(b))
+			err = cmp.UnmarshalText(b)
+			require.NoError(t, err)
+			require.Equal(t, level, cmp)
+		})
+	}
+}

logrus.go

@@ -14,24 +14,11 @@ type Level uint32
 
 // Convert the Level to a string. E.g. PanicLevel becomes "panic".
 func (level Level) String() string {
-	switch level {
-	case TraceLevel:
-		return "trace"
-	case DebugLevel:
-		return "debug"
-	case InfoLevel:
-		return "info"
-	case WarnLevel:
-		return "warning"
-	case ErrorLevel:
-		return "error"
-	case FatalLevel:
-		return "fatal"
-	case PanicLevel:
-		return "panic"
+	if b, err := level.MarshalText(); err == nil {
+		return string(b)
+	} else {
+		return "unknown"
 	}
-
-	return "unknown"
 }
 
 // ParseLevel takes a string level and returns the Logrus log level constant.
@@ -69,6 +56,27 @@ func (level *Level) UnmarshalText(text []byte) error {
 	return nil
 }
 
+func (level Level) MarshalText() ([]byte, error) {
+	switch level {
+	case TraceLevel:
+		return []byte("trace"), nil
+	case DebugLevel:
+		return []byte("debug"), nil
+	case InfoLevel:
+		return []byte("info"), nil
+	case WarnLevel:
+		return []byte("warning"), nil
+	case ErrorLevel:
+		return []byte("error"), nil
+	case FatalLevel:
+		return []byte("fatal"), nil
+	case PanicLevel:
+		return []byte("panic"), nil
+	}
+
+	return nil, fmt.Errorf("not a valid lorus level %q", level)
+}
+
 // A constant exposing all logging levels
 var AllLevels = []Level{
 	PanicLevel,

logrus_test.go

@@ -508,19 +508,6 @@ func TestParseLevel(t *testing.T) {
 	assert.Equal(t, "not a valid logrus Level: \"invalid\"", err.Error())
 }
 
-func TestUnmarshalText(t *testing.T) {
-	var u Level
-	for _, level := range AllLevels {
-		t.Run(level.String(), func(t *testing.T) {
-			assert.NoError(t, u.UnmarshalText([]byte(level.String())))
-			assert.Equal(t, level, u)
-		})
-	}
-	t.Run("invalid", func(t *testing.T) {
-		assert.Error(t, u.UnmarshalText([]byte("invalid")))
-	})
-}
-
 func TestGetSetLevelRace(t *testing.T) {
 	wg := sync.WaitGroup{}
 	for i := 0; i < 100; i++ {