feat: allow logging output to use JSON formatter (#1705)

Co-authored-by: nils måsén <nils@piksel.se>

internal/flags/flags.go

@@ -85,6 +85,12 @@ func RegisterSystemFlags(rootCmd *cobra.Command) {
 		envBool("WATCHTOWER_LABEL_ENABLE"),
 		"Watch containers where the com.centurylinklabs.watchtower.enable label is true")
 
+	flags.StringP(
+		"log-format",
+		"l",
+		viper.GetString("WATCHTOWER_LOG_FORMAT"),
+		"Sets what logging format to use for console output. Possible values: Auto, LogFmt, Pretty, JSON")
+
 	flags.BoolP(
 		"debug",
 		"d",
@@ -409,6 +415,7 @@ func SetDefaults() {
 	viper.SetDefault("WATCHTOWER_NOTIFICATION_EMAIL_SUBJECTTAG", "")
 	viper.SetDefault("WATCHTOWER_NOTIFICATION_SLACK_IDENTIFIER", "watchtower")
 	viper.SetDefault("WATCHTOWER_LOG_LEVEL", "info")
+	viper.SetDefault("WATCHTOWER_LOG_FORMAT", "auto")
 }
 
 // EnvConfig translates the command-line options into environment variables
@@ -611,6 +618,46 @@ func ProcessFlagAliases(flags *pflag.FlagSet) {
 
 }
 
+// SetupLogging reads only the flags that is needed to set up logging and applies them to the global logger
+func SetupLogging(f *pflag.FlagSet) error {
+	logFormat, _ := f.GetString(`log-format`)
+	noColor, _ := f.GetBool("no-color")
+
+	switch strings.ToLower(logFormat) {
+	case "auto":
+		// This will either use the "pretty" or "logfmt" format, based on whether the standard out is connected to a TTY
+		log.SetFormatter(&log.TextFormatter{
+			DisableColors: noColor,
+			// enable logrus built-in support for https://bixense.com/clicolors/
+			EnvironmentOverrideColors: true,
+		})
+	case "json":
+		log.SetFormatter(&log.JSONFormatter{})
+	case "logfmt":
+		log.SetFormatter(&log.TextFormatter{
+			DisableColors: true,
+			FullTimestamp: true,
+		})
+	case "pretty":
+		log.SetFormatter(&log.TextFormatter{
+			// "Pretty" format combined with `--no-color` will only change the timestamp to the time since start
+			ForceColors:   !noColor,
+			FullTimestamp: false,
+		})
+	default:
+		return fmt.Errorf("invalid log format: %s", logFormat)
+	}
+
+	rawLogLevel, _ := f.GetString(`log-level`)
+	if logLevel, err := log.ParseLevel(rawLogLevel); err != nil {
+		return fmt.Errorf("invalid log level: %e", err)
+	} else {
+		log.SetLevel(logLevel)
+	}
+
+	return nil
+}
+
 func flagIsEnabled(flags *pflag.FlagSet, name string) bool {
 	value, err := flags.GetBool(name)
 	if err != nil {

internal/flags/flags_test.go

@@ -177,6 +177,57 @@ func TestProcessFlagAliasesLogLevelFromEnvironment(t *testing.T) {
 	assert.Equal(t, `debug`, logLevel)
 }
 
+func TestLogFormatFlag(t *testing.T) {
+	cmd := new(cobra.Command)
+
+	SetDefaults()
+	RegisterDockerFlags(cmd)
+	RegisterSystemFlags(cmd)
+
+	// Ensure the default value is Auto
+	require.NoError(t, cmd.ParseFlags([]string{}))
+	require.NoError(t, SetupLogging(cmd.Flags()))
+	assert.IsType(t, &logrus.TextFormatter{}, logrus.StandardLogger().Formatter)
+
+	// Test JSON format
+	require.NoError(t, cmd.ParseFlags([]string{`--log-format`, `JSON`}))
+	require.NoError(t, SetupLogging(cmd.Flags()))
+	assert.IsType(t, &logrus.JSONFormatter{}, logrus.StandardLogger().Formatter)
+
+	// Test Pretty format
+	require.NoError(t, cmd.ParseFlags([]string{`--log-format`, `pretty`}))
+	require.NoError(t, SetupLogging(cmd.Flags()))
+	assert.IsType(t, &logrus.TextFormatter{}, logrus.StandardLogger().Formatter)
+	textFormatter, ok := (logrus.StandardLogger().Formatter).(*logrus.TextFormatter)
+	assert.True(t, ok)
+	assert.True(t, textFormatter.ForceColors)
+	assert.False(t, textFormatter.FullTimestamp)
+
+	// Test LogFmt format
+	require.NoError(t, cmd.ParseFlags([]string{`--log-format`, `logfmt`}))
+	require.NoError(t, SetupLogging(cmd.Flags()))
+	textFormatter, ok = (logrus.StandardLogger().Formatter).(*logrus.TextFormatter)
+	assert.True(t, ok)
+	assert.True(t, textFormatter.DisableColors)
+	assert.True(t, textFormatter.FullTimestamp)
+
+	// Test invalid format
+	require.NoError(t, cmd.ParseFlags([]string{`--log-format`, `cowsay`}))
+	require.Error(t, SetupLogging(cmd.Flags()))
+}
+
+func TestLogLevelFlag(t *testing.T) {
+	cmd := new(cobra.Command)
+
+	SetDefaults()
+	RegisterDockerFlags(cmd)
+	RegisterSystemFlags(cmd)
+
+	// Test invalid format
+	require.NoError(t, cmd.ParseFlags([]string{`--log-level`, `gossip`}))
+	require.Error(t, SetupLogging(cmd.Flags()))
+}
+
 func TestProcessFlagAliasesSchedAndInterval(t *testing.T) {
 	logrus.StandardLogger().ExitFunc = func(_ int) { panic(`FATAL`) }
 	cmd := new(cobra.Command)