cmd: configurable logrus formatters

We lost the nice context-stack [jobname][taskname][...] at the beginning
of each log line when switching to logrus.

Define some field names that define these contexts.
Write a human-friendly formatter that presents these field names like
the solution we had before logrus.

Write some other formatters for logfmt and json output along the way.

Limit ourselves to stdout logging for now.

cmd/logrus.go

@@ -2,52 +2,123 @@ package cmd
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
-	"os"
+	"strings"
+	"time"
 )
 
-type CLIFormatter struct {
+const (
+	logJobField     string = "job"
+	logTaskField    string = "task"
+	logFSField      string = "filesystem"
+	logMapFromField string = "map_from"
+	logMapToField   string = "map_to"
+	logIncFromField string = "inc_from"
+	logIncToField   string = "inc_to"
+)
+
+type NoFormatter struct{}
+
+func (f NoFormatter) Format(e *logrus.Entry) ([]byte, error) {
+	return []byte(e.Message), nil
 }
 
-func (f CLIFormatter) Format(e *logrus.Entry) (out []byte, err error) {
-	var buf bytes.Buffer
-	fmt.Fprintf(&buf, "%s\n", e.Message)
-	return buf.Bytes(), nil
-}
+type HumanFormatter struct{}
 
-var stdhookStderrLevels []logrus.Level = []logrus.Level{
-	logrus.PanicLevel, logrus.FatalLevel, logrus.ErrorLevel, logrus.WarnLevel,
-}
-
-type Stdhook struct {
-}
-
-func NewStdHook() *Stdhook {
-	return &Stdhook{}
-}
-
-func (h *Stdhook) Levels() []logrus.Level {
-	// Accept all so we can filter the output later
-	return []logrus.Level{
-		logrus.PanicLevel, logrus.FatalLevel, logrus.ErrorLevel, logrus.WarnLevel,
-		logrus.InfoLevel, logrus.DebugLevel,
+func (f HumanFormatter) shortLevel(l logrus.Level) string {
+	switch l {
+	case logrus.DebugLevel:
+		return "DBG"
+	case logrus.InfoLevel:
+		return "INF"
+	case logrus.WarnLevel:
+		return "WRN"
+	case logrus.ErrorLevel:
+		return "ERR"
+	case logrus.PanicLevel:
+		return "PNC"
 	}
+	panic("incomplete implementation")
 }
 
-func (h *Stdhook) Fire(entry *logrus.Entry) error {
-	s, err := entry.String()
-	if err != nil {
-		return err
-	}
-	for _, l := range stdhookStderrLevels {
-		if l == entry.Level {
-			fmt.Fprint(os.Stderr, s)
-			return nil
+func (f HumanFormatter) Format(e *logrus.Entry) (out []byte, err error) {
+
+	var line bytes.Buffer
+
+	fmt.Fprintf(&line, "[%s]", f.shortLevel(e.Level))
+
+	prefixFields := []string{logJobField, logTaskField, logFSField}
+	prefixed := make(map[string]bool, len(prefixFields)+2)
+	for _, field := range prefixFields {
+		val, ok := e.Data[field].(string)
+		if ok {
+			fmt.Fprintf(&line, "[%s]", val)
+			prefixed[field] = true
+		} else {
+			break
 		}
 	}
-	fmt.Fprint(os.Stdout, s)
-	return nil
+	// even more prefix fields
+	mapFrom, mapFromOk := e.Data[logMapFromField].(string)
+	mapTo, mapToOk := e.Data[logMapToField].(string)
+	if mapFromOk && mapToOk {
+		fmt.Fprintf(&line, "[%s => %s]", mapFrom, mapTo)
+		prefixed[logMapFromField], prefixed[logMapToField] = true, true
+	}
+	incFrom, incFromOk := e.Data[logIncFromField].(string)
+	incTo, incToOk := e.Data[logIncToField].(string)
+	if incFromOk && incToOk {
+		fmt.Fprintf(&line, "[%s => %s]", incFrom, incTo)
+		prefixed[logIncFromField], prefixed[logIncToField] = true, true
+	}
+
+	fmt.Fprintf(&line, ": %s", e.Message)
+
+	for field, value := range e.Data {
+
+		if prefixed[field] {
+			continue
+		}
+
+		if strings.ContainsAny(field, " \t") {
+			return nil, errors.Errorf("field must not contain whitespace: '%s'", field)
+		}
+		fmt.Fprintf(&line, " %s=\"%s\"", field, value)
+	}
+
+	fmt.Fprintf(&line, "\n")
+
+	return line.Bytes(), nil
+}
+
+type JSONFormatter struct{}
+
+func (f JSONFormatter) Format(e *logrus.Entry) ([]byte, error) {
+	data := make(logrus.Fields, len(e.Data)+3)
+	for k, v := range e.Data {
+		switch v := v.(type) {
+		case error:
+			// Otherwise errors are ignored by `encoding/json`
+			// https://github.com/sirupsen/logrus/issues/137
+			data[k] = v.Error()
+		default:
+			_, err := json.Marshal(v)
+			if err != nil {
+				return nil, errors.Errorf("field is not JSON encodable: %s", k)
+			}
+			data[k] = v
+		}
+	}
+
+	data["msg"] = e.Message
+	data["time"] = e.Time.Format(time.RFC3339)
+	data["level"] = e.Level
+
+	return json.Marshal(data)
+
 }
 
 type nopWriter int