Basic ordered JSON logger inspired by go-kit

orderedlogger.go

package olog

import (
	"encoding"
	"encoding/json"
	"fmt"

	"github.com/go-kit/kit/log"
	"github.com/mickep76/mapslice-json"
)

type orderedJSONLogger struct {
	enc *json.Encoder
}

func (l *orderedJSONLogger) Log(keyvals ...interface{}) error {
	n := (len(keyvals) + 1) / 2 // +1 to handle case when len is odd
	message := make(mapslice.MapSlice, 0, n)

	// Assemble entry fields
	for i := 0; i < len(keyvals); i += 2 {
		k := keyvals[i]
		var v interface{} = log.ErrMissingValue
		if i+1 < len(keyvals) {
			v = keyvals[i+1]
		}
		message = addLogField(message, k, v)
	}

	return l.enc.Encode(message)
}

func addLogField(dst mapslice.MapSlice, k, v interface{}) mapslice.MapSlice {
	var key string
	switch x := k.(type) {
	case string:
		key = x
	case fmt.Stringer:
		key = safeString(x)
	default:
		key = fmt.Sprint(x)
	}

	// We want json.Marshaler and encoding.TextMarshaler to take priority over
	// err.Error() and v.String(). But json.Marshal (called later) does that
	// by default, so we force a no-op in those 2 cases.
	switch x := v.(type) {
	case json.Marshaler:
	case encoding.TextMarshaler:
	case error:
		v = safeError(x)
	case fmt.Stringer:
		v = safeString(x)
	}

	return append(dst, mapslice.MapItem{Key: key, Value: v})
}