fs: generalise machinery for putting extra values when using --use-json-log

fs/accounting/stats.go

@@ -377,7 +377,7 @@ func (s *StatsInfo) Transferred() []TransferSnapshot {
 func (s *StatsInfo) Log() {
 	if fs.Config.UseJSONLog {
 		out, _ := s.RemoteStats()
-		fs.LogLevelPrintf(fs.Config.StatsLogLevel, nil, "%T\n", map[string]interface{}(out))
+		fs.LogLevelPrintf(fs.Config.StatsLogLevel, nil, "%v%v\n", s, fs.LogValue("stats", out))
 	} else {
 		fs.LogLevelPrintf(fs.Config.StatsLogLevel, nil, "%v\n", s)
 	}

fs/log.go

@@ -3,7 +3,6 @@ package fs
 import (
 	"fmt"
 	"log"
-	"strings"
 
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -75,6 +74,27 @@ var LogPrint = func(level LogLevel, text string) {
 	_ = log.Output(4, text)
 }
 
+// LogValueItem describes keyed item for a JSON log entry
+type LogValueItem struct {
+	key   string
+	value interface{}
+}
+
+// LogValue should be used as an argument to any logging calls to
+// augment the JSON output with more structured information.
+//
+// key is the dictionary parameter used to store value.
+func LogValue(key string, value interface{}) LogValueItem {
+	return LogValueItem{key: key, value: value}
+}
+
+// String returns an empty string so LogValueItem entries won't show
+// in the textual representation of logs. They need to be put in so
+// the number of parameters of the log call matches.
+func (j LogValueItem) String() string {
+	return ""
+}
+
 // LogPrintf produces a log string from the arguments passed in
 func LogPrintf(level LogLevel, o interface{}, text string, args ...interface{}) {
 	out := fmt.Sprintf(text, args...)
@@ -87,9 +107,10 @@ func LogPrintf(level LogLevel, o interface{}, text string, args ...interface{})
 				"objectType": fmt.Sprintf("%T", o),
 			}
 		}
-		if strings.HasPrefix(out, "map[") {
-			fields["json"] = args[0]
-			out = ""
+		for _, arg := range args {
+			if item, ok := arg.(LogValueItem); ok {
+				fields[item.key] = item.value
+			}
 		}
 		switch level {
 		case LogLevelDebug: