log: optionally print pid in logs - #5593

This option is useful to troubleshoot `rclone mount --daemon`

docs/content/docs.md

@@ -1024,7 +1024,7 @@ have a signal to rotate logs.
 
 ### --log-format LIST ###
 
-Comma separated list of log format options. `date`, `time`, `microseconds`, `longfile`, `shortfile`, `UTC`.  The default is "`date`,`time`". 
+Comma separated list of log format options. Accepted options are `date`, `time`, `microseconds`, `pid`, `longfile`, `shortfile`, `UTC`. Any other keywords will be silently ignored. `pid` will tag log messages with process identifier which useful with `rclone mount --daemon`. Other accepted options are explained in the [go documentation](https://pkg.go.dev/log#pkg-constants). The default log format is "`date`,`time`".
 
 ### --log-level LEVEL ###
 

fs/log.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"os"
 
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -80,9 +81,15 @@ func (l *LogLevel) UnmarshalJSON(in []byte) error {
 	})
 }
 
+// LogPrintPid enables process pid in log
+var LogPrintPid = false
+
 // LogPrint sends the text to the logger of level
 var LogPrint = func(level LogLevel, text string) {
 	text = fmt.Sprintf("%-6s: %s", level, text)
+	if LogPrintPid {
+		text = fmt.Sprintf("[%d] %s", os.Getpid(), text)
+	}
 	_ = log.Output(4, text)
 }
 

fs/log/log.go

@@ -113,6 +113,8 @@ func InitLogging() {
 	}
 	log.SetFlags(flags)
 
+	fs.LogPrintPid = strings.Contains(flagsStr, ",pid,")
+
 	// Log file output
 	if Opt.File != "" {
 		f, err := os.OpenFile(Opt.File, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0640)