Fix race condition

internal/log.go

@@ -24,6 +24,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync"
 
 	"github.com/matrix-org/util"
 
@@ -37,6 +38,7 @@ import (
 // this unfortunately results in us adding the same hook multiple times.
 // This map ensures we only ever add one level hook.
 var stdLevelLogAdded = make(map[logrus.Level]bool)
+var levelLogAddedMu = &sync.Mutex{}
 
 type utcFormatter struct {
 	logrus.Formatter

internal/log_unix.go

@@ -85,6 +85,8 @@ func checkSyslogHookParams(params map[string]interface{}) {
 }
 
 func setupStdLogHook(level logrus.Level) {
+	levelLogAddedMu.Lock()
+	defer levelLogAddedMu.Unlock()
 	if stdLevelLogAdded[level] {
 		return
 	}