glog: don't hold mutex when sync'ing

glog_file.go

@@ -369,23 +369,30 @@ func (s *fileSink) Flush() error {
 
 // flush flushes all logs of severity threshold or greater.
 func (s *fileSink) flush(threshold logsink.Severity) error {
-s.mu.Lock()
-defer s.mu.Unlock()
-
 var firstErr error
 updateErr := func(err error) {
 if err != nil && firstErr == nil {
 firstErr = err
 }
 }
 
-// Flush from fatal down, in case there's trouble flushing.
-for sev := logsink.Fatal; sev >= threshold; sev-- {
-file := s.file[sev]
-if file != nil {
-updateErr(file.Flush())
-updateErr(file.Sync())
+// Remember where we flushed, so we can call sync without holding
+// the lock.
+var files []flushSyncWriter
+func() {
+s.mu.Lock()
+defer s.mu.Unlock()
+// Flush from fatal down, in case there's trouble flushing.
+for sev := logsink.Fatal; sev >= threshold; sev-- {
+if file := s.file[sev]; file != nil {
+updateErr(file.Flush())
+files = append(files, file)
+}
 }
+}()
+
+for _, file := range files {
+updateErr(file.Sync())
 }
 
 return firstErr