multi.go
func (b *multiLogger) Log(level Level, calldepth int, rec *Record) (err error) {
	for _, backend := range b.backends {
		if backend.IsEnabledFor(level, rec.Module) {
			// Shallow copy of the record for the formatted cache on Record and get the
			// record formatter from the backend.
			r2 := *rec
			if e := backend.Log(level, calldepth+1, &r2); e != nil {
				err = e
			}
		}
	}
	return
}
func (b *multiLogger) Log(level Level, calldepth int, rec *Record) (err error) {
	for _, backend := range b.backends {
		if backend.IsEnabledFor(level, rec.Module) {
			// Shallow copy of the record for the formatted cache on Record and get the
			// record formatter from the backend.
			r2 := *rec
			if e := backend.Log(level, calldepth+1, &r2); e != nil {
				err = e
			}
		}
	}
	return
}
// Log passes the log record to all backends.