fix https://github.com/sagemathinc/cocalc/issues/2382 -- race condition when opening syncstring.
fix https://github.com/sagemathinc/cocalc/issues/2382 -- race condition when opening syncstring.

sagemathinc/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc