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devdraw: use QLock to serialize sleep on refresh to prevent double sleep

--- a/sys/src/9/port/devdraw.c

+++ b/sys/src/9/port/devdraw.c

@@ -73,6 +73,7 @@

 	CScreen*	cscreen;

 	Refresh*	refresh;

 	Rendez		refrend;

+	QLock		refq;

 	uchar*		readdata;

 	int		nreaddata;

 	int		busy;

@@ -1227,10 +1228,17 @@

 				break;

 			dunlock();

 			if(waserror()){

-				dlock();	/* restore lock for waserror() above */

+				dlock();

 				nexterror();

 			}

+			eqlock(&cl->refq);

+			if(waserror()){

+				qunlock(&cl->refq);

+				nexterror();

+			}

 			sleep(&cl->refrend, drawrefactive, cl);

+			poperror();

+			qunlock(&cl->refq);

 			poperror();

 			dlock();

 		}