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kernel: make allocb() wait instead of panic() when possible

as long as our process doesnt hold any locks or ilocks, we
can try to wait for the memory to become available instead of
panicing.

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

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

@@ -64,10 +64,16 @@

 	 */

 	if(up == nil)

 		panic("allocb without up: %#p", getcallerpc(&size));

-	if((b = _allocb(size)) == nil){

-		xsummary();

-		mallocsummary();

-		panic("allocb: no memory for %d bytes", size);

+	while((b = _allocb(size)) == nil){

+		if(up->nlocks.ref || m->ilockdepth || !islo()){

+			xsummary();

+			mallocsummary();

+			panic("allocb: no memory for %d bytes", size);

+		}

+		if(!waserror()){

+			resrcwait("no memory for allocb");

+			poperror();

+		}

 	}

 	setmalloctag(b, getcallerpc(&size));

-- 

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