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libdraw: don't loop forever when getting eof on /dev/cons in keyboard ioproc

--- a/sys/src/libdraw/keyboard.c

+++ b/sys/src/libdraw/keyboard.c

@@ -19,7 +19,7 @@

 void

 _ioproc(void *arg)

 {

-	int m, n;

+	int m, n, nerr;

 	char buf[20];

 	Rune r;

 	Keyboardctl *kc;

@@ -27,18 +27,27 @@

 	kc = arg;

 	threadsetname("kbdproc");

 	n = 0;

-loop:

+	nerr = 0;

 	while(kc->consfd >= 0){

+		m = read(kc->consfd, buf+n, sizeof buf-n);

+		if(m <= 0){

+			yield();	/* if error is due to exiting, we'll exit here */

+			if(kc->consfd < 0)

+				break;

+			fprint(2, "keyboard: short read: %r\n");

+			if(m<0 || ++nerr>10)

+				threadexits("read error");

+			continue;

+		}

+		nerr = 0;

+		n += m;

 		while(n>0 && fullrune(buf, n)){

 			m = chartorune(&r, buf);

 			n -= m;

 			memmove(buf, buf+m, n);

 			if(send(kc->c, &r) < 0)

-				goto loop;

+				break;

 		}

-		if((m = read(kc->consfd, buf+n, sizeof buf-n)) <= 0)

-			goto loop;

-		n += m;

 	}

 	chanfree(kc->c);

 	free(kc->file);