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vt: correctly handle terminators in osc codes

when we get an osc control code, we were gobbling the '\a' terminator,
and assuming any \033 code was a terminator, not just \033\\; this was
not correct, and would lead us to attempting to read an unbounded amount
of input for an osc code.

--- a/sys/src/cmd/vt/vt.c

+++ b/sys/src/cmd/vt/vt.c

@@ -891,7 +891,7 @@

 			n = 1;

 			c = 0;

 			while (!cs->raw && host_avail() && x+n<=xmax && n<BUFS

-			    && (c = nextcmd())>=' ' && c<'\177') {

+			    && (c = nextchar())>=' ' && c<'\177') {

 				buf[n++] = c;

 				c = 0;

 			}

@@ -1027,14 +1027,41 @@

 	return 1;

 }

+static int

+readosc(char *buf, int nbuf)

+{

+	Rune ch;

+	int i;

+	i = 0;

+	while(1){

+		ch = nextchar();

+		if(ch == '\a')

+			break;

+		if(ch == '\033'){

+			ch = nextchar();

+			if(ch == '\\')

+				break;

+			if(i < nbuf - UTFmax - 1)

+				buf[i++] = '\033';

+		}

+		if(cursctl(ch))

+			continue;

+		if(i < nbuf - UTFmax - 1)

+			i += runetochar(buf+i, &ch);

+	}

+	buf[i] = 0;

+	return i;

+}

+

 // handle ESC], Operating System Command

 static void

 osc(void)

 {

-	Rune ch, buf[BUFS+1];

-	int fd, osc, got, i;

+	char buf[BUFS];

+	int fd, osc, got;

 	char *o, *s;

+	Rune ch;

 	osc = number(&ch, &got);

 	if(got) {

@@ -1042,30 +1069,15 @@

 		case 0:

 		case 1:

 		case 2: /* set title */

-			i = 0;

-

-			while((ch = nextcmd()) != '\a') {

-				if(i < nelem(buf) - 1) {

-					buf[i++] = ch;

-				}

-			}

-			buf[i] = 0;

+			readosc(buf, sizeof(buf));

 			if((fd = open("/dev/label", OWRITE)) >= 0) {

-				fprint(fd, "%S", buf);

+				fprint(fd, "%s", buf);

 				close(fd);

 			}

 			break;

 		case 7: /* set pwd */

-			i = 0;

-

-			while((ch = nextcmd()) != '\033'){

-				if(i < sizeof(osc7cwd)-UTFmax-1)

-					i += runetochar(osc7cwd+i, &ch);

-			}

-			nextcmd();

-			osc7cwd[i] = 0;

-

+			readosc(osc7cwd, sizeof(osc7cwd));

 			/* file://hostname/path → /n/hostname/path */

 			if(strncmp(osc7cwd, "file://", 7) == 0){

 				osc7cwd[0] = '/';

-- 

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