block pthread cancellation in openpty function

being a nonstandard function, this isn't strictly necessary, but it's
inexpensive and avoids unpleasant surprises. eventually I would like
all functions in libc to be safe against cancellation, either ignoring
it or acting on it cleanly.

src/misc/openpty.c

@@ -3,33 +3,38 @@
 #include <unistd.h>
 #include <pty.h>
 #include <stdio.h>
+#include <pthread.h>
 
 /* Nonstandard, but vastly superior to the standard functions */
 
 int openpty(int *pm, int *ps, char *name, const struct termios *tio, const struct winsize *ws)
 {
-	int m, s, n=0;
+	int m, s, n=0, cs;
 	char buf[20];
 
 	m = open("/dev/ptmx", O_RDWR|O_NOCTTY);
 	if (m < 0) return -1;
 
-	if (ioctl(m, TIOCSPTLCK, &n) || ioctl (m, TIOCGPTN, &n)) {
-		close(m);
-		return -1;
-	}
+	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+	if (ioctl(m, TIOCSPTLCK, &n) || ioctl (m, TIOCGPTN, &n))
+		goto fail;
 
 	if (!name) name = buf;
 	snprintf(name, sizeof buf, "/dev/pts/%d", n);
-	if ((s = open(name, O_RDWR|O_NOCTTY)) < 0) {
-		close(m);
-		return -1;
-	}
+	if ((s = open(name, O_RDWR|O_NOCTTY)) < 0)
+		goto fail;
 
 	if (tio) tcsetattr(s, TCSANOW, tio);
 	if (ws) ioctl(s, TIOCSWINSZ, ws);
 
 	*pm = m;
 	*ps = s;
+
+	pthread_setcancelstate(cs, 0);
 	return 0;
+fail:
+	close(m);
+	pthread_setcancelstate(cs, 0);
+	return -1;
 }