Fix ttywrite()

ttywrite was assuming that if it could not write then it could
read, but this is not necessarily true, there are some situations
where you cannot read or write. The correct behaviour is to detect
if you can read or/and write.

st.c

@@ -415,7 +415,7 @@ static int32_t tdefcolor(int *, int *, int);
 static void tdeftran(char);
 static inline int match(uint, uint);
 static void ttynew(void);
-static void ttyread(void);
+static size_t ttyread(void);
 static void ttyresize(void);
 static void ttysend(char *, size_t);
 static void ttywrite(const char *, size_t);
@@ -1464,7 +1464,7 @@ ttynew(void)
 	}
 }
 
-void
+size_t
 ttyread(void)
 {
 	static char buf[BUFSIZ];
@@ -1489,14 +1489,16 @@ ttyread(void)
 
 	/* keep any uncomplete utf8 char for the next call */
 	memmove(buf, ptr, buflen);
+
+	return ret;
 }
 
 void
 ttywrite(const char *s, size_t n)
 {
-	fd_set wfd;
-	struct timespec tv;
+	fd_set wfd, rfd;
 	ssize_t r;
+	size_t lim = 256;
 
 	/*
 	 * Remember that we are using a pty, which might be a modem line.
@@ -1506,38 +1508,34 @@ ttywrite(const char *s, size_t n)
 	 */
 	while (n > 0) {
 		FD_ZERO(&wfd);
+		FD_ZERO(&rfd);
 		FD_SET(cmdfd, &wfd);
-		tv.tv_sec = 0;
-		tv.tv_nsec = 0;
+		FD_SET(cmdfd, &rfd);
 
 		/* Check if we can write. */
-		if (pselect(cmdfd+1, NULL, &wfd, NULL, &tv, NULL) < 0) {
+		if (pselect(cmdfd+1, &rfd, &wfd, NULL, NULL, NULL) < 0) {
 			if (errno == EINTR)
 				continue;
 			die("select failed: %s\n", strerror(errno));
 		}
-		if(!FD_ISSET(cmdfd, &wfd)) {
-			/* No, then free some buffer space. */
-			ttyread();
-		} else {
+		if (FD_ISSET(cmdfd, &rfd))
+			lim = ttyread();
+		if (FD_ISSET(cmdfd, &wfd)) {
 			/*
 			 * Only write 256 bytes at maximum. This seems to be a
 			 * reasonable value for a serial line. Bigger values
 			 * might clog the I/O.
 			 */
-			r = write(cmdfd, s, (n < 256)? n : 256);
-			if (r < 0) {
-				die("write error on tty: %s\n",
-						strerror(errno));
-			}
+			if ((r = write(cmdfd, s, (n < 256)? n : 256)) < 0)
+				goto write_error;
 			if (r < n) {
 				/*
 				 * We weren't able to write out everything.
 				 * This means the buffer is getting full
 				 * again. Empty it.
 				 */
-				if (n < 256)
-					ttyread();
+				if (n < lim)
+					lim = ttyread();
 				n -= r;
 				s += r;
 			} else {
@@ -1546,6 +1544,10 @@ ttywrite(const char *s, size_t n)
 			}
 		}
 	}
+	return;
+
+write_error:
+	die("write error on tty: %s\n", strerror(errno));
 }
 
 void