Index: tty_pts.c
===================================================================
--- tty_pts.c	(revision 195892)
+++ tty_pts.c	(working copy)
@@ -204,8 +204,10 @@
 		error = uiomove(ib, iblen, uio);
 
 		tty_lock(tp);
-		if (error != 0)
+		if (error != 0) {
+			iblen = 0;
 			goto done;
+		}
 
 		/*
 		 * When possible, avoid the slow path. rint_bypass()
@@ -260,6 +262,12 @@
 
 done:	ttydisc_rint_done(tp);
 	tty_unlock(tp);
+
+	/*
+	 * Don't account for the part of the buffer that we couldn't
+	 * pass to the TTY.
+	 */
+	uio->uio_resid += iblen;
 	return (error);
 }
 
Index: tty_ttydisc.c
===================================================================
--- tty_ttydisc.c	(revision 195892)
+++ tty_ttydisc.c	(working copy)
@@ -1060,6 +1060,8 @@
 
 	ret = ttyinq_write(&tp->t_inq, buf, len, 0);
 	ttyinq_canonicalize(&tp->t_inq);
+	if (ret < len)
+		tty_hiwat_in_block(tp);
 
 	return (ret);
 }