Backport two small fixes from the MPSAFE TTY branch in Perforce:

- Implement IMAXBEL. It turned out the IMAXBEL termios switch was marked
  as supported, while it had not been implemented.

- Don't go into the high watermark when in canonical mode, no data has
  been canonicalized and the input buffer is full. This caused the
  terminal to lock up. This prevented users from pressing
  backspace/^U/etc in such cases.

  This could easily be simulated by pasting a very big amount of data in
  a shell with sh(1) in canonical mode.

Obtained from:	//depot/projects/mpsafetty/...

2 files changed, 29 insertions, 8 deletions

diff --git a/sys/kern/tty_ttydisc.c b/sys/kern/tty_ttydisc.c
index b52fc18..6244166 100644
--- a/sys/kern/tty_ttydisc.c
+++ b/sys/kern/tty_ttydisc.c
@@ -305,7 +305,6 @@ int
 ttydisc_read(struct tty *tp, struct uio *uio, int ioflag)
 {
 	int error;
-	size_t c;
 
 	tty_lock_assert(tp, MA_OWNED);
 
@@ -322,8 +321,8 @@ ttydisc_read(struct tty *tp, struct uio *uio, int ioflag)
 	else
 		error = ttydisc_read_raw_interbyte_timer(tp, uio, ioflag);
 
-	c = ttyinq_bytesleft(&tp->t_inq);
-	if (c >= tp->t_inlow) {
+	if (ttyinq_bytesleft(&tp->t_inq) >= tp->t_inlow ||
+	    ttyinq_bytescanonicalized(&tp->t_inq) == 0) {
 		/* Unset the input watermark when we've got enough space. */
 		tty_hiwat_in_unblock(tp);
 	}
@@ -1001,7 +1000,20 @@ parmrk:
 print:
 	/* See if we can store this on the input queue. */
 	if (ttyinq_write_nofrag(&tp->t_inq, ob, ol, quote) != 0) {
-		/* We cannot. Enable the input watermark. */
+		if (CMP_FLAG(i, IMAXBEL))
+			ttyoutq_write_nofrag(&tp->t_outq, "\a", 1);
+
+		/*
+		 * Prevent a deadlock here. It may be possible that a
+		 * user has entered so much data, there is no data
+		 * available to read(), but the buffers are full anyway.
+		 *
+		 * Only enter the high watermark if the device driver
+		 * can actually transmit something.
+		 */
+		if (ttyinq_bytescanonicalized(&tp->t_inq) == 0)
+			return (0);
+
 		tty_hiwat_in_block(tp);
 		return (-1);
 	}
diff --git a/sys/sys/ttydisc.h b/sys/sys/ttydisc.h
index dd170e6..0d43666 100644
--- a/sys/sys/ttydisc.h
+++ b/sys/sys/ttydisc.h
@@ -84,10 +84,19 @@ ttydisc_write_poll(struct tty *tp)
 static __inline size_t
 ttydisc_rint_poll(struct tty *tp)
 {
-
-	tty_lock_assert(tp, MA_OWNED);
-
-	return ttyinq_bytesleft(&tp->t_inq);
+	size_t l;
+
+	/*
+	 * XXX: Still allow character input when there's no space in the
+	 * buffers, but we haven't entered the high watermark. This is
+	 * to allow backspace characters to be inserted when in
+	 * canonical mode.
+	 */
+	l = ttyinq_bytesleft(&tp->t_inq);
+	if (l == 0 && (tp->t_flags & TF_HIWAT_IN) == 0)
+		return (1);
+	
+	return (l);
 }
 
 static __inline size_t