broken flow control in 2.9.1 tty code

[eric j. haug]

Subject: Flow control in cooked,tandem mode broken
Index:	<src/sys/dev>/<tty.c, ttyold.c, ttynew.c> 2.9bsd

Description:
	The cooked with tandem mode turned on tty code is broken.
Repeat-By:
	Login to a 2.9 system through a computer of some sort.  Start up:
	(stty tandem;cat > tmpfile;stty -tandem) and feed the tty line
	at a fast rate.  Eventually a ^S will be sent, but a ^Q will never
	be sent.

Fix:
	It is not obvious what the author of the changed code had in mind.
	And there is a conflict between the ttyold and ttynew code.
	The t_delct is never set in the ttynew code.
	NOTE the commented code in the WORKING section, it does not work
	It is left for comment.

add to tty.c
#ifdef	WORKING_CODE
	/* using TTHIWAT(tp) as limit breaks something else
	if ((tp->t_flags&(CBREAK|RAW) ? tp->t_rawq.c_cc : tp->t_canq.c_cc) > TTYHOG/2) {
the above seems like a good idea but fails somehow */

	if ( x >= TTYHOG/2 ) {
#else
	/*
	 * Block further input iff:
	 * Current input > threshold AND input is available to user program
	 */
	if (x >= TTYHOG/2 && (tp->t_delct>0 || (tp->t_flags&(RAW|CBREAK)))) {
#endif
add to ttyold.c
#ifdef	WORKING_CODE
	/* see notes in tty.c ttyblock()
	if (tp->t_state&TBLOCK)
		if ((tp->t_flags&(CBREAK|RAW) ? tp->t_rawq.c_cc : tp->t_canq.c_cc)<TTYHOG/5) {
the above seemed like a good idea but fails*/

	if ( tp->t_state & TBLOCK &&(tp->t_rawq.c_cc < TTYHOG/5 
			|| ((tp->t_flags&(CBREAK|RAW)==0) && tp->t_canq.c_cc==0))) {
#else
 	/* Unblock output iff:
 	 * is blocked
 	 * AND (input < threshold OR (cooked mode AND delim_count == 0))
 	 * This makes tandem mode usable for line-mode input.
 	 */
	if (tp->t_state&TBLOCK && ((tp->t_rawq.c_cc < TTYHOG/5) ||
	   (tp->t_delct==0 && ((tp->t_flags&(CBREAK|RAW)) == 0)))) {
#endif

add to ttynew.c
#ifdef	WORKING_CODE
	/* see notes in ttyblock() tty.c
	if (tp->t_state&TBLOCK)
		if ((tp->t_flags&(CBREAK|RAW) ? tp->t_rawq.c_cc : tp->t_canq.c_cc)<TTYHOG/5) {
the above seemed like a good idea but fails somehow */

	if ( tp->t_state & TBLOCK &&(tp->t_rawq.c_cc < TTYHOG/5 
			|| ((tp->t_flags&(CBREAK|RAW)==0) && tp->t_canq.c_cc==0))) {
#else
 	/*
 	 * Resume output iff:
 	 * is blocked
 	 * AND (input < threshold OR (cooked mode AND delim_count == 0))
 	 * This makes tandem mode usable for line-mode input.
 	 */
	if (tp->t_state&TBLOCK && ((tp->t_rawq.c_cc < TTYHOG/5) ||
	   (tp->t_delct==0 && ((tp->t_flags&(CBREAK|RAW)) == 0)))) {
#endif