To replace c0-*, add a high watermark to the pty event, and also backoff

when the any of the ttys the pane is going to write to has buffered
enough data.

tmux.h

@@ -56,6 +56,16 @@ extern char   **environ;
  */
 #define UTF8_SIZE 9
 
+/*
+ * READ_SIZE is the maximum size of data to hold from a pty (the event high
+ * watermark). READ_BACKOFF is the amount of data waiting to be output to a tty
+ * before pty reads will be backed off. READ_TIME is how long to back off
+ * before the next read (in microseconds) if a tty is above READ_BACKOFF.
+ */
+#define READ_SIZE 1024
+#define READ_BACKOFF 512
+#define READ_TIME 100
+
 /* Fatal errors. */
 #define fatal(msg) log_fatal("%s: %s", __func__, msg);
 #define fatalx(msg) log_fatalx("%s: %s", __func__, msg);
@@ -847,6 +857,7 @@ struct window_pane {
 
 	int		 fd;
 	struct bufferevent *event;
+	struct event	 timer;
 
 	struct input_ctx *ictx;
 
@@ -1595,8 +1606,9 @@ void	tty_draw_line(struct tty *, const struct window_pane *, struct screen *,
 int	tty_open(struct tty *, char **);
 void	tty_close(struct tty *);
 void	tty_free(struct tty *);
-void	tty_write(
-	    void (*)(struct tty *, const struct tty_ctx *), struct tty_ctx *);
+void	tty_write(void (*)(struct tty *, const struct tty_ctx *),
+	    struct tty_ctx *);
+int	tty_client_ready(struct client *, struct window_pane *wp);
 void	tty_cmd_alignmenttest(struct tty *, const struct tty_ctx *);
 void	tty_cmd_cell(struct tty *, const struct tty_ctx *);
 void	tty_cmd_clearendofline(struct tty *, const struct tty_ctx *);

tty.c

@@ -723,9 +723,23 @@ tty_draw_line(struct tty *tty, const struct window_pane *wp,
 	tty_update_mode(tty, tty->mode, s);
 }
 
+int
+tty_client_ready(struct client *c, struct window_pane *wp)
+{
+	if (c->session == NULL || c->tty.term == NULL)
+		return (0);
+	if (c->flags & CLIENT_SUSPENDED)
+		return (0);
+	if (c->tty.flags & TTY_FREEZE)
+		return (0);
+	if (c->session->curw->window != wp->window)
+		return (0);
+	return (1);
+}
+
 void
-tty_write(
-    void (*cmdfn)(struct tty *, const struct tty_ctx *), struct tty_ctx *ctx)
+tty_write(void (*cmdfn)(struct tty *, const struct tty_ctx *),
+    struct tty_ctx *ctx)
 {
 	struct window_pane	*wp = ctx->wp;
 	struct client		*c;
@@ -740,13 +754,7 @@ tty_write(
 		return;
 
 	TAILQ_FOREACH(c, &clients, entry) {
-		if (c->session == NULL || c->tty.term == NULL)
-			continue;
-		if (c->flags & CLIENT_SUSPENDED)
-			continue;
-		if (c->tty.flags & TTY_FREEZE)
-			continue;
-		if (c->session->curw->window != wp->window)
+		if (!tty_client_ready(c, wp))
 			continue;
 
 		ctx->xoff = wp->xoff;

window.c

@@ -58,6 +58,7 @@ u_int	next_window_pane_id;
 u_int	next_window_id;
 u_int	next_active_point;
 
+void	window_pane_timer_callback(int, short, void *);
 void	window_pane_read_callback(struct bufferevent *, void *);
 void	window_pane_error_callback(struct bufferevent *, short, void *);
 
@@ -740,6 +741,9 @@ window_pane_destroy(struct window_pane *wp)
 {
 	window_pane_reset_mode(wp);
 
+	if (event_initialized(&wp->timer))
+		evtimer_del(&wp->timer);
+
 	if (wp->fd != -1) {
 		bufferevent_free(wp->event);
 		close(wp->fd);
@@ -867,28 +871,53 @@ window_pane_spawn(struct window_pane *wp, int argc, char **argv,
 
 	wp->event = bufferevent_new(wp->fd, window_pane_read_callback, NULL,
 	    window_pane_error_callback, wp);
+
+	bufferevent_setwatermark(wp->event, EV_READ, 0, READ_SIZE);
 	bufferevent_enable(wp->event, EV_READ|EV_WRITE);
 
 	free(cmd);
 	return (0);
 }
 
+void
+window_pane_timer_callback(unused int fd, unused short events, void *data)
+{
+	window_pane_read_callback(NULL, data);
+}
+
 void
 window_pane_read_callback(unused struct bufferevent *bufev, void *data)
 {
-	struct window_pane     *wp = data;
-	char   		       *new_data;
-	size_t			new_size;
+	struct window_pane	*wp = data;
+	struct evbuffer		*evb = wp->event->input;
+	char			*new_data;
+	size_t			 new_size, available;
+	struct client		*c;
+	struct timeval		 tv;
+
+	if (event_initialized(&wp->timer))
+		evtimer_del(&wp->timer);
+
+	log_debug("%%%u has %zu bytes", wp->id, EVBUFFER_LENGTH(evb));
 
-	new_size = EVBUFFER_LENGTH(wp->event->input) - wp->pipe_off;
+	TAILQ_FOREACH(c, &clients, entry) {
+		if (!tty_client_ready(c, wp))
+			continue;
+
+		available = EVBUFFER_LENGTH(c->tty.event->output);
+		if (available > READ_BACKOFF)
+			goto start_timer;
+	}
+
+	new_size = EVBUFFER_LENGTH(evb) - wp->pipe_off;
 	if (wp->pipe_fd != -1 && new_size > 0) {
-		new_data = EVBUFFER_DATA(wp->event->input);
+		new_data = EVBUFFER_DATA(evb);
 		bufferevent_write(wp->pipe_event, new_data, new_size);
 	}
 
 	input_parse(wp);
 
-	wp->pipe_off = EVBUFFER_LENGTH(wp->event->input);
+	wp->pipe_off = EVBUFFER_LENGTH(evb);
 
 	/*
 	 * If we get here, we're not outputting anymore, so set the silence
@@ -897,11 +926,22 @@ window_pane_read_callback(unused struct bufferevent *bufev, void *data)
 	wp->window->flags |= WINDOW_SILENCE;
 	if (gettimeofday(&wp->window->silence_timer, NULL) != 0)
 		fatal("gettimeofday failed.");
+	return;
+
+start_timer:
+	log_debug("%%%u backing off (%s %zu > %d)", wp->id, c->ttyname,
+	    available, READ_BACKOFF);
+
+	tv.tv_sec = 0;
+	tv.tv_usec = READ_TIME;
+
+	evtimer_set(&wp->timer, window_pane_timer_callback, wp);
+	evtimer_add(&wp->timer, &tv);
 }
 
 void
-window_pane_error_callback(
-    unused struct bufferevent *bufev, unused short what, void *data)
+window_pane_error_callback(unused struct bufferevent *bufev, unused short what,
+    void *data)
 {
 	struct window_pane *wp = data;