Change the way backoff works. Instead of stopping reading from the pty

when the client tty backs up too much, just stop updating the tty and
only update the internal screen. Then when the tty recovers, force a
redraw.

This prevents a dodgy client from causing other clients to go into
backoff while still allowing tmux to be responsive (locally) when seeing
lots of output.

server-client.c

@@ -492,6 +492,50 @@ server_client_check_exit(struct client *c)
 	c->flags &= ~CLIENT_EXIT;
 }
 
+/*
+ * Check if the client should backoff. During backoff, data from external
+ * programs is not written to the terminal. When the existing data drains, the
+ * client is redrawn.
+ *
+ * There are two backoff phases - both the tty and client have backoff flags -
+ * the first to allow existing data to drain and the latter to ensure backoff
+ * is disabled until the redraw has finished and prevent the redraw triggering
+ * another backoff.
+ */
+void
+server_client_check_backoff(struct client *c)
+{
+	struct tty	*tty = &c->tty;
+	size_t		 used;
+
+	used = EVBUFFER_LENGTH(tty->event->output);
+
+	/*
+	 * If in the second backoff phase (redrawing), don't check backoff
+	 * until the redraw has completed (or enough of it to drop below the
+	 * backoff threshold).
+	 */
+	if (c->flags & CLIENT_BACKOFF) {
+		if (used > BACKOFF_THRESHOLD)
+			return;
+		c->flags &= ~CLIENT_BACKOFF;
+		return;
+	}
+
+	/* Once drained, allow data through again and schedule redraw. */
+	if (tty->flags & TTY_BACKOFF) {
+		if (used != 0)
+			return;
+		tty->flags &= ~TTY_BACKOFF;
+		c->flags |= (CLIENT_BACKOFF|CLIENT_REDRAWWINDOW|CLIENT_STATUS);
+		return;
+	}
+
+	/* If too much data, start backoff. */
+	if (used > BACKOFF_THRESHOLD)
+		tty->flags |= TTY_BACKOFF;
+}
+
 /* Check for client redraws. */
 void
 server_client_check_redraw(struct client *c)
@@ -520,6 +564,10 @@ server_client_check_redraw(struct client *c)
 	if (c->flags & CLIENT_REDRAW) {
 		screen_redraw_screen(c, 0, 0);
 		c->flags &= ~(CLIENT_STATUS|CLIENT_BORDERS);
+	} else if (c->flags & CLIENT_REDRAWWINDOW) {
+		TAILQ_FOREACH(wp, &c->session->curw->window->panes, entry)
+			screen_redraw_pane(c, wp);
+		c->flags &= ~CLIENT_REDRAWWINDOW;
 	} else {
 		TAILQ_FOREACH(wp, &c->session->curw->window->panes, entry) {
 			if (wp->flags & PANE_REDRAW)

server-window.c

@@ -29,31 +29,6 @@ int	server_window_check_activity(struct session *, struct winlink *);
 int	server_window_check_content(
 	    struct session *, struct winlink *, struct window_pane *);
 
-/* Check if this window should suspend reading. */
-int
-server_window_backoff(struct window_pane *wp)
-{
-	struct client	*c;
-	u_int		 i;
-
-	if (!window_pane_visible(wp))
-		return (0);
-
-	for (i = 0; i < ARRAY_LENGTH(&clients); i++) {
-		c = ARRAY_ITEM(&clients, i);
-		if (c == NULL || c->session == NULL)
-			continue;
-		if ((c->flags & (CLIENT_SUSPENDED|CLIENT_DEAD)) != 0)
-			continue;
-		if (c->session->curw->window != wp->window)
-			continue;
-
-		if (EVBUFFER_LENGTH(c->tty.event->output) > BACKOFF_THRESHOLD)
-			return (1);
-	}
-	return (0);
-}
-
 /* Window functions that need to happen every loop. */
 void
 server_window_loop(void)
@@ -69,17 +44,6 @@ server_window_loop(void)
 		if (w == NULL)
 			continue;
 
-		TAILQ_FOREACH(wp, &w->panes, entry) {
-			if (wp->fd == -1)
-				continue;
-			if (!(wp->flags & PANE_FREEZE)) {
-				if (server_window_backoff(wp))
-					bufferevent_disable(wp->event, EV_READ);
-				else
-					bufferevent_enable(wp->event, EV_READ);
-			}
-		}
-
 		for (j = 0; j < ARRAY_LENGTH(&sessions); j++) {
 			s = ARRAY_ITEM(&sessions, j);
 			if (s == NULL)

tmux.h

@@ -59,8 +59,8 @@ extern char   **environ;
 /* Automatic name refresh interval, in milliseconds. */
 #define NAME_INTERVAL 500
 
-/* Maximum data to buffer for output before suspending reading from panes. */
-#define BACKOFF_THRESHOLD 1024
+/* Maximum data to buffer for output before suspending writing to a tty. */
+#define BACKOFF_THRESHOLD 16384
 
 /*
  * Maximum sizes of strings in message data. Don't forget to bump
@@ -1017,6 +1017,7 @@ struct tty {
 #define TTY_UTF8 0x8
 #define TTY_STARTED 0x10
 #define TTY_OPENED 0x20
+#define TTY_BACKOFF 0x40
 	int		 flags;
 
 	int		 term_flags;
@@ -1126,6 +1127,8 @@ struct client {
 #define CLIENT_DEAD 0x200
 #define CLIENT_BORDERS 0x400
 #define CLIENT_READONLY 0x800
+#define CLIENT_BACKOFF 0x1000
+#define CLIENT_REDRAWWINDOW 0x2000
 	int		 flags;
 
 	struct event	 identify_timer;

tty.c

@@ -578,7 +578,9 @@ tty_write(void (*cmdfn)(
 			continue;
 
 		if (c->session->curw->window == wp->window) {
-			if (c->tty.flags & TTY_FREEZE || c->tty.term == NULL)
+			if (c->tty.term == NULL)
+				continue;
+			if (c->tty.flags & (TTY_FREEZE|TTY_BACKOFF))
 				continue;
 			cmdfn(&c->tty, ctx);
 		}