image: never let a Kitty image drop to zero placements mid-redraw

Every redraw that replaces an image's placements (a pane revealed after
an overlapping floating pane closes, a full window redraw, etc.)
deleted all of an image's existing placements before creating their
replacements. That briefly left the image with none at all - and some
Kitty implementations free an image's underlying pixel data once it
has no placements left, so the replacement placements just placed
would then reference already-discarded data and render as nothing: an
empty hole where the image used to be. Not reproducible with SIXEL,
which has no separate placement/data lifecycle to race.

kitty_redraw_start() now only marks placements intersecting a redraw
area as stale instead of deleting them immediately; a new
kitty_redraw_finish() (called via image_redraw_finish() after a
redraw's replacement placements have already been created) deletes
whatever is still marked. The image now always has at least one live
placement throughout the transition.

image-kitty.c

@@ -131,6 +131,7 @@ struct kitty_placement_cache {
 	u_int				 y;
 	u_int				 width;
 	u_int				 height;
+	int				 pending_delete;
 	struct kitty_placement_cache	*next;
 };
 
@@ -177,13 +178,49 @@ kitty_free_placements(struct kitty_image_cache *entry)
 	entry->placements = NULL;
 }
 
-/* Delete Kitty placements intersecting a redraw area. */
+/*
+ * Mark Kitty placements intersecting a redraw area as stale, without
+ * deleting them yet - see kitty_redraw_finish(). Deleting immediately here
+ * would, for an image whose only placements are in this area, leave it
+ * with none at all until the replacement is placed - some Kitty
+ * implementations free an image's pixel data once it has no placements
+ * left, which would leave the replacement referencing already-discarded
+ * data and render as nothing.
+ */
 void
 kitty_redraw_start(struct tty *tty, u_int x, u_int y, u_int width,
     u_int height)
 {
-	struct kitty_output		 *ko = tty->image_data;
-	struct kitty_image_cache	 *entry;
+	struct kitty_output		*ko = tty->image_data;
+	struct kitty_image_cache	*entry;
+	struct kitty_placement_cache	*placement;
+
+	if (ko == NULL)
+		return;
+	for (entry = ko->images; entry != NULL; entry = entry->next) {
+		for (placement = entry->placements; placement != NULL;
+		    placement = placement->next) {
+			if (placement->x >= x + width ||
+			    placement->x + placement->width <= x ||
+			    placement->y >= y + height ||
+			    placement->y + placement->height <= y)
+				continue;
+			placement->pending_delete = 1;
+		}
+	}
+}
+
+/*
+ * Delete placements marked stale by kitty_redraw_start() - called once any
+ * replacement placements have already been created, so an image already
+ * placed elsewhere in the same redraw is never left with none at all in
+ * between the two.
+ */
+void
+kitty_redraw_finish(struct tty *tty)
+{
+	struct kitty_output		*ko = tty->image_data;
+	struct kitty_image_cache	*entry;
 	struct kitty_placement_cache	**pp, *placement;
 	char				  s[64];
 
@@ -191,15 +228,12 @@ kitty_redraw_start(struct tty *tty, u_int x, u_int y, u_int width,
 		return;
 	for (entry = ko->images; entry != NULL; entry = entry->next) {
 		for (pp = &entry->placements; (placement = *pp) != NULL; ) {
-			if (placement->x >= x + width ||
-			    placement->x + placement->width <= x ||
-			    placement->y >= y + height ||
-			    placement->y + placement->height <= y) {
+			if (!placement->pending_delete) {
 				pp = &placement->next;
 				continue;
-		}
-		xsnprintf(s, sizeof s,
-		    "\033_Ga=d,d=i,i=%u,p=%u,q=2\033\\", entry->kitty_id,
+			}
+			xsnprintf(s, sizeof s,
+			    "\033_Ga=d,d=i,i=%u,p=%u,q=2\033\\", entry->kitty_id,
 			    placement->id);
 			tty_puts(tty, s);
 			*pp = placement->next;

image.c

@@ -156,6 +156,20 @@ image_redraw_start(struct tty *tty, u_int x, u_int y, u_int width,
 		sixel_redraw_start(tty, x, y, width, height);
 }
 
+/*
+ * Delete placements marked stale by image_redraw_start() - called once any
+ * replacement placements a redraw is making have already been created. Only
+ * meaningful for Kitty (see kitty_redraw_finish()) - SIXEL has no separate
+ * placement/data distinction for sixel_redraw_start()'s plain erase to
+ * leave dangling.
+ */
+void
+image_redraw_finish(struct tty *tty)
+{
+	if (tty->image_backend == &image_backend_kitty)
+		kitty_redraw_finish(tty);
+}
+
 /* Write out any image output the backend is still holding back. */
 void
 image_draw_flush(struct tty *tty)

screen-redraw.c

@@ -1806,6 +1806,10 @@ redraw_draw_pane_lines(struct redraw_draw_ctx *dctx, struct window_pane *wp,
 		image_draw_flush(&scene->c->tty);
 #endif
 	}
+#ifdef ENABLE_IMAGES
+	if (flags & REDRAW_PANE)
+		image_redraw_finish(&scene->c->tty);
+#endif
 }
 
 /* Draw lines. */
@@ -2139,8 +2143,13 @@ redraw_draw(struct client *c, struct window_pane *wp, int flags)
 
 	if (wp != NULL)
 		redraw_draw_pane_lines(&dctx, wp, flags);
-	else
+	else {
 		redraw_draw_lines(&dctx, flags);
+#ifdef ENABLE_IMAGES
+		if (flags & REDRAW_PANE)
+			image_redraw_finish(tty);
+#endif
+	}
 #ifdef ENABLE_IMAGES
 	if ((flags & REDRAW_PANE) &&
 	    (image_backend_flags(tty) &
@@ -2541,6 +2550,10 @@ redraw_draw_damage_rect(struct redraw_draw_ctx *dctx, u_int x, u_int y,
 		image_draw_flush(&scene->c->tty);
 #endif
 	}
+#ifdef ENABLE_IMAGES
+	if (!skip_images)
+		image_redraw_finish(&scene->c->tty);
+#endif
 
 	/* SIXEL image output may disturb status cells; compose them last. */
 	for (yy = y; yy < y + sy; yy++) {

tmux.h

@@ -4341,6 +4341,7 @@ void		 image_redraw_area(struct screen_write_ctx *, u_int, u_int,
 void		 image_redraw_all(struct screen_write_ctx *);
 void		 image_redraw_scroll(struct screen_write_ctx *, u_int);
 void		 image_redraw_start(struct tty *, u_int, u_int, u_int, u_int);
+void		 image_redraw_finish(struct tty *);
 void		 image_draw_flush(struct tty *);
 int		 image_backend_flags(struct tty *);
 int		 image_tty_update(struct tty *);
@@ -4398,6 +4399,7 @@ void		 kitty_free_state(void *);
 void		 kitty_draw_rect(struct tty *,
 			     const struct image_rect *, const struct tty_style_ctx *);
 void		 kitty_redraw_start(struct tty *, u_int, u_int, u_int, u_int);
+void		 kitty_redraw_finish(struct tty *);
 void		 kitty_free_output_state(struct tty *, int);
 #endif