From 2414e23dd6e144be521688915038b236c0aef6ff Mon Sep 17 00:00:00 2001
From: Andri Yngvason <andri@yngvason.is>
Date: Sun, 6 Dec 2020 21:04:56 +0000
Subject: [PATCH] Use double-buffering

Without double-buffering there's a slight chance that the vnc code could
start writing to the buffer before the compositor is done with it.
---
 src/main.c | 35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/src/main.c b/src/main.c
index 00c1913..859692d 100644
--- a/src/main.c
+++ b/src/main.c
@@ -51,7 +51,8 @@ struct window {
 	struct xdg_surface* xdg_surface;
 	struct xdg_toplevel* xdg_toplevel;
 
-	struct buffer* buffer;
+	struct buffer* front_buffer;
+	struct buffer* back_buffer;
 };
 
 static struct wl_display* wl_display;
@@ -278,8 +279,8 @@ static int init_signal_handler(void)
 
 static void window_attach(struct window* w, int x, int y)
 {
-	w->buffer->is_attached = true;
-	wl_surface_attach(w->wl_surface, w->buffer->wl_buffer, x, y);
+	w->back_buffer->is_attached = true;
+	wl_surface_attach(w->wl_surface, w->back_buffer->wl_buffer, x, y);
 }
 
 static void window_commit(struct window* w)
@@ -287,6 +288,14 @@ static void window_commit(struct window* w)
 	wl_surface_commit(w->wl_surface);
 }
 
+static void window_swap(struct window* w, struct vnc_client* client)
+{
+	struct buffer* tmp = w->front_buffer;
+	w->front_buffer = w->back_buffer;
+	w->back_buffer = tmp;
+	vnc_client_set_fb(client, window->back_buffer->pixels);
+}
+
 static void window_damage(struct window* w, int x, int y, int width, int height)
 {
 	wl_surface_damage(w->wl_surface, x, y, width, height);
@@ -364,8 +373,11 @@ wl_surface_failure:
 
 static void window_destroy(struct window* w)
 {
-	if (w->buffer)
-		buffer_destroy(w->buffer);
+	if (w->front_buffer)
+		buffer_destroy(w->front_buffer);
+
+	if (w->back_buffer)
+		buffer_destroy(w->back_buffer);
 
 	xdg_toplevel_destroy(w->xdg_toplevel);
 	xdg_surface_destroy(w->xdg_surface);
@@ -442,8 +454,12 @@ int on_vnc_client_alloc_fb(struct vnc_client* client)
 	if (!window)
 		return -1;
 
-	window->buffer = buffer_create(width, height, stride, wl_shm_format);
-	vnc_client_set_fb(client, window->buffer->pixels);
+	window->front_buffer = buffer_create(width, height, stride,
+			wl_shm_format);
+	window->back_buffer = buffer_create(width, height, stride,
+			wl_shm_format);
+
+	vnc_client_set_fb(client, window->back_buffer->pixels);
 	return 0;
 }
 
@@ -452,8 +468,8 @@ void on_vnc_client_update_fb(struct vnc_client* client)
 	if (!pixman_region_not_empty(&client->damage))
 		return;
 
-	if (window->buffer->is_attached)
-		fprintf(stderr, "Oops, buffer is still attached.\n");
+	if (window->back_buffer->is_attached)
+		fprintf(stderr, "Oops, back-buffer is still attached.\n");
 
 	window_attach(window, 0, 0);
 
@@ -471,6 +487,7 @@ void on_vnc_client_update_fb(struct vnc_client* client)
 	}
 
 	window_commit(window);
+	window_swap(window, client);
 }
 
 void on_vnc_client_event(void* obj)