libweston: Check output placement

Make sure we don't enable an output that overlaps with other enabled
outputs.

We should probably do something similar when moving outputs, but we can't
realistically do that right now, so at least leave a comment explaining
why we're ignoring that case.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

libweston/compositor.c

@@ -6608,6 +6608,18 @@ weston_output_set_position(struct weston_output *output, int x, int y)
 WL_EXPORT void
 weston_output_move(struct weston_output *output, int x, int y)
 {
+	/* XXX: we should probably perform some sanity checking here
+	 * as we do for weston_output_enable, and allow moves to fail.
+	 *
+	 * However, while a front-end is rearranging outputs it may
+	 * pass through indeterminate states where outputs overlap
+	 * or are discontinuous, and this may be ok as long as no
+	 * input processing or rendering occurs at that time.
+	 *
+	 * Ultimately, we probably need a way to pass complete output
+	 * config atomically to libweston.
+	 */
+
 	output->compositor->output_flow_dirty = true;
 	weston_output_set_position(output, x, y);
 }
@@ -7230,6 +7242,45 @@ weston_output_create_heads_string(struct weston_output *output)
 	return str;
 }
 
+static bool
+weston_outputs_overlap(struct weston_output *a, struct weston_output *b)
+{
+	bool overlap;
+	pixman_region32_t intersection;
+
+	pixman_region32_init(&intersection);
+	pixman_region32_intersect(&intersection, &a->region, &b->region);
+	overlap = pixman_region32_not_empty(&intersection);
+	pixman_region32_fini(&intersection);
+
+	return overlap;
+}
+
+/* This only works if the output region is current!
+ *
+ * That means we shouldn't expect it to return usable results unless
+ * the output is at least undergoing enabling.
+ */
+static bool
+weston_output_placement_ok(struct weston_output *output)
+{
+	struct weston_compositor *c = output->compositor;
+	struct weston_output *iter;
+
+	wl_list_for_each(iter, &c->output_list, link) {
+		if (!iter->enabled)
+			continue;
+
+		if (weston_outputs_overlap(iter, output)) {
+			weston_log("Error: output '%s' overlaps enabled output '%s'.\n",
+				   output->name, iter->name);
+			return false;
+		}
+	}
+
+	return true;
+}
+
 /** Constructs a weston_output object that can be used by the compositor.
  *
  * \param output The weston_output object that needs to be enabled. Must not
@@ -7309,6 +7360,11 @@ weston_output_enable(struct weston_output *output)
 	weston_output_transform_scale_init(output, output->transform, output->scale);
 
 	weston_output_init_geometry(output, output->x, output->y);
+
+	/* At this point we have a valid region so we can check placement. */
+	if (!weston_output_placement_ok(output))
+		return -1;
+
 	weston_output_damage(output);
 
 	wl_list_init(&output->animation_list);