Add optional headless fallback output

This allows configuring a headless fallback output that
is automatically created whenever there is no other output
around. It is destroyed when a new output is discovered.

It can be enabled by setting the environment variable
LABWC_FALLBACK_OUTPUT to the desired output name.

The feature benefits applications like wayvnc the most
as there is always an output available to connect to.

Co-Authored-By: Simon Long <simon@raspberrypi.com>

docs/environment

@@ -68,3 +68,18 @@
 
 # XDG_CURRENT_DESKTOP=wlroots
 
+##
+## This causes a virtual output to be created automatically whenever there
+## are no outputs around. This helps for cases like wayvnc so there is always
+## an output available to connect to. The name can be chosen freely but there
+## must be no duplicate output names, for this reason using VIRTUAL-x or a
+## physical connector name like HDMI-A-1 is not recommended as wlroots may
+## want to create outputs with those names later on which would then fail.
+##
+## Using an output name that starts with NOOP- has the additional benefit
+## that wayvnc will detect it being a virtual output and allow clients to
+## resize the output to match the client resolution.
+##
+
+# LABWC_FALLBACK_OUTPUT=NOOP-fallback
+

include/output-virtual.h

@@ -8,5 +8,6 @@ struct wlr_output;
 void output_virtual_add(struct server *server, const char *output_name,
 		struct wlr_output **store_wlr_output);
 void output_virtual_remove(struct server *server, const char *output_name);
+void output_virtual_update_fallback(struct server *server);
 
 #endif

src/output-virtual.c

@@ -1,10 +1,14 @@
 // SPDX-License-Identifier: GPL-2.0-only
 
+#include <stdlib.h>
 #include <wlr/backend/headless.h>
 #include <wlr/types/wlr_output.h>
+#include "common/string-helpers.h"
 #include "labwc.h"
 #include "output-virtual.h"
 
+static struct wlr_output *fallback_output = NULL;
+
 void
 output_virtual_add(struct server *server, const char *output_name,
 		struct wlr_output **store_wlr_output)
@@ -77,7 +81,8 @@ output_virtual_remove(struct server *server, const char *output_name)
 {
 	struct output *output;
 	wl_list_for_each(output, &server->outputs, link) {
-		if (!wlr_output_is_headless(output->wlr_output)) {
+		if (!wlr_output_is_headless(output->wlr_output)
+				|| output->wlr_output == fallback_output) {
 			continue;
 		}
 
@@ -100,3 +105,32 @@ output_virtual_remove(struct server *server, const char *output_name)
 		}
 	}
 }
+
+void
+output_virtual_update_fallback(struct server *server)
+{
+	struct wl_list *layout_outputs = &server->output_layout->outputs;
+	const char *fallback_output_name = getenv("LABWC_FALLBACK_OUTPUT");
+
+	if (!fallback_output && wl_list_empty(layout_outputs)
+			&& !string_null_or_empty(fallback_output_name)) {
+		wlr_log(WLR_DEBUG, "adding fallback output %s", fallback_output_name);
+
+		output_virtual_add(server, fallback_output_name, &fallback_output);
+	} else if (fallback_output && (wl_list_length(layout_outputs) > 1
+			|| string_null_or_empty(fallback_output_name))) {
+		wlr_log(WLR_DEBUG, "destroying fallback output %s",
+			fallback_output->name);
+
+		/*
+		 * We must reset fallback_output to NULL before destroying it.
+		 *
+		 * Otherwise we may end up trying to destroy the same wlr_output
+		 * twice due to wlr_output_destroy() removing the output from the
+		 * layout which in turn causes us to be called again.
+		 */
+		struct wlr_output *wlr_output = fallback_output;
+		fallback_output = NULL;
+		wlr_output_destroy(wlr_output);
+	}
+}

src/output.c

@@ -24,6 +24,7 @@
 #include "labwc.h"
 #include "layers.h"
 #include "node.h"
+#include "output-virtual.h"
 #include "regions.h"
 #include "view.h"
 #include "xwayland.h"
@@ -694,6 +695,12 @@ handle_output_layout_change(struct wl_listener *listener, void *data)
 {
 	struct server *server =
 		wl_container_of(listener, server, output_layout_change);
+
+	/* Prevents unnecessary layout recalculations */
+	server->pending_output_layout_change++;
+	output_virtual_update_fallback(server);
+	server->pending_output_layout_change--;
+
 	do_output_layout_change(server);
 }
 

src/server.c

@@ -27,6 +27,7 @@
 #include "labwc.h"
 #include "layers.h"
 #include "menu/menu.h"
+#include "output-virtual.h"
 #include "regions.h"
 #include "resize_indicator.h"
 #include "theme.h"
@@ -69,6 +70,7 @@ handle_sighup(int signal, void *data)
 {
 	session_environment_init();
 	reload_config_and_theme();
+	output_virtual_update_fallback(g_server);
 	return 0;
 }
 
@@ -490,6 +492,9 @@ server_start(struct server *server)
 		exit(EXIT_FAILURE);
 	}
 
+	/* Potentially set up the initial fallback output */
+	output_virtual_update_fallback(server);
+
 	if (setenv("WAYLAND_DISPLAY", socket, true) < 0) {
 		wlr_log_errno(WLR_ERROR, "unable to set WAYLAND_DISPLAY");
 	} else {