Fixup headless output names

We use the headless backend to create a special fallback output
used when no other output is connected. However this messes up the
"real" headless output names users have come to expect (e.g.
currently the first headless output will be named "HEADLESS-2"
instead of "HEADLESS-1").

Fix this by setting the output name with [1].

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/3395

sway/desktop/output.c

@@ -5,6 +5,7 @@
 #include <time.h>
 #include <wayland-server-core.h>
 #include <wlr/backend/drm.h>
+#include <wlr/backend/headless.h>
 #include <wlr/render/wlr_renderer.h>
 #include <wlr/types/wlr_buffer.h>
 #include <wlr/types/wlr_drm_lease_v1.h>
@@ -833,13 +834,22 @@ static void handle_present(struct wl_listener *listener, void *data) {
 	output->refresh_nsec = output_event->refresh;
 }
 
+static unsigned int last_headless_num = 0;
+
 void handle_new_output(struct wl_listener *listener, void *data) {
 	struct sway_server *server = wl_container_of(listener, server, new_output);
 	struct wlr_output *wlr_output = data;
+
 	if (wlr_output == root->fallback_output->wlr_output) {
 		return;
 	}
 
+	if (wlr_output_is_headless(wlr_output)) {
+		char name[64];
+		snprintf(name, sizeof(name), "HEADLESS-%u", ++last_headless_num);
+		wlr_output_set_name(wlr_output, name);
+	}
+
 	sway_log(SWAY_DEBUG, "New output %p: %s (non-desktop: %d)",
 			wlr_output, wlr_output->name, wlr_output->non_desktop);
 

sway/server.c

@@ -227,6 +227,7 @@ bool server_init(struct sway_server *server) {
 
 	struct wlr_output *wlr_output =
 			wlr_headless_add_output(server->headless_backend, 800, 600);
+	wlr_output_set_name(wlr_output, "FALLBACK");
 	root->fallback_output = output_create(wlr_output);
 
 	// This may have been set already via -Dtxn-timeout