Add a noop output using the noop backend

This is intended for use when no real outputs are available.

include/render.c

@@ -1,5 +1,6 @@
 #define WLR_USE_UNSTABLE
 #include <wlr/backend.h>
+#include <wlr/backend/noop.h>
 #include <wlr/backend/multi.h>
 #include <wlr/render/wlr_renderer.h>
 
@@ -22,3 +23,11 @@ bool river_wlr_backend_is_multi(struct wlr_backend *backend) {
 struct wlr_session *river_wlr_backend_get_session(struct wlr_backend *backend) {
     return wlr_backend_get_session(backend);
 }
+
+struct wlr_backend *river_wlr_noop_backend_create(struct wl_display *display) {
+    return wlr_noop_backend_create(display);
+}
+
+struct wlr_output *river_wlr_noop_add_output(struct wlr_backend *backend) {
+    return wlr_noop_add_output(backend);
+}

include/render.h

@@ -27,5 +27,7 @@ struct wlr_renderer *river_wlr_backend_get_renderer(struct wlr_backend *backend)
 bool river_wlr_backend_start(struct wlr_backend *backend);
 bool river_wlr_backend_is_multi(struct wlr_backend *backend);
 struct wlr_session *river_wlr_backend_get_session(struct wlr_backend *backend);
+struct wlr_backend *river_wlr_noop_backend_create(struct wl_display *display);
+struct wlr_output *river_wlr_noop_add_output(struct wlr_backend *backend);
 
 #endif

src/root.zig

@@ -19,6 +19,9 @@ pub const Root = struct {
     wlr_output_layout: *c.wlr_output_layout,
     outputs: std.TailQueue(Output),
 
+    /// This output is used when no real outputs are available.
+    noop_output: Output,
+
     /// Number of pending configures sent in the current transaction.
     /// A value of 0 means there is no current transaction.
     pending_configures: u32,
@@ -37,6 +40,10 @@ pub const Root = struct {
 
         self.outputs = std.TailQueue(Output).init();
 
+        const noop_wlr_output = c.river_wlr_noop_add_output(server.noop_backend) orelse
+            return error.CantAddNoopOutput;
+        try self.noop_output.init(self, noop_wlr_output);
+
         self.pending_configures = 0;
 
         self.transaction_timer = null;

src/server.zig

@@ -18,6 +18,7 @@ pub const Server = struct {
     wl_display: *c.wl_display,
     wl_event_loop: *c.wl_event_loop,
     wlr_backend: *c.wlr_backend,
+    noop_backend: *c.wlr_backend,
     wlr_renderer: *c.wlr_renderer,
 
     wlr_xdg_shell: *c.wlr_xdg_shell,
@@ -47,12 +48,16 @@ pub const Server = struct {
 
         // The wlr_backend abstracts the input/output hardware. Autocreate chooses
         // the best option based on the environment, for example DRM when run from
-        // a tty or wayland if WAYLAND_DISPLAY is set.
-        //
-        // This frees itself.when the wl_display is destroyed.
+        // a tty or wayland if WAYLAND_DISPLAY is set. This frees itself when the
+        // wl_display is destroyed.
         self.wlr_backend = c.river_wlr_backend_autocreate(self.wl_display) orelse
             return error.CantCreateWlrBackend;
 
+        // This backend is used to create a noop output for use when no actual
+        // outputs are available. This frees itself when the wl_display is destroyed.
+        self.noop_backend = c.river_wlr_noop_backend_create(self.wl_display) orelse
+            return error.CantCreateNoopBackend;
+
         // If we don't provide a renderer, autocreate makes a GLES2 renderer for us.
         // The renderer is responsible for defining the various pixel formats it
         // supports for shared memory, this configures that for clients.