backend: add WLR_BACKEND env variable

backend/backend.c

@@ -6,6 +6,7 @@
 #include <unistd.h>
 #include <wayland-server.h>
 #include <wlr/backend/drm.h>
+#include <wlr/backend/headless.h>
 #include <wlr/backend/interface.h>
 #include <wlr/backend/libinput.h>
 #include <wlr/backend/multi.h>
@@ -101,6 +102,35 @@ static struct wlr_backend *attempt_x11_backend(struct wl_display *display,
 }
 #endif
 
+static struct wlr_backend *attempt_headless_backend(
+		struct wl_display *display) {
+	struct wlr_backend *backend = wlr_headless_backend_create(display);
+	if (backend == NULL) {
+		return NULL;
+	}
+
+	size_t outputs = parse_outputs_env("WLR_HEADLESS_OUTPUTS");
+	for (size_t i = 0; i < outputs; ++i) {
+		wlr_headless_add_output(backend, 1280, 720);
+	}
+
+	return backend;
+}
+
+static struct wlr_backend *attempt_backend_by_name(struct wl_display *display,
+		const char *name) {
+	if (strcmp(name, "wayland") == 0) {
+		return attempt_wl_backend(display);
+#ifdef WLR_HAS_X11_BACKEND
+	} else if (strcmp(name, "x11") == 0) {
+		return attempt_x11_backend(display, NULL);
+#endif
+	} else if (strcmp(name, "headless") == 0) {
+		return attempt_headless_backend(display);
+	}
+	return NULL;
+}
+
 struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
 	struct wlr_backend *backend = wlr_multi_backend_create(display);
 	if (!backend) {
@@ -108,6 +138,18 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
 		return NULL;
 	}
 
+	const char *name = getenv("WLR_BACKEND");
+	if (name) {
+		struct wlr_backend *subbackend = attempt_backend_by_name(display, name);
+		if (subbackend) {
+			wlr_multi_backend_add(backend, subbackend);
+			return backend;
+		} else {
+			wlr_log(L_ERROR, "unrecognized backend '%s'", name);
+			return NULL;
+		}
+	}
+
 	if (getenv("WAYLAND_DISPLAY") || getenv("_WAYLAND_DISPLAY") ||
 			getenv("WAYLAND_SOCKET")) {
 		struct wlr_backend *wl_backend = attempt_wl_backend(display);

docs/env_vars.md

@@ -5,6 +5,7 @@ wlroots specific
 * *WLR_DRM_DEVICES*: specifies the DRM devices (as a colon separated list) instead of auto probing them. The first existing device in this list is considered the primary DRM device.
 * *WLR_DRM_NO_ATOMIC*: set to 1 to use legacy DRM interface instead of atomic mode setting
 * *WLR_LIBINPUT_NO_DEVICES*: set to 1 to not fail without any input devices
+* *WLR_BACKEND*: force a specific backend to be used (one of: wayland, x11, headless)
 * *WLR_WL_OUTPUTS*: when using the wayland backend specifies the number of outputs
 * *WLR_X11_OUTPUTS*: when using the X11 backend specifies the number of outputs