backend: allow multiple backends in WLR_BACKENDS

backend/backend.c

@@ -1,3 +1,4 @@
+#define _POSIX_C_SOURCE 200809L
 #include <assert.h>
 #include <errno.h>
 #include <libinput.h>
@@ -138,16 +139,32 @@ 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 {
+	char *names = getenv("WLR_BACKENDS");
+	if (names) {
+		names = strdup(names);
+		if (names == NULL) {
+			wlr_log(L_ERROR, "allocation failed");
+			return NULL;
+		}
+
+		char *saveptr;
+		char *name = strtok_r(names, ",", &saveptr);
+		while (name != NULL) {
+			struct wlr_backend *subbackend =
+				attempt_backend_by_name(display, name);
+			if (subbackend == NULL) {
 				wlr_log(L_ERROR, "unrecognized backend '%s'", name);
 				return NULL;
 			}
+
+			if (!wlr_multi_backend_add(backend, subbackend)) {
+				return NULL;
+			}
+
+			name = strtok_r(NULL, ",", &saveptr);
+		}
+
+		return backend;
 	}
 
 	if (getenv("WAYLAND_DISPLAY") || getenv("_WAYLAND_DISPLAY") ||

docs/env_vars.md

@@ -2,20 +2,26 @@ wlroots reads these environment variables
 
 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_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_BACKENDS*: comma-separated list of backends to use (available backends:
+  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
 
 rootston specific
 ------------------
-* *XKB_DEFAULT_RULES*, *XKB_DEFAULT_MODEL*, *XKB_DEFAULT_LAYOUT*, *XKB_DEFAULT_VARIANT*, *XKB_DEFAULT_OPTIONS*: xkb setup
+* *XKB_DEFAULT_RULES*, *XKB_DEFAULT_MODEL*, *XKB_DEFAULT_LAYOUT*,
+  *XKB_DEFAULT_VARIANT*, *XKB_DEFAULT_OPTIONS*: xkb setup
 
 generic
 -------
 * *DISPLAY*: if set probe X11 backend in *wlr_backend_autocreate*
-* *WAYLAND_DISPLAY*, *_WAYLAND_DISPLAY*, *WAYLAND_SOCKET*: if set probe Wayland backend in *wlr_backend_autocreate*
+* *WAYLAND_DISPLAY*, *_WAYLAND_DISPLAY*, *WAYLAND_SOCKET*: if set probe Wayland
+  backend in *wlr_backend_autocreate*
 * *XCURSOR_PATH*: directory where xcursors are located
 * *XDG_RUNTIME_DIR*: used to store non persistent temporary files