options: reject invalid backend earlier

Unsupported backends were rejected when we try to initialize them, now
we reject them in the option parsing phase.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/options.c

@@ -10,6 +10,7 @@
 #include <unistd.h>
 #include <xcb/render.h>        // for xcb_render_fixed_t, XXX
 
+#include "backend/backend.h"
 #include "common.h"
 #include "config.h"
 #include "log.h"
@@ -480,7 +481,7 @@ bool get_early_config(int argc, char *const *argv, char **config_file, bool *all
 /**
  * Process arguments and configuration files.
  */
-void get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
+bool get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
              bool fading_enable, bool conv_kern_hasneg, win_option_mask_t *winopt_mask) {
 
 	int o = 0, longopt_idx = -1;
@@ -822,6 +823,13 @@ void get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
 		log_warn("--monitor-repaint has no effect when backend is not xrender");
 	}
 
+	if (opt->experimental_backends && !backend_list[opt->backend]) {
+		log_error("Backend \"%s\" is not available as part of the experimental "
+		          "backends.",
+		          BACKEND_STRS[opt->backend]);
+		return false;
+	}
+
 	// Range checking and option assignments
 	opt->fade_delta = max2(opt->fade_delta, 1);
 	opt->shadow_radius = max2(opt->shadow_radius, 0);
@@ -880,6 +888,8 @@ void get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
 		log_warn("A convolution kernel with negative values may not work "
 		         "properly under X Render backend.");
 	}
+
+	return true;
 }
 
 // vim: set noet sw=8 ts=8 :

src/options.h

@@ -27,8 +27,11 @@ bool get_early_config(int argc, char *const *argv, char **config_file, bool *all
  *   fading_enable
  *   conv_kern_hasneg
  *   winopt_mask
+ * Returns:
+ *   Whether configuration are processed successfully.
  */
-void get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
+bool must_use get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
-             bool fading_enable, bool conv_kern_hasneg, win_option_mask_t *winopt_mask);
+                      bool fading_enable, bool conv_kern_hasneg,
+                      win_option_mask_t *winopt_mask);
 
 // vim: set noet sw=8 ts=8:

src/picom.c

@@ -724,13 +724,8 @@ static bool initialize_backend(session_t *ps) {
 	if (ps->o.experimental_backends) {
 		assert(!ps->backend_data);
 		// Reinitialize win_data
-		if (backend_list[ps->o.backend]) {
+		assert(backend_list[ps->o.backend]);
 		ps->backend_data = backend_list[ps->o.backend]->init(ps);
-		} else {
-			log_error("Backend \"%s\" is not available as part of the "
-			          "experimental backends.",
-			          BACKEND_STRS[ps->o.backend]);
-		}
 		if (!ps->backend_data) {
 			log_fatal("Failed to initialize backend, aborting...");
 			quit(ps);
@@ -1751,7 +1746,11 @@ static session_t *session_init(int argc, char **argv, Display *dpy,
 	}
 
 	// Parse all of the rest command line options
-	get_cfg(&ps->o, argc, argv, shadow_enabled, fading_enable, hasneg, winopt_mask);
+	if (!get_cfg(&ps->o, argc, argv, shadow_enabled, fading_enable, hasneg, winopt_mask)) {
+		log_fatal("Failed to get configuration, usually mean you have specified "
+		          "invalid options.");
+		return NULL;
+	}
 
 	if (ps->o.logpath) {
 		auto l = file_logger_new(ps->o.logpath);