Allow mapping to modifier keycodes

src/descriptor.c

@@ -236,10 +236,9 @@ int parse_descriptor(char *s,
 	if (!parse_key_sequence(s, &seq)) {
 		desc->op = OP_KEYSEQ;
 
-		if (keycode_to_mod(seq.code)) {
-			err("modifier keycodes are not directly mappable, you probably want layer(<modifier name>) (e.g layer(alt))");
-			return -1;
-		}
+		if (keycode_to_mod(seq.code))
+			fprintf(stderr,
+				"WARNING: mapping modifier keycodes directly may produce unintended results, you probably want layer(<modifier name>) instead\n");
 
 		desc->args[0].sequence = seq;
 	} else if (!parse_macro_fn(&config->macros[macro_idx], s)) {

src/keyd.c

@@ -226,22 +226,23 @@ static void send_repetitions()
 
 void set_mods(uint16_t mods)
 {
+	static uint16_t state = 0;
 	size_t i;
 
-	for (i = 0; i < sizeof modifier_table / sizeof modifier_table[0]; i++) {
-		struct modifier_table_ent *m = &modifier_table[i];
+	uint16_t diff = mods ^ state;
 
-		if (m->mask & mods) {
-			if (!keystate[m->code1] && !keystate[m->code2])
-				send_key(m->code1, 1);
-		} else {
-			if (keystate[m->code1])
-				send_key(m->code1, 0);
+	if (MOD_CTRL & diff)
+		send_key(KEY_LEFTCTRL, !!(MOD_CTRL & mods));
+	if (MOD_ALT_GR & diff)
+		send_key(KEY_RIGHTALT, !!(MOD_ALT_GR & mods));
+	if (MOD_SHIFT & diff)
+		send_key(KEY_LEFTSHIFT, !!(MOD_SHIFT & mods));
+	if (MOD_SUPER & diff)
+		send_key(KEY_LEFTMETA, !!(MOD_SUPER & mods));
+	if (MOD_ALT & diff)
+		send_key(KEY_LEFTALT, !!(MOD_ALT & mods));
 
-			if (keystate[m->code2])
-				send_key(m->code2, 0);
-		}
-	}
+	state = mods;
 }
 
 /* Block on the given keyboard nodes until no keys are depressed. */

t/layer4.t

@@ -0,0 +1,13 @@
+s down
+meta down
+c down
+c up
+meta up
+s up
+
+shift down
+meta down
+c down
+c up
+meta up
+shift up

t/run.sh

@@ -4,7 +4,7 @@
 
 cleanup() {
 	sudo pkill keyd
-	sleep .5s
+	sleep 1s
 	sudo systemctl restart keyd
 
 	trap - EXIT

t/test.conf

@@ -23,6 +23,7 @@ l = layer(test)
 t = overload(o, esc, 2)
 m = macro(C-h one)
 - = layout(layout2)
+s = leftshift
 
 [layout2:layout]