commands: implement cmd_append_layout

Wires the JSON loader to a config/runtime command. Defers at
config-read time so workspaces exist when the layout is applied.

include/sway/commands.h

@@ -105,6 +105,7 @@ sway_cmd cmd_exec_validate;
 sway_cmd cmd_exec_process;
 
 sway_cmd cmd_allow_tearing;
+sway_cmd cmd_append_layout;
 sway_cmd cmd_assign;
 sway_cmd cmd_bar;
 sway_cmd cmd_bindcode;

sway/commands.c

@@ -42,6 +42,7 @@ struct cmd_results *checkarg(int argc, const char *name, enum expected_args type
 
 /* Keep alphabetized */
 static const struct cmd_handler handlers[] = {
+	{ "append_layout", cmd_append_layout },
 	{ "assign", cmd_assign },
 	{ "bar", cmd_bar },
 	{ "bindcode", cmd_bindcode },

sway/commands/append_layout.c

@@ -0,0 +1,47 @@
+#include "log.h"
+#include "stringop.h"
+#include "sway/commands.h"
+#include "sway/config.h"
+#include "sway/input/input-manager.h"
+#include "sway/input/seat.h"
+#include "sway/tree/load_layout.h"
+#include "sway/tree/workspace.h"
+
+struct cmd_results *cmd_append_layout(int argc, char **argv) {
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, "append_layout", EXPECTED_AT_LEAST, 1))) {
+		return error;
+	}
+
+	if (config->reading || !config->active) {
+		return cmd_results_new(CMD_DEFER, NULL);
+	}
+
+	struct sway_seat *seat = input_manager_current_seat();
+	struct sway_workspace *ws = seat_get_focused_workspace(seat);
+	if (!ws) {
+		return cmd_results_new(CMD_FAILURE,
+				"append_layout: no focused workspace");
+	}
+
+	char *path = join_args(argv, argc);
+	if (!path) {
+		return cmd_results_new(CMD_FAILURE, "append_layout: out of memory");
+	}
+	if (!expand_path(&path)) {
+		free(path);
+		return cmd_results_new(CMD_FAILURE,
+				"append_layout: failed to expand path");
+	}
+
+	char *err = NULL;
+	bool ok = load_layout_from_file(ws, path, &err);
+	free(path);
+	if (!ok) {
+		struct cmd_results *res = cmd_results_new(CMD_FAILURE, "%s",
+				err ? err : "append_layout: unknown error");
+		free(err);
+		return res;
+	}
+	return cmd_results_new(CMD_SUCCESS, NULL);
+}

sway/meson.build

@@ -43,6 +43,7 @@ sway_sources = files(
 	'config/input.c',
 
 	'commands/allow_tearing.c',
+	'commands/append_layout.c',
 	'commands/assign.c',
 	'commands/bar.c',
 	'commands/bind.c',