add logger for config errors and <core><errorCommand> to display it.

include/common/log.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_LOG_H
+#define LABWC_LOG_H
+
+#include <wlr/util/log.h>
+#include "common/buf.h"
+
+struct buf *log_get_buf(void);
+void log_set_error(enum wlr_log_importance importance);
+void log_reset(void);
+void nag_show(void);
+void nag_show_callback(void *data);
+
+#define nag_log(verb, fmt, ...) \
+do { \
+	wlr_log(verb, fmt, ##__VA_ARGS__);						\
+	log_set_error(verb);									\
+	buf_add_fmt(log_get_buf(), fmt "\n", ##__VA_ARGS__);	\
+} while (0)
+
+#endif /* LABWC_LOG_H */

include/common/spawn.h

@@ -22,6 +22,19 @@ void spawn_async_no_shell(char const *command);
  */
 void spawn_sync_no_shell(char const *command);
 
+/**
+ * spawn_piped_async_no_shell - execute asynchronously
+ * @command: command to be executed
+ * @pipe_fd_w: set to the write end of a pipe
+ *             connected to stdin of the command
+ * Notes:
+ * The returned pid_t has to be waited for to
+ * not produce zombies and the pipe_fd_w has to
+ * be closed. spawn_piped_close() can be used
+ * to ensure both.
+ */
+pid_t spawn_piped_async_no_shell(const char *command, int *pipe_fd_w);
+
 /**
  * spawn_piped - execute asynchronously
  * @command: command to be executed

include/config/rcxml.h

@@ -87,6 +87,7 @@ struct rcxml {
 	bool xwayland_persistence;
 	bool primary_selection;
 	char *prompt_command;
+	char *error_command;
 
 	/* placement */
 	enum lab_placement_policy placement_policy;