kike: optionally write a PID file

kike.c

@@ -31,6 +31,16 @@
 
 static struct config_item g_config_table[] =
 {
+	// TODO: expand ~[blah] in resolve_config_filename()
+	// TODO: expand the path at least? Use XDG_RUNTIME_DIR if relative.
+	//   The last fallback is: "$XDG_DATA_HOME defines the base directory
+	//   relative to which user specific data files should be stored.
+	//   If $XDG_DATA_HOME is either not set or empty, a default equal to
+	//   $HOME/.local/share should be used."
+	//
+	//   Note that when using XDG_RUNTIME_DIR, it either needs to have access
+	//   time bumped every 6 hours, or have the sticky bit set.
+	{ "pid_file",        NULL,              "Full path for the PID file"     },
 	{ "server_name",     NULL,              "Server name"                    },
 	{ "server_info",     "My server",       "Brief server description"       },
 	{ "motd",            NULL,              "MOTD filename"                  },
@@ -3661,6 +3671,51 @@ irc_initialize_server_name (struct server_context *ctx, struct error **e)
 	return true;
 }
 
+static bool
+irc_lock_pid_file (struct server_context *ctx, struct error **e)
+{
+	const char *path = str_map_find (&ctx->config, "pid_file");
+	if (!path)
+		return true;
+
+	int fd = open (path, O_RDWR | O_CREAT,
+		S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH /* 644 */);
+	if (fd < 0)
+	{
+		error_set (e, "can't open `%s': %s", path, strerror (errno));
+		return false;
+	}
+
+	struct flock lock =
+	{
+		.l_type = F_WRLCK,
+		.l_start = 0,
+		.l_whence = SEEK_SET,
+		.l_len = 0,
+	};
+	if (fcntl (fd, F_SETLK, &lock))
+	{
+		error_set (e, "can't lock `%s': %s", path, strerror (errno));
+		return false;
+	}
+
+	struct str pid;
+	str_init (&pid);
+	str_append_printf (&pid, "%ld", (long) getpid ());
+
+	if (ftruncate (fd, 0)
+	 || write (fd, pid.str, pid.len) != (ssize_t) pid.len)
+	{
+		error_set (e, "can't write to `%s': %s", path, strerror (errno));
+		return false;
+	}
+	str_free (&pid);
+
+	// Intentionally not closing the file descriptor; it must stay alive
+	// for the entire life of the application
+	return true;
+}
+
 static int
 irc_listen (struct addrinfo *gai_iter)
 {
@@ -3781,7 +3836,6 @@ on_signal_pipe_readable (const struct pollfd *fd, struct server_context *ctx)
 static void
 daemonize (void)
 {
-	// TODO: create and lock a PID file?
 	print_status ("daemonizing...");
 
 	if (chdir ("/"))
@@ -3804,7 +3858,10 @@ daemonize (void)
 	openlog (PROGRAM_NAME, LOG_NDELAY | LOG_NOWAIT | LOG_PID, 0);
 	g_log_message_real = log_message_syslog;
 
-	// XXX: we may close our own descriptors this way, crippling ourselves
+	// XXX: we may close our own descriptors this way, crippling ourselves;
+	//   there is no real guarantee that we will start with all three
+	//   descriptors open.  In theory we could try to enumerate the descriptors
+	//   at the start of main().
 	for (int i = 0; i < 3; i++)
 		xclose (i);
 
@@ -3887,6 +3944,7 @@ main (int argc, char *argv[])
 	 || !irc_initialize_motd (&ctx, &e)
 	 || !irc_initialize_catalog (&ctx, &e)
 	 || !irc_parse_config (&ctx, &e)
+	 || !irc_lock_pid_file (&ctx, &e)
 	 || !irc_setup_listen_fds (&ctx, &e))
 	{
 		print_error ("%s", e->message);