libctrl: Add support for 'deferred control commands'

Sometimes a control interface command cannot be processed
and responded immediately, but we need to process it asynchronously.

In order to support this, we introduce the 'ctrl_cmd_def', which
represents such a deferred command.  It is created by the service
implementing the command using ctrl_cmd_def_make(), and a response is
later sent using ctrl_cmd_def_send().

ctrl_cmd_def_is_zombie() must be called to handle the case where
the control connection has disconnected/died between receiving the
command and sending the response.

include/osmocom/ctrl/control_cmd.h

@@ -45,6 +45,9 @@ struct ctrl_connection {
 
 	/* Pending commands for this connection */
 	struct llist_head cmds;
+
+	/* Pending deferred commands for this connection */
+	struct llist_head def_cmds;
 };
 
 struct ctrl_cmd {
@@ -75,6 +78,18 @@ struct ctrl_cmd_map {
 	enum ctrl_type type;
 };
 
+/* deferred control command, i.e. responded asynchronously */
+struct ctrl_cmd_def {
+	struct llist_head list;		/* ctrl_connection.def_cmds */
+	struct ctrl_cmd *cmd;
+	void *data;			/* opaque user data */
+};
+
+struct ctrl_cmd_def *
+ctrl_cmd_def_make(const void *ctx, struct ctrl_cmd *cmd, void *data, unsigned int secs);
+int ctrl_cmd_def_is_zombie(struct ctrl_cmd_def *cd);
+int ctrl_cmd_def_send(struct ctrl_cmd_def *cd);
+
 int ctrl_cmd_exec(vector vline, struct ctrl_cmd *command, vector node, void *data);
 int ctrl_cmd_install(enum ctrl_node_type node, struct ctrl_cmd_element *cmd);
 int ctrl_cmd_send(struct osmo_wqueue *queue, struct ctrl_cmd *cmd);

src/ctrl/control_cmd.c

@@ -487,3 +487,63 @@ err:
 	msgb_free(msg);
 	return NULL;
 }
+
+struct ctrl_cmd_def *
+ctrl_cmd_def_make(const void *ctx, struct ctrl_cmd *cmd, void *data, unsigned int secs)
+{
+	struct ctrl_cmd_def *cd;
+
+	if (!cmd->ccon)
+		return NULL;
+
+	cd = talloc_zero(ctx, struct ctrl_cmd_def);
+
+	cd->cmd = cmd;
+	cd->data = data;
+
+	/* add to per-connection list of deferred commands */
+	llist_add(&cd->list, &cmd->ccon->def_cmds);
+
+	return cd;
+}
+
+int ctrl_cmd_def_is_zombie(struct ctrl_cmd_def *cd)
+{
+	/* luckily we're still alive */
+	if (cd->cmd)
+		return 0;
+
+	/* if we are a zombie, make sure we really die */
+	llist_del(&cd->list);
+	talloc_free(cd);
+
+	return 1;
+}
+
+int ctrl_cmd_def_send(struct ctrl_cmd_def *cd)
+{
+	struct ctrl_cmd *cmd = cd->cmd;
+
+	int rc;
+
+	/* Deferred commands can only be responses to GET/SET or ERROR, but
+	 * never TRAP or anything else */
+	switch (cmd->type) {
+	case CTRL_TYPE_GET:
+		cmd->type = CTRL_TYPE_GET_REPLY;
+		break;
+	case CTRL_TYPE_SET:
+		cmd->type = CTRL_TYPE_SET_REPLY;
+		break;
+	default:
+		cmd->type = CTRL_TYPE_ERROR;
+	}
+
+	rc = ctrl_cmd_send(&cmd->ccon->write_queue, cmd);
+
+	talloc_free(cmd);
+	llist_del(&cd->list);
+	talloc_free(cd);
+
+	return rc;
+}

src/ctrl/control_if.c

@@ -128,6 +128,8 @@ struct ctrl_cmd *ctrl_cmd_trap(struct ctrl_cmd *cmd)
 
 static void control_close_conn(struct ctrl_connection *ccon)
 {
+	struct ctrl_cmd_def *cd, *cd2;
+
 	osmo_wqueue_clear(&ccon->write_queue);
 	close(ccon->write_queue.bfd.fd);
 	osmo_fd_unregister(&ccon->write_queue.bfd);
@@ -135,6 +137,19 @@ static void control_close_conn(struct ctrl_connection *ccon)
 	if (ccon->closed_cb)
 		ccon->closed_cb(ccon);
 	msgb_free(ccon->pending_msg);
+
+	/* clean up deferred commands */
+	llist_for_each_entry_safe(cd, cd2, &ccon->def_cmds, list) {
+		/* delete from list of def_cmds for this ccon */
+		llist_del(&cd->list);
+		/* not strictly needed as this is a slave to the ccon which we
+		 * are about to free anyway */
+		talloc_free(cd->cmd);
+		/* set the CMD to null, this is the indication to the user that
+		 * the connection for this command has gone */
+		cd->cmd = NULL;
+	}
+
 	talloc_free(ccon);
 }
 
@@ -338,6 +353,8 @@ static struct ctrl_connection *ctrl_connection_alloc(void *ctx)
 	/* Error handling here? */
 
 	INIT_LLIST_HEAD(&ccon->cmds);
+	INIT_LLIST_HEAD(&ccon->def_cmds);
+
 	return ccon;
 }