mod_server_info: New module to manage the serverinfo disco extension form

This allows multiple modules to populate the form dynamically. Currently the
form is "owned" by mod_server_contact_info, which prevents other modules from
contributing to it.

A further commit will port mod_server_contact_info to use this module.

plugins/mod_server_info.lua

@@ -0,0 +1,55 @@
+local dataforms = require "prosody.util.dataforms";
+
+local server_info_config = module:get_option("server_info", {});
+local server_info_custom_fields = module:get_option_array("server_info_extensions");
+
+-- Source: http://xmpp.org/registrar/formtypes.html#http:--jabber.org-network-serverinfo
+local form_layout = dataforms.new({
+	{ var = "FORM_TYPE"; type = "hidden"; value = "http://jabber.org/network/serverinfo" };
+});
+
+if server_info_custom_fields then
+	for _, field in ipairs(server_info_custom_fields) do
+		table.insert(form_layout, field);
+	end
+end
+
+local generated_form;
+
+function update_form()
+	local new_form = form_layout:form(server_info_config, "result");
+	if generated_form then
+		module:remove_item("extension", generated_form);
+	end
+	generated_form = new_form;
+	module:add_item("extension", generated_form);
+end
+
+function add_fields(event)
+	local fields = event.item;
+	for _, field in ipairs(fields) do
+		table.insert(form_layout, field);
+	end
+	update_form();
+end
+
+function remove_fields(event)
+	local removed_fields = event.item;
+	for _, removed_field in ipairs(removed_fields) do
+		local removed_var = removed_field.var or removed_field.name;
+		for i, field in ipairs(form_layout) do
+			local var = field.var or field.name
+			if var == removed_var then
+				table.remove(form_layout, i);
+				break;
+			end
+		end
+	end
+	update_form();
+end
+
+module:handle_items("server-info-fields", add_fields, remove_fields);
+
+function module.load()
+	update_form();
+end