mod_external_services: Validate services added via events

While writing developer documentation it became obvious that i was silly
to have one item format for config and items API, and another format for
the event API.

Then there's the stanza format, but that's a common pattern.

This change reduces the possible input formats to two and allows other
modules the benefit of the processing and validation performed on items
from the config.

plugins/mod_external_services.lua

@@ -4,6 +4,7 @@ local base64 = require "util.encodings".base64;
 local hashes = require "util.hashes";
 local st = require "util.stanza";
 local jid = require "util.jid";
+local array = require "util.array";
 
 local default_host = module:get_option_string("external_service_host", module.host);
 local default_port = module:get_option_number("external_service_port");
@@ -105,6 +106,14 @@ function module.load()
 	end
 end
 
+-- Ensure only valid items are added in events
+local services_mt = {
+	__index = getmetatable(array()).__index;
+	__newindex = function (self, i, v)
+		rawset(self, i, assert(prepare(v), "Invalid service entry added"));
+	end;
+}
+
 local function handle_services(event)
 	local origin, stanza = event.origin, event.stanza;
 	local action = stanza.tags[1];
@@ -127,6 +136,8 @@ local function handle_services(event)
 		end);
 	end
 
+	setmetatable(services, services_mt);
+
 	module:fire_event("external_service/services", {
 			origin = origin;
 			stanza = stanza;
@@ -177,6 +188,9 @@ local function handle_credentials(event)
 			});
 	end
 
+	setmetatable(services, services_mt);
+	setmetatable(requested_credentials, services_mt);
+
 	module:fire_event("external_service/credentials", {
 			origin = origin;
 			stanza = stanza;