Adding a prosody module to support sip-style call flows.

When combined with mod_muc_poltergeist mod_muc_call allows
for enabling call features using a proper ext_events.lib.lua
implementation. By default when the module is configured only
stub implementations are used for ext_events.lib.lua as these
are unique between deployments.

resources/prosody-plugins/ext_events.lib.lua

@@ -0,0 +1,17 @@
+-- invite will perform the trigger for external call invites.
+-- This trigger is left unimplemented. The implementation is expected
+-- to be specific to the deployment.
+local function invite(stanza, url)
+   module:log(
+	  "warn",
+	  "A module has been configured that triggers external events."
+   )
+   module:log("warn", "Implement this lib to trigger external events.")
+end
+
+
+local ext_events = {
+   invite = invite
+}
+
+return ext_events

resources/prosody-plugins/mod_muc_call.lua

@@ -0,0 +1,64 @@
+local ext_events = module:require "ext_events"
+local jid = require "util.jid"
+
+-- Options and configuration
+local poltergeist_component = module:get_option_string(
+   "poltergeist_component",
+   module.host
+);
+local muc_domain_base = module:get_option_string("muc_mapper_domain_base");
+if not muc_domain_base then
+   module:log(
+	  "warn",
+	  "No 'muc_domain_base' option set, unable to send call events."
+   );
+   return
+end
+
+-- Status strings that trigger call events.
+local invited_status = "Invited"
+
+-- url_from_room_jid will determine the url for a conference
+-- provided a room jid. It is required that muc domain mapping
+-- is enabled and configured. There are two url formats that are supported.
+-- The following urls are examples of the supported formats.
+--     https://meet.jit.si/jitsi/ProductiveMeeting
+--     https://meet.jit.si/MoreProductiveMeeting
+-- The urls are derived from portions of the room jid.
+local function url_from_room_jid(room_jid)
+   local node, _, _ = jid.split(room_jid)
+   if not node then return nil end
+
+   local target_subdomain, target_node = node:match("^%[([^%]]+)%](.+)$")
+
+   if not(target_node or target_subdomain) then
+	  return "https://"..muc_domain_base.."/"..node
+   else
+	  return
+		 "https://"..muc_domain_base.."/"..target_subdomain.."/"..target_node
+   end
+end
+
+-- Listening for all muc presences stanza events. If a presence stanza is from
+-- a poltergeist then it will be further processed to determine if a call
+-- event should be triggered. Call events are triggered by status strings
+-- the status strings supported are:
+--    -------------------------
+--    Status     | Event Type
+--    _________________________
+--    "Invited"  | Invite
+module:hook("muc-broadcast-presence", function (event)
+    -- Detect if the presence is for a poltergeist or not.
+    if not
+	   (jid.bare(event.occupant.jid) == poltergeist_component)
+	   or
+	   event.stanza == nil
+	then
+	   return
+    end
+
+    if event.stanza:get_child_text("status") == invited_status then
+	   local url = assert(url_from_room_jid(event.stanza.attr.from))
+	   ext_events.invite(event.stanza, url)
+    end
+end, -101);