--- a/mod_pubsub_serverinfo/README.markdown	Wed Jan 03 07:53:55 2024 +0100
+++ b/mod_pubsub_serverinfo/README.markdown	Wed Jan 03 23:05:14 2024 +0100
@@ -5,4 +5,39 @@
 
 Exposes server information over Pub/Sub per ProtoXEP: PubSub Server Information.
 
-This initial version only announces support (used to 'opt-in', per the XEP). It does not publish any data. This is intended to be a future addition to this mod.
+This version, announces support (used to 'opt-in', per the XEP) and publishes the name of the local domain via a Pub/Sub node. The published data
+will contain an unnamed 'remote-domain' element for each inbound or outgoing s2s connection.
+
+Features yet to be implemented:
+- For 'remote-domain' elements, add domain name _only if_ through service discovery that domain advertises the 'urn:xmpp:serverinfo:0' feature.
+
+Installation
+============
+
+Enable this module in the global or a virtual host.
+
+The default configuration requires the existence of a Pub/Sub component that uses the 'pubsub' subdomain of the host in which the module is enabled:
+
+    Component "pubsub.example.org" "pubsub"
+
+The module will create a node and publish data, using a JID that matches the XMPP domain name of the host. Ensure that this actor is an admin of the
+Pub/Sub service:
+
+    admins = { "example.org" }
+
+Configuration
+=============
+
+The Pub/Sub service on which data is published, by default, is a component addressed as the `pubsub` subdomain of the domain of the virtual host that
+the module is loaded under. To change this, apply this configuration setting:
+
+    pubsub_serverinfo_service = "anotherpubsub.example.org"
+
+The Pub/Sub node on which data is published is, by default, a leaf-node named `serverinfo`. To change this, apply this configuration setting:
+
+    pubsub_serverinfo_node = "foobar"
+
+To prevent a surplus of event notifications, this module will only publish new data after a certain period of time has expired. The default duration
+is 300 seconds (5 minutes). To change this simply put in the config:
+
+    pubsub_serverinfo_publication_interval = 180 -- or any other number of seconds

--- a/mod_pubsub_serverinfo/mod_pubsub_serverinfo.lua	Wed Jan 03 07:53:55 2024 +0100
+++ b/mod_pubsub_serverinfo/mod_pubsub_serverinfo.lua	Wed Jan 03 23:05:14 2024 +0100
@@ -1,1 +1,168 @@
-module:add_feature("urn:xmpp:serverinfo:0");
+local st = require "util.stanza";
+local new_id = require"util.id".medium;
+local dataform = require "util.dataforms".new;
+
+local local_domain = module:get_host();
+local service = module:get_option(module.name .. "_service") or "pubsub." .. local_domain;
+local node = module:get_option(module.name .. "_node") or "serverinfo";
+local actor = module.host .. "/modules/" .. module.name;
+local publication_interval = module:get_option(module.name .. "_publication_interval") or 300;
+
+local opt_in_reports
+
+function module.load()
+	-- Will error out with a 'conflict' if the node already exists. TODO: create the node only when it's missing.
+	create_node():next()
+
+	module:add_feature("urn:xmpp:serverinfo:0");
+
+	module:add_extension(dataform {
+		{ name = "FORM_TYPE", type = "hidden", value = "http://jabber.org/network/serverinfo" },
+		{ name = "serverinfo-pubsub-node", type = "text-single" },
+	}:form({ ["serverinfo-pubsub-node"] = ("xmpp:%s?;node=%s"):format(service, node) }, "result"));
+
+	module:add_timer(10, publish_serverinfo);
+end
+
+function module.unload()
+	-- This removes all subscribers, which may or may not be desirable, depending on the reason for the unload.
+	delete_node(); -- Should this block, to delay unload() until the node is deleted?
+end
+
+-- Returns a promise
+function create_node()
+	local request = st.iq({ type = "set", to = service, from = actor, id = new_id() })
+		:tag("pubsub", { xmlns = "http://jabber.org/protocol/pubsub" })
+			:tag("create", { node = node }):up()
+			:tag("configure")
+				:tag("x", { xmlns = "jabber:x:data", type = "submit" })
+					:tag("field", { var = "FORM_TYPE", type = "hidden"})
+						:text_tag("value", "http://jabber.org/protocol/pubsub#node_config")
+						:up()
+					:tag("field", { var = "pubsub#max_items" })
+						:text_tag("value", "1")
+						:up()
+					:tag("field", { var = "pubsub#persist_items" })
+						:text_tag("value", "0")
+	return module:send_iq(request);
+end
+
+-- Returns a promise
+function delete_node()
+	local request = st.iq({ type = "set", to = service, from = actor, id = new_id() })
+		:tag("pubsub", { xmlns = "http://jabber.org/protocol/pubsub" })
+			:tag("delete", { node = node });
+
+	return module:send_iq(request);
+end
+
+function publish_serverinfo()
+	-- Iterate over s2s sessions, adding them to a multimap, where the key is the local domain name,
+	-- mapped to a collection of remote domain names. De-duplicate all remote domain names by using
+	-- them as an index in a table.
+	local domains_by_host = {}
+	for session, _ in pairs(prosody.incoming_s2s) do
+		if session ~= nil and session.from_host ~= nil and local_domain == session.to_host then
+			local sessions = domains_by_host[session.to_host]
+			if sessions == nil then sessions = {} end; -- instantiate a new entry if none existed
+			sessions[session.from_host] = true
+			domains_by_host[session.to_host] = sessions
+		end
+	end
+
+	-- At an earlier stage, the code iterated voer all prosody.hosts - but that turned out to be to noisy.
+	-- for host, data in pairs(prosody.hosts) do
+	local host = local_domain
+	local data = prosody.hosts[host]
+	if data ~= nil then
+		local sessions = domains_by_host[host]
+		if sessions == nil then sessions = {} end; -- instantiate a new entry if none existed
+		if data.s2sout ~= nil then
+			for _, session in pairs(data.s2sout) do
+				if session.to_host ~= nil then
+					sessions[session.to_host] = true
+					domains_by_host[host] = sessions
+				end
+			end
+		end
+	end
+
+	-- Build the publication stanza.
+	local request = st.iq({ type = "set", to = service, from = actor, id = new_id() })
+		:tag("pubsub", { xmlns = "http://jabber.org/protocol/pubsub" })
+			:tag("publish", { node = node, xmlns = "http://jabber.org/protocol/pubsub" })
+				:tag("item", { id = "current", xmlns = "http://jabber.org/protocol/pubsub" })
+					:tag("serverinfo", { xmlns = "urn:xmpp:serverinfo:0" })
+
+	request:tag("domain", { name = local_domain })
+		:tag("federation")
+
+	local remotes = domains_by_host[host]
+
+	if remotes ~= nil then
+		for remote, _ in pairs(remotes) do
+			-- include a domain name for remote domains, but only if they advertise support.
+			if does_opt_in(remote) then
+				request:tag("remote-domain", { name = remote }):up()
+			else
+				request:tag("remote-domain"):up()
+			end
+		end
+	end
+
+	request:up():up()
+
+	module:send_iq(request):next()
+
+	return publication_interval;
+end
+
+local opt_in_cache = {}
+
+function does_opt_in(remoteDomain)
+
+	-- try to read answer from cache.
+	local cached_value = opt_in_cache[remoteDomain]
+	if cached_value ~= nil and os.difftime(cached_value.expires, os.time()) > 0 then
+		return cached_value.opt_in;
+	end
+
+	-- TODO worry about not having multiple requests in flight to the same domain.cached_value
+
+	-- Cache could not provide an answer. Perform service discovery.
+    local discoRequest = st.iq({ type = "get", to = remoteDomain, from = actor, id = new_id() })
+    	:tag("query", { xmlns = "http://jabber.org/protocol/disco#info" })
+
+	module:send_iq(discoRequest):next(
+		function(response)
+			if response.stanza ~= nil and response.stanza.attr.type == "result" then
+				local query = response.stanza:get_child("query", "http://jabber.org/protocol/disco#info")
+				if query ~= nil then
+					for feature in query:childtags("feature", "http://jabber.org/protocol/disco#info") do
+						if feature.attr.var == 'urn:xmpp:serverinfo:0' then
+							opt_in_cache[remoteDomain] = {
+								opt_in = true;
+								expires = os.time() + 3600;
+							}
+							return; -- prevent 'false' to be cached, down below.
+						end
+					end
+				end
+			end
+			opt_in_cache[remoteDomain] = {
+				opt_in = false;
+				expires = os.time() + 3600;
+			}
+		end,
+		function(response)
+			opt_in_cache[remoteDomain] = {
+				opt_in = false;
+				expires = os.time() + 3600;
+			}
+		end
+	);
+
+	-- return 'false' for now. Better luck next time...
+	return false;
+
+end