--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_auto_moved/mod_auto_moved.lua	Sun Sep 12 18:49:56 2021 +0100
@@ -0,0 +1,90 @@
+local id = require "util.id";
+local jid = require "util.jid";
+local promise = require "util.promise";
+local rm = require "core.rostermanager";
+local st = require "util.stanza";
+
+local errors = require "util.error".init(module.name, {
+	["statement-not-found"] = { type = "cancel", condition = "item-not-found" };
+	["statement-mismatch"] = { type = "cancel", condition = "conlict" };
+});
+
+module:hook("presence/bare", function (event)
+	local origin, stanza = event.origin, event.stanza;
+	if stanza.attr.type ~= "subscribe" then
+		return; -- We're only interested in subscription requests
+	end
+	local moved = stanza:get_child("moved", "urn:xmpp:moved:1");
+	if not moved then
+		return; -- We're only interested in stanzas with a moved notification
+	end
+
+	local verification = stanza:get_child("moved-verification", "https://prosody.im/protocol/moved");
+	if verification then
+		return; -- We already attempted to verify this stanza
+	end
+
+	module:log("debug", "Received moved notification from %s", stanza.attr.from);
+
+	local old_jid = moved:get_child_text("old-jid");
+	if not old_jid then
+		return; -- Failed to read old JID
+	end
+
+	local to_user = jid.node(stanza.attr.to);
+	local new_jid_unverified = jid.bare(stanza.attr.from);
+
+	if not rm.is_contact_subscribed(to_user, module.host, old_jid) then
+		return; -- Old JID was not an existing contact, ignore
+	end
+
+	if rm.is_contact_pending_in(to_user, module.host, new_jid_unverified)
+	or rm.is_contact_subscribed(to_user, module.host, new_jid_unverified) then
+		return; -- New JID already subscribed or pending, ignore
+	end
+
+	local moved_statement_query = st.iq({ to = old_jid, type = "get", id = id.short() })
+		:tag("pubsub", { xmlns = "http://jabber.org/protocol/pubsub" })
+			:tag("items", { node = "urn:xmpp:moved:1" })
+				:tag("item", { id = "current" }):up()
+			:up()
+		:up();
+	-- TODO: Catch and handle <gone/> errors per note in XEP-0283.
+	module:send_iq(moved_statement_query):next(function (reply)
+		module:log("debug", "Statement reply: %s", reply.stanza);
+		local moved_statement = reply.stanza:find("{http://jabber.org/protocol/pubsub}pubsub/items/{http://jabber.org/protocol/pubsub}item/{urn:xmpp:moved:1}moved");
+		if not moved_statement then
+			return promise.reject(errors.new("statement-not-found")); -- No statement found
+		end
+
+		local new_jid = jid.prep(moved_statement:get_child_text("new-jid"));
+		if new_jid ~= new_jid_unverified then
+			return promise.reject(errors.new("statement-mismatch")); -- Verification failed; JIDs do not match
+		end
+
+		-- Verified!
+		module:log("info", "Verified moved notification <%s> -> <%s>", old_jid, new_jid);
+
+		-- Add incoming subscription and respond
+		rm.set_contact_pending_in(to_user, module.host, new_jid);
+		rm.subscribed(to_user, module.host, new_jid);
+		module:send(st.presence({ to = new_jid, from = to_user.."@"..module.host, type = "subscribed" }));
+		rm.roster_push(to_user, module.host, new_jid);
+
+		-- Request outgoing subscription if old JID had one
+		if rm.is_user_subscribed(to_user, module.host, old_jid) then
+			module:log("debug", "Requesting subscription to new JID");
+			rm.set_contact_pending_out(to_user, module.host, new_jid);
+			module:send(st.presence({ to = new_jid, from = to_user.."@"..module.host, type = "subscribe" }));
+		end
+	end):catch(function (err)
+		module:log("debug", "Failed to verify moved statement for <%s> -> <%s>: %s", old_jid, new_jid_unverified, require "util.serialization".serialize(err, "debug"));
+		stanza:reset()
+			:tag("moved-verification", { xmlns = "https://prosody.im/protocol/moved", status = "failed" })
+			:up();
+		module:send(stanza, origin);
+	end);
+
+	-- Halt processing of the stanza, for now
+	return true;
+end, 1);