diff -r 987b203bb091 -r 3a96070f4a14 mod_muc_moderation/mod_muc_moderation.lua
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_muc_moderation/mod_muc_moderation.lua	Sat Feb 22 21:11:31 2020 +0100
@@ -0,0 +1,116 @@
+-- Imports
+local dt = require "util.datetime";
+local id = require "util.id";
+local jid = require "util.jid";
+local st = require "util.stanza";
+
+-- Plugin dependencies
+local mod_muc = module:depends "muc";
+
+local muc_util = module:require "muc/util";
+local valid_roles = muc_util.valid_roles;
+
+local muc_log_archive = module:open_store("muc_log", "archive");
+
+if not muc_log_archive.set then
+	module:log("warn", "Selected archive storage module does not support message replacement, no tombstones will be saved");
+end
+
+-- Namespaces
+local xmlns_fasten = "urn:xmpp:fasten:0";
+local xmlns_moderate = "urn:xmpp:message-moderate:0";
+local xmlns_retract = "urn:xmpp:message-retract:0";
+
+-- Discovering support
+module:hook("muc-disco#info", function (event)
+	event.reply:tag("feature", { var = xmlns_moderate }):up();
+end);
+
+-- Main handling
+module:hook("iq-set/bare/" .. xmlns_fasten .. ":apply-to", function (event)
+	local stanza, origin = event.stanza, event.origin;
+
+	-- Collect info we need
+	local apply_to = stanza.tags[1];
+	local moderate_tag = apply_to:get_child("moderate", xmlns_moderate);
+	if not moderate_tag then return end -- some other kind of fastening?
+
+	local reason = moderate_tag:get_child_text("reason");
+
+	local room_jid = stanza.attr.to;
+	local room_node = jid.split(room_jid);
+	local room = mod_muc.get_room_from_jid(room_jid);
+
+	local stanza_id = apply_to.attr.id;
+
+	-- Permissions
+	local actor = stanza.attr.from;
+	local actor_nick = room:get_occupant_jid(actor);
+	local affiliation = room:get_affiliation(actor);
+	local role = room:get_role(actor_nick) or room:get_default_role(affiliation);
+	if valid_roles[role or "none"] < valid_roles.moderator then
+		origin.send(st.error_reply(stanza, "auth", "forbidden", "Insufficient privileges"));
+		return true;
+	end
+
+	-- Original stanza to base tombstone on
+	local original, err;
+	if muc_log_archive.get then
+		original, err = muc_log_archive:get(room_node, stanza_id);
+	else
+		-- COMPAT missing :get API
+		err = "item-not-found";
+		for i, item in muc_log_archive:find(room_node, { key = stanza_id, limit = 1 }) do
+			if i == stanza_id then
+				original, err = item, nil;
+			end
+		end
+	end
+	if not original then
+		if err == "item-not-found" then
+			origin.send(st.error_reply(stanza, "modify", "item-not-found"));
+		else
+			origin.send(st.error_reply(stanza, "wait", "internal-server-error"));
+		end
+		return true;
+	end
+
+	-- Replacements
+	local tombstone = st.message({ from = original.attr.from, type = "groupchat", id = original.attr.id })
+		:tag("moderated", { xmlns = xmlns_moderate, by = actor_nick })
+			:tag("retracted", { xmlns = xmlns_retract, stamp = dt.datetime() }):up();
+
+	local announcement = st.message({ from = room_jid, type = "groupchat", id = id.medium(), })
+		:tag("apply-to", { xmlns = xmlns_fasten, id = stanza_id })
+			:tag("moderated", { xmlns = xmlns_moderate, by = actor_nick })
+			:tag("retract", { xmlns = xmlns_retract }):up();
+
+	if reason then
+		tombstone:text_tag("reason", reason);
+		announcement:text_tag("reason", reason);
+	end
+
+	if muc_log_archive.set then
+		-- Tombstone
+		local was_replaced = muc_log_archive:set(room_node, stanza_id, tombstone);
+		if not was_replaced then
+			origin.send(st.error_reply(stanza, "wait", "internal-server-error"));
+			return true;
+		end
+	end
+
+	-- Done, tell people about it
+	module:log("info", "Message with id '%s' in room %s moderated by %s, reason: %s", stanza_id, room_jid, actor, reason);
+	module:log("debug", ":broadcast(%s)", announcement);
+	room:broadcast(announcement);
+
+	origin.send(st.reply(stanza));
+	return true;
+end);
+
+module:hook("muc-message-is-historic", function (event)
+	-- Ensure moderation messages are stored
+	if event.stanza.attr.from == event.room.jid then
+		return event.stanza:get_child("apply-to", xmlns_fasten);
+	end
+end, 1);