diff -r f74c7c518bb2 -r a7c0c70e64b6 mod_ping_muc/mod_ping_muc.lua
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_ping_muc/mod_ping_muc.lua	Thu Dec 02 23:43:11 2021 +0100
@@ -0,0 +1,66 @@
+local id = require "util.id";
+local jid = require "util.jid";
+local set = require "util.set";
+local st = require "util.stanza";
+
+module:depends "track_muc_joins";
+module:add_feature("https://modules.prosody.im/mod_" .. module.name);
+
+local local_sessions = prosody.hosts[module.host].sessions;
+
+module:hook_global("s2s-destroyed", function(event)
+	local s2s_session = event.session;
+	if s2s_session.direction == "outgoing" and s2s_session.from_host ~= module.host then
+		return
+	elseif s2s_session.direction == "incoming" and s2s_session.to_host ~= module.host then
+		return
+	end
+
+	local related_hosts = set.new({ s2s_session.direction == "outgoing" and s2s_session.to_host or s2s_session.from_host });
+
+	if s2s_session.hosts then
+		-- While rarely used, multiplexing is still supported
+		for host, state in pairs(s2s_session.hosts) do if state.authed then related_hosts:add(host); end end
+	end
+
+	for _, user_session in pairs(local_sessions) do
+		for _, session in pairs(user_session.sessions) do
+			if session.rooms_joined then
+				for room, info in pairs(session.rooms_joined) do
+					local nick = info.nick or info;
+					local room_nick = room .. "/" .. nick;
+					if related_hosts:contains(jid.host(room)) then
+						-- User is in a MUC room for which the s2s connection was lost. Now what?
+
+						-- Self-ping
+						-- =========
+						--
+						-- Response of <iq type=result> means the user is still in the room
+						-- (and self-ping is supported), so we do nothing.
+						--
+						-- An error reply either means the user has fallen out of the room,
+						-- or that self-ping is unsupported. In the later case, whether the
+						-- user is still joined is indeterminate and we might as well
+						-- pretend they fell out.
+						module:send_iq(st.iq({ type = "get"; id = id.medium(); from = session.full_jid; to = room_nick })
+								:tag("ping", { xmlns = "urn:xmpp:ping"; }))
+						:catch(function(err)
+							module:send(
+								st.presence({ type = "unavailable"; id = id.medium(); to = session.full_jid; from = room_nick })
+									:tag("x", { xmlns = "http://jabber.org/protocol/muc#user" })
+										:tag("item", { affiliation = "none"; role = "none" })
+											:text_tag("reason", err.text or "Connection to remote server lost")
+										:up()
+										:tag("status", { code = "110" }):up()
+										:tag("status", { code = "307" }):up()
+										:tag("status", { code = "333" }):up()
+									:reset());
+						end);
+						-- TODO do this with some delay?
+					end
+				end
+
+			end
+		end
+	end
+end);