--- a/plugins/mod_mam/mod_mam.lua	Thu Mar 14 08:09:49 2019 +0000
+++ b/plugins/mod_mam/mod_mam.lua	Fri Mar 22 17:32:56 2019 +0100
@@ -33,7 +33,7 @@
 local tostring = tostring;
 local time_now = os.time;
 local m_min = math.min;
-local timestamp, timestamp_parse = require "util.datetime".datetime, require "util.datetime".parse;
+local timestamp, timestamp_parse, datestamp = import( "util.datetime", "datetime", "parse", "date");
 local default_max_items, max_max_items = 20, module:get_option_number("max_archive_query_results", 50);
 local strip_tags = module:get_option_set("dont_archive_namespaces", { "http://jabber.org/protocol/chatstates" });
 
@@ -46,13 +46,8 @@
 end
 local use_total = module:get_option_boolean("mam_include_total", true);
 
-local cleanup;
-
-local function schedule_cleanup(username)
-	if cleanup and not cleanup[username] then
-		table.insert(cleanup, username);
-		cleanup[username] = true;
-	end
+function schedule_cleanup()
+	-- replaced by non-noop later if cleanup is enabled
 end
 
 -- Handle prefs.
@@ -96,7 +91,6 @@
 	local qid = query.attr.queryid;
 
 	get_prefs(origin.username, true);
-	schedule_cleanup(origin.username);
 
 	-- Search query parameters
 	local qwith, qstart, qend;
@@ -212,6 +206,7 @@
 local function shall_store(user, who)
 	-- TODO Cache this?
 	if not um.user_exists(user, host) then
+		module:log("debug", "%s@%s does not exist", user, host)
 		return false;
 	end
 	local prefs = get_prefs(user);
@@ -329,6 +324,9 @@
 local cleanup_after = module:get_option_string("archive_expires_after", "1w");
 local cleanup_interval = module:get_option_number("archive_cleanup_interval", 4 * 60 * 60);
 if cleanup_after ~= "never" then
+	local cleanup_storage = module:open_store("archive_cleanup");
+	local cleanup_map = module:open_store("archive_cleanup", "map");
+
 	local day = 86400;
 	local multipliers = { d = day, w = day * 7, m = 31 * day, y = 365.2425 * day };
 	local n, m = cleanup_after:lower():match("(%d+)%s*([dwmy]?)");
@@ -346,33 +344,47 @@
 		return false;
 	end
 
-	-- Set of known users to do message expiry for
-	-- Populated either below or when new messages are added
-	cleanup = {};
+	-- For each day, store a set of users that have new messages. To expire
+	-- messages, we collect the union of sets of users from dates that fall
+	-- outside the cleanup range.
+
+	function schedule_cleanup(username, date)
+		cleanup_map:set(date or datestamp(), username, true);
+	end
 
-	-- Iterating over users is not supported by all authentication modules
-	-- Catch and ignore error if not supported
-	pcall(function ()
-		-- If this works, then we schedule cleanup for all known users on startup
-		for user in um.users(module.host) do
-			schedule_cleanup(user);
+	cleanup_runner = require "util.async".runner(function ()
+		local users = {};
+		local cut_off = datestamp(os.time() - cleanup_after);
+		for date in cleanup_storage:users() do
+			if date <= cut_off then
+				module:log("debug", "Messages from %q should be expired", date);
+				local messages_this_day = cleanup_storage:get(date);
+				if messages_this_day then
+					for user in pairs(messages_this_day) do
+						users[user] = true;
+					end
+					if date < cut_off then
+						-- Messages from the same day as the cut-off might not have expired yet,
+						-- but all earlier will have, so clear storage for those days.
+						cleanup_storage:set(date, nil);
+					end
+				end
+			end
 		end
+		local sum, num_users = 0, 0;
+		for user in pairs(users) do
+			local ok, err = archive:delete(user, { ["end"] = os.time() - cleanup_after; })
+			if ok then
+				num_users = num_users + 1;
+				sum = sum + (tonumber(ok) or 0);
+			end
+		end
+		module:log("info", "Deleted %d expired messages for %d users", sum, num_users);
 	end);
 
-	-- At odd intervals, delete old messages for one user
-	module:add_timer(math.random(10, 60), function()
-		local user = table.remove(cleanup, 1);
-		if user then
-			module:log("debug", "Removing old messages for user %q", user);
-			local ok, err = archive:delete(user, { ["end"] = os.time() - cleanup_after; })
-			if not ok then
-				module:log("warn", "Could not expire archives for user %s: %s", user, err);
-			elseif type(ok) == "number" then
-				module:log("debug", "Removed %d messages", ok);
-			end
-			cleanup[user] = nil;
-		end
-		return math.random(cleanup_interval, cleanup_interval * 2);
+	cleanup_task = module:add_timer(1, function ()
+		cleanup_runner:run(true);
+		return cleanup_interval;
 	end);
 else
 	module:log("debug", "Archive expiry disabled");