perf: buffer mod_vnc_lastactivity DB writes and flush periodically

The store :set() calls go through mod_storage_sql (LuaDBI), a blocking C
client. These hooks fire on the presence/auth hot path, so inline writes
stall the event loop under presence storms. Buffer writes in memory and
flush every 5 seconds (configurable via vnc_lastactivity_flush_interval).
The IQ handlers already read from in-memory map/remote_act_cache first,
so read consistency is preserved. Final flush on server-stopping and
host-deactivating prevents data loss on graceful shutdown.

Part-of: <http://gitlab.vnc.biz/uxf/vnctalk-prosody/-/merge_requests/3>

vnctalk/mod_vnc_lastactivity.lua

@@ -26,6 +26,44 @@ local activity_store = module:open_store("activity", "map");
 local remote_activity_store = module:open_store("remote_activity", "map");
 local remote_avatar_update_store = module:open_store("remote_avatar", "map");
 
+-- The store :set() calls go through mod_storage_sql (LuaDBI), which is a
+-- blocking C client -- every write blocks the event loop for a DB round-trip.
+-- These hooks fire on the presence/auth hot path, so doing the write inline
+-- stalls the whole server under presence storms. Instead we buffer the
+-- last-value-wins writes in memory (the IQ handlers already read from the
+-- in-memory map/caches) and flush them periodically from a timer.
+local FLUSH_INTERVAL = module:get_option_number("vnc_lastactivity_flush_interval", 5);
+
+-- Pending writes, coalesced by user/contact. Only the most recent value per
+-- key is kept, matching the "last value wins" semantics of the stores.
+local pending_activity = {};        -- [username]     = stamp  (store key "last")
+local pending_remote_activity = {}; -- [contact_bare] = stamp  (store key "last")
+local pending_remote_avatar = {};   -- [contact_bare] = stamp  (store key "avatar")
+
+local function flush_pending_writes()
+  for username, value in pairs(pending_activity) do
+    local ok, err = activity_store:set(username, "last", value);
+    if not ok then
+      module:log("warn", "flush activity_store:set(%s) failed: %s", username, tostring(err));
+    end
+  end
+  for contact_bare, value in pairs(pending_remote_activity) do
+    local ok, err = remote_activity_store:set(contact_bare, "last", value);
+    if not ok then
+      module:log("warn", "flush remote_activity_store:set(%s) failed: %s", contact_bare, tostring(err));
+    end
+  end
+  for contact_bare, value in pairs(pending_remote_avatar) do
+    local ok, err = remote_avatar_update_store:set(contact_bare, "avatar", value);
+    if not ok then
+      module:log("warn", "flush remote_avatar_update_store:set(%s) failed: %s", contact_bare, tostring(err));
+    end
+  end
+  for k in pairs(pending_activity) do pending_activity[k] = nil; end
+  for k in pairs(pending_remote_activity) do pending_remote_activity[k] = nil; end
+  for k in pairs(pending_remote_avatar) do pending_remote_avatar[k] = nil; end
+end
+
 local function getTableLng(tbl)
   local getN = 0
   for n in pairs(tbl) do
@@ -107,8 +145,8 @@ local map = {};
 module:hook("authentication-success", function(event)
 	local session = event.session;
 	if session.username then
-    local pers = activity_store:set(session.username, "last", 0);
-	end
+    pending_activity[session.username] = 0;
+  end
 end, 10);
 
 
@@ -139,7 +177,7 @@ module:hook("presence/bare", function(event)
           remote_act_sessions[contact_bare] = nil;
           remote_act_cache[contact_bare] = t;
           -- module:log("info", "remote_act_cache for %s is now %s", contact_bare, remote_act_cache[contact_bare]);
-          remote_activity_store:set(contact_bare, "last", t);
+          pending_remote_activity[contact_bare] = t;
         end
       end
     else
@@ -161,7 +199,7 @@ module:hook("presence/bare", function(event)
 		if (x_photo_update) then
 			-- module:log("info", "avatarupdate - need to store %s update for %s", x_photo_update, contact_bare);
 			local t = os.time();
-			remote_avatar_update_store:set(contact_bare, "avatar", t);
+			pending_remote_avatar[contact_bare] = t;
 		end
 	end
 
@@ -188,7 +226,7 @@ module:hook("pre-presence/bare", function(event)
     end
     if lastsession then
        -- module:log("info", "last session gone - storing to activity_store for %s => %s", event.origin.username, s);
-      local pers = activity_store:set(event.origin.username, "last", s);
+      pending_activity[event.origin.username] = s;
     end
 	end
 end, 10);
@@ -413,3 +451,20 @@ end
 module.restore = function(data)
 	map = data.map or {};
 end
+
+-- Flush buffered writes on a fixed cadence instead of on every presence/auth
+-- event. Returning FLUSH_INTERVAL reschedules the timer. The DB I/O still
+-- blocks the event loop while it runs, but once per interval rather than per
+-- stanza.
+module:add_timer(FLUSH_INTERVAL, function ()
+	flush_pending_writes();
+	return FLUSH_INTERVAL;
+end);
+
+-- Best-effort final flush so buffered writes are not lost on stop/reload.
+module:hook_global("server-stopping", function ()
+	flush_pending_writes();
+end);
+module:hook("host-deactivating", function ()
+	flush_pending_writes();
+end);