# HG changeset patch
# User Kim Alvefur <zash@zash.se>
# Date 1575138880 -3600
# Node ID 658b759a1f7abe226eb31f84540d97a4bb9e6c1d
# Parent  7341d2f4749a8035dd2e9ae6a5ea04e992cba5ed
mod_carbons: Improve performance by delaying creation of carbon payload

If there are no other sessions which also enabled carbons then the
carbons wrapper is not used and the potentially expensive clone
operation was a waste of cycles.

diff -r 7341d2f4749a -r 658b759a1f7a plugins/mod_carbons.lua
--- a/plugins/mod_carbons.lua	Sat Nov 30 16:40:04 2019 +0100
+++ b/plugins/mod_carbons.lua	Sat Nov 30 19:34:40 2019 +0100
@@ -74,17 +74,7 @@
 		return
 	end
 
-	-- Create the carbon copy and wrap it as per the Stanza Forwarding XEP
-	local copy = st.clone(stanza);
-	if c2s and not orig_to then
-		stanza.attr.to = bare_from;
-	end
-	copy.attr.xmlns = "jabber:client";
-	local carbon = st.message{ from = bare_jid, type = orig_type, }
-		:tag(c2s and "sent" or "received", { xmlns = xmlns_carbons })
-			:tag("forwarded", { xmlns = xmlns_forward })
-				:add_child(copy):reset();
-
+	local carbon;
 	user_sessions = user_sessions and user_sessions.sessions;
 	for _, session in pairs(user_sessions) do
 		-- Carbons are sent to resources that have enabled it
@@ -93,6 +83,20 @@
 		and session ~= target_session
 		-- and isn't among the top resources that would receive the message per standard routing rules
 		and (c2s or session.priority ~= top_priority) then
+			if not carbon then
+				-- Create the carbon copy and wrap it as per the Stanza Forwarding XEP
+				local copy = st.clone(stanza);
+				if c2s and not orig_to then
+					stanza.attr.to = bare_from;
+				end
+				copy.attr.xmlns = "jabber:client";
+				carbon = st.message{ from = bare_jid, type = orig_type, }
+					:tag(c2s and "sent" or "received", { xmlns = xmlns_carbons })
+						:tag("forwarded", { xmlns = xmlns_forward })
+							:add_child(copy):reset();
+
+			end
+
 			carbon.attr.to = session.full_jid;
 			module:log("debug", "Sending carbon to %s", session.full_jid);
 			session.send(carbon);