--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_isr/README.md	Sun Aug 28 17:30:52 2022 +0100
@@ -0,0 +1,7 @@
+---
+labels:
+- Stage-Alpha
+summary: "XEP-0397: Instant Stream Resumption"
+---
+
+Experimental and incomplete implementation of [XEP-0397: Instant Stream Resumption].

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_isr/mod_isr.lua	Sun Aug 28 17:30:52 2022 +0100
@@ -0,0 +1,55 @@
+local st = require "util.stanza";
+
+local mod_smacks = module:depends("smacks");
+
+local xmlns_sasl2 = "urn:xmpp:sasl:1";
+local xmlns_sm = "urn:xmpp:sm:3";
+local xmlns_isr = "https://xmpp.org/extensions/isr/0";
+local xmlns_errors = "urn:ietf:params:xml:ns:xmpp-stanzas";
+
+module:hook_tag(xmlns_sasl2, "authenticate", function (session, auth)
+	local isr_resume = auth:get_child("inst-resume", xmlns_isr);
+	if not isr_resume then return end
+	local is_using_token = isr_resume.attr["with-isr-token"] ~= "false";
+	if is_using_token then
+		-- TODO: If authing with token, set session.sasl_handler to our own
+		-- event.session.sasl_handler = ...
+		error("not yet implemented");
+	end
+
+	-- Cache resume element for future processing after SASL success
+	session.isr_sm_resume = isr_resume:get_child("resume", "urn:xmpp:sm:3");
+end, 100);
+
+module:hook("sasl2/c2s/success", function (event)
+	local session = event.session;
+	local sm_resume = session.isr_sm_resume;
+	if sm_resume then
+		session.isr_sm_resume = nil;
+		local resumed, err = mod_smacks.do_resume(session, sm_resume);
+		if not resumed then
+			local failed = st.stanza("failed", { xmlns = xmlns_sm, h = ("%d"):format(err.context.h) })
+				:tag(err.condition, { xmlns = xmlns_errors });
+			event.success:add_child(failed);
+		else
+			event.session = resumed.session;
+			event.isr_resumed = resumed;
+			event.success:tag("resumed", { xmlns = xmlns_sm,
+				h = ("%d"):format(event.session.handled_stanza_count);
+				previd = resumed.id; }):up();
+		end
+	end
+end, 100);
+
+module:hook("sasl2/c2s/success", function (event)
+	-- The authenticate response has already been sent at this point
+	local resumed = event.isr_resumed;
+	if resumed then
+		resumed.finish(); -- Finish resume and sync stanzas
+	end
+end, -1100);
+
+module:hook("sasl2/c2s/failure", function (event)
+	event.session.isr_sm_resume = nil;
+end);
+