From 8d38a56221dff0df80fad41dc61bbc4131d96169 Mon Sep 17 00:00:00 2001
From: Bryce Kwon <bryce@brycekwon.com>
Date: Wed, 4 Nov 2026 23:13:43 -1000
Subject: Put a Lua filter's print output where it runs

LuaJIT and Lua 5.1 leave print's line in stdio's buffer until cgit
exits, after the end of the page.
---
 MANUAL.txt             |  3 +++
 source/filter.c        | 31 +++++++++++++++++++++++++++++++
 tests/t0201-filters.sh | 20 ++++++++++++++++++++
 3 files changed, 54 insertions(+)

diff --git a/MANUAL.txt b/MANUAL.txt
index e17d5fd..4b1e8f5 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -844,6 +844,9 @@ specification with one of these strings.
 		Url escapes for an argument and writes 'str' to the webpage.
 	'html_include(file)'::
 		Includes 'file' in webpage.
+	'print(...)'::
+		Writes its arguments to the webpage unescaped, a tab between
+		each and a newline at the end, as Lua's own print lays them out.
 
 
 Parameters are provided to filters as follows.
diff --git a/source/filter.c b/source/filter.c
index 6cc5e80..82d8725 100644
--- a/source/filter.c
+++ b/source/filter.c
@@ -293,6 +293,36 @@ static int script_html_include(lua_State *lua_state)
 	return emit_unfiltered(lua_state, html_include);
 }
 
+/*
+ * The print of LuaJIT and Lua 5.1 leaves its line in stdio's buffer, which
+ * reaches stdout only when cgit exits, after the end of the page. This one
+ * builds the same line, each value through tostring with a tab between them,
+ * and writes it as html does.
+ */
+static int script_print(lua_State *lua_state)
+{
+	int i, count = lua_gettop(lua_state);
+	luaL_Buffer line;
+
+	lua_getglobal(lua_state, "tostring");
+	luaL_buffinit(lua_state, &line);
+	for (i = 1; i <= count; i++) {
+		if (i > 1)
+			luaL_addchar(&line, '\t');
+		lua_pushvalue(lua_state, count + 1);
+		lua_pushvalue(lua_state, i);
+		lua_call(lua_state, 1, 1);
+		if (!lua_isstring(lua_state, -1))
+			return luaL_error(lua_state, "'tostring' must return a string to 'print'");
+		luaL_addvalue(&line);
+	}
+	luaL_addchar(&line, '\n');
+	luaL_pushresult(&line);
+	// emit_unfiltered takes its text from the first slot.
+	lua_replace(lua_state, 1);
+	return emit_unfiltered(lua_state, html);
+}
+
 static const struct {
 	const char *name;
 	lua_CFunction fn;
@@ -303,6 +333,7 @@ static const struct {
 	{ "html_url_path", script_html_url_path },
 	{ "html_url_arg", script_html_url_arg },
 	{ "html_include", script_html_include },
+	{ "print", script_print },
 };
 
 static void init_lua_filter(struct lua_filter *filter)
diff --git a/tests/t0201-filters.sh b/tests/t0201-filters.sh
index 8d8743a..4b39f18 100755
--- a/tests/t0201-filters.sh
+++ b/tests/t0201-filters.sh
@@ -168,4 +168,24 @@ test_expect_success CGIT_LUA 'a lua filter can encode an argument and include a
 	grep "a+b%26c|included <b>bold</b>" tmp
 '
 
+# The print of LuaJIT and Lua 5.1 leaves its line in stdio's buffer, which only
+# reached the page as cgit exited, behind the closing tag. A print lands where
+# the filter runs.
+test_expect_success CGIT_LUA 'a lua filter prints where it runs' '
+	cat >print.lua <<-\EOF &&
+	function filter_open(...) end
+	function filter_write(str)
+		print("printed", 1, nil, true)
+		html(str)
+	end
+	function filter_close() return 0 end
+	EOF
+	sed "s|loud.lua|print.lua|" loudrc >printrc &&
+	cgit_rc printrc "url=foo/commit" >tmp &&
+	tab=$(printf "\t") &&
+	grep "<div class=.commit-subject.>printed${tab}1${tab}nil${tab}true$" tmp &&
+	sed -n "/<\/html>/,\$p" tmp >tail &&
+	! grep printed tail
+'
+
 test_done
-- 
cgit 2.12.1