[Tarantool-patches] [PATCH luajit] LJ_GC64: Fix lua_concat().

[Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

From: Sergey Bronnikov <sergeyb@tarantool.org>

Reported by Mathias Westerdahl.

(cherry picked from commit 633f265f67f322cbe2c5fd11d3e46d968ac220f7)

Lua 5.1 Reference Manual [1] defines a function `lua_concat`, that:

> void lua_concat (lua_State *L, int n);
>
> Concatenates the n values at the top of the stack, pops them, and leaves
> the result at the top.

Without the patch `lua_concat()` behaved incorrectly with userdata with
defined `__concat` metamethod. The problem is GC64-specific.

Assuming we have three literals and a userdata with defined "__concat"
metamethod on top of the Lua stack:

1 [string]
2 [string]
3 [string]
4 [string]
5 [userdata] <--- top

On attempt to concatenate *two* items on top Lua stack, `lua_concat()`
concatenates *four* items and leaves result at the top:

1 [string]
2 [string][string][string][userdata] <--- top

The problem is in incorrect calculation of `n` counter in a loop in
implementation of function `lua_concat`. Without a fix `n` is equal to 3
at the end of the first iteration and therefore it goes to the next
iteration of concatenation. In a fixed implementation of `lua_concat()`
`n` is equal to 1 at the end of the first loop iteration, decremented in
a loop postcondition and breaks a loop.

The patch fixes incorrect behaviour.

1. https://www.lua.org/manual/5.1/manual.html

Sergey Bronnikov:
* added the description and the test for the problem

Part of tarantool/tarantool#8825
---
PR: https://github.com/tarantool/tarantool/pull/9176
Branch: https://github.com/tarantool/luajit/commits/ligurio/lj-881-fix-concat
Issues:
	- https://github.com/LuaJIT/LuaJIT/issues/881
	- https://github.com/tarantool/tarantool/issues/8825

 src/lj_api.c                                  |   2 +-
 .../lj-881-fix-lua-concat.test.c              | 116 ++++++++++++++++++
 2 files changed, 117 insertions(+), 1 deletion(-)
 create mode 100644 test/tarantool-c-tests/lj-881-fix-lua-concat.test.c

diff --git a/src/lj_api.c b/src/lj_api.c
index 05e02029..3bacad33 100644
--- a/src/lj_api.c
+++ b/src/lj_api.c
@@ -801,7 +801,7 @@ LUA_API void lua_concat(lua_State *L, int n)
 	L->top -= n;
 	break;
       }
-      n -= (int)(L->top - top);
+      n -= (int)(L->top - (top - 2*LJ_FR2));
       L->top = top+2;
       jit_secure_call(L, top, 1+1);
       L->top -= 1+LJ_FR2;
diff --git a/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c b/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c
new file mode 100644
index 00000000..464ed27e
--- /dev/null
+++ b/test/tarantool-c-tests/lj-881-fix-lua-concat.test.c
@@ -0,0 +1,116 @@
+#include <string.h>
+
+#include "lua.h"
+#include "lauxlib.h"
+
+#include "test.h"
+#include "utils.h"
+
+/*
+ * This test demonstrates LuaJIT's incorrect behaviour, when
+ * calling `lua_concat()` with userdata with __concat metamethod.
+ * See https://github.com/LuaJIT/LuaJIT/issues/881 for details.
+ */
+
+#define TYPE_NAME_INFO "Info"
+
+typedef struct Info
+{
+	int value;
+} Info;
+
+static void lua_pushinfo(lua_State *L, Info *info)
+{
+	Info *infop = (Info *)lua_newuserdata(L, sizeof(Info));
+	*infop = *info;
+
+	luaL_getmetatable(L, TYPE_NAME_INFO);
+	lua_setmetatable(L, -2);
+}
+
+static int INFO_concat(lua_State *L)
+{
+	const char *s = luaL_checkstring(L, 1);
+	Info *info = (Info*)luaL_checkudata(L, 2, TYPE_NAME_INFO);
+	lua_pushfstring(L, "%s[%s.value=%d]", s, TYPE_NAME_INFO, info->value);
+	return 1;
+}
+
+static const luaL_Reg INFO_methods[] =
+{
+	{0, 0}
+};
+
+static const luaL_Reg INFO_meta[] =
+{
+	{"__concat",    INFO_concat},
+	{0, 0}
+};
+
+static int lua_concat_testcase(void *test_state)
+{
+	lua_State *L = test_state;
+
+	/* Create methods table, add it to the globals. */
+	luaL_register(L, TYPE_NAME_INFO, INFO_methods);
+	int methods_idx = lua_gettop(L);
+	/* Create metatable and add it to the Lua registry. */
+	luaL_newmetatable(L, TYPE_NAME_INFO);
+
+	int metatable_idx = lua_gettop(L);
+	/* Fill metatable. */
+	luaL_register(L, 0, INFO_meta);
+
+	lua_pushliteral(L, "__metatable");
+	/* Duplicate methods table. */
+	lua_pushvalue(L, methods_idx);
+	lua_settable(L, metatable_idx);
+	lua_pop(L, 2);
+
+	assert_true(lua_gettop(L) == 0);
+
+	Info info;
+	info.value = 7;
+
+	lua_pushliteral(L, "C");
+	lua_pushliteral(L, "B");
+	lua_pushliteral(L, "A");
+	lua_pushinfo(L, &info);
+
+	int top = 4;
+	assert_true(lua_gettop(L) == top);
+
+	/**
+	 * void lua_concat (lua_State *L, int n);
+	 *
+	 * Concatenates the n values at the top of the stack,
+	 * pops them, and leaves the result at the top. If n is 1,
+	 * the result is the single value on the stack; if n is 0,
+	 * the result is the empty string [1].
+	 *
+	 * 1. https://www.lua.org/manual/5.1/manual.html
+	 */
+	int n = 2;
+	lua_concat(L, n);
+
+	const char *str = lua_tostring(L, -1);
+	assert_true(top - n + 1 == lua_gettop(L));
+	assert_true(strcmp(str, "A[Info.value=7]") == 0);
+
+	/* Cleanup. */
+	lua_settop(L, 0);
+
+	return TEST_EXIT_SUCCESS;
+}
+
+int main(void)
+{
+	lua_State *L = utils_lua_init();
+	const struct test_unit tgroup[] = {
+		test_unit_def(lua_concat_testcase),
+	};
+	const int test_result = test_run_group(tgroup, L);
+	utils_lua_close(L);
+
+	return test_result;
+}
-- 
2.34.1