[Tarantool-patches] [PATCH luajit 4/4] Restore cur_L for specific Lua/C API use case.

[Maxim Kokryashkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

From: Mike Pall <mike>

Thanks to Peter Cawley.

(cherry-picked from commit e86990f7f24a94b0897061f25a84547fe1108bed)

Consider the following Lua C API function:

```
static int error_after_coroutine_return(lua_State *L)
{
	lua_State *innerL = lua_newthread(L);
	luaL_loadstring(innerL, "print('inner coro')");
	lua_pcall(innerL, 0, 0, 0);
	luaL_error(L, "my fancy error");
	return 0;
}
```

And the following Lua script:
```
local libcur_L = require('libcur_L')

local function onesnap_f(var)
  if var then
    return 1
  else
    return 0
  end
end

-- Compile function to trace with snapshot.
if jit then jit.opt.start('hotloop=1') end
onesnap_f(true)
onesnap_f(true)

local r, s = pcall(libcur_L.error_after_coroutine_return)
onesnap_f(false)
```

This is the only case when `cur_L` is not restored, according to
the analysis done in https://github.com/LuaJIT/LuaJIT/issues/1066.

This patch changes the error-catching routine, so now the patch
sets the actual cur_L there.
Now it is possible to throw errors on non-executing coroutines,
which is a violation of the Lua C API. So, even though it is now
possible, that behavior should be avoided anyway.

Maxim Kokryashkin:
* added the description for the problem

Resolves tarantool/tarantool#6323
---
 src/lj_err.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/lj_err.c b/src/lj_err.c
index 46fb81ee..1a9a2f2b 100644
--- a/src/lj_err.c
+++ b/src/lj_err.c
@@ -174,12 +174,15 @@ static void *err_unwind(lua_State *L, void *stopcf, int errcode)
     case FRAME_PCALL:  /* FF pcall() frame. */
     case FRAME_PCALLH:  /* FF pcall() frame inside hook. */
       if (errcode) {
+	global_State *g;
 	if (errcode == LUA_YIELD) {
 	  frame = frame_prevd(frame);
 	  break;
 	}
+	g = G(L);
+	setgcref(g->cur_L, obj2gco(L));
 	if (frame_typep(frame) == FRAME_PCALL)
-	  hook_leave(G(L));
+	  hook_leave(g);
 	L->base = frame_prevd(frame) + 1;
 	L->cframe = cf;
 	unwindstack(L, L->base);
-- 
2.42.0