core: fix cur_L restoration on error throw

This change is a kind of revertion of commits
ed412cd ('Update cur_L on exceptional
path') and 97699d9 ('Fix cur_L tracking
on exceptional path').

When an error is thrown on the coroutine that is not the one being
currently executed, `cur_L` is not set up. Hence, when the running trace
exits at assertion guard right after the error is caught, Lua state is
restored from the incorrect `cur_L`. As a result the resulting stack is
inconsistent and the crash occurs.

Aforementioned patches fix the behaviour only for x86/x64
architectures. This patch updates the `cur_L` within `lj_err_throw()` to
the given lua_State, where the error is raised, since this is the only
coroutine that can proceed later. Also, it removes unnecessary
restorations of `cur_L` at C and FF exception handlers in the VM.

Nevertheless, throwing an error at non-currently executed coroutine is a
violation of Lua/C API. So, in the nearest possible future this patch
should be replaced within the corresponding assert.

Resolves tarantool/tarantool#6189
Relates to tarantool/tarantool#6323
Follows up tarantool/tarantool#1516

src/lj_err.c

@@ -509,6 +509,12 @@ LJ_NOINLINE void LJ_FASTCALL lj_err_throw(lua_State *L, int errcode)
   global_State *g = G(L);
   lj_trace_abort(g);
   setmref(g->jit_base, NULL);
+  /*
+  ** FIXME: Throwing error not on the currently executing coroutine is
+  ** a violation of Lua/C API. The next line should be changed to assert.
+  ** Set the running lua_State as the one where the error can be handled.
+  */
+  setgcref(g->cur_L, obj2gco(L));
   L->status = LUA_OK;
 #if LJ_UNWIND_EXT
   err_raise_ext(errcode);

src/vm_x64.dasc

@@ -513,7 +513,6 @@ static void build_subroutines(BuildCtx *ctx)
   |->vm_unwind_c_eh:			// Landing pad for external unwinder.
   |  mov L:DISPATCH, SAVE_L
   |  mov GL:RB, L:DISPATCH->glref
-  |  mov GL:RB->cur_L, L:DISPATCH
   |  mov dword GL:RB->vmstate, ~LJ_VMST_CFUNC
   |  mov DISPATCH, L:DISPATCH->glref	// Setup pointer to dispatch table.
   |  add DISPATCH, GG_G2DISP
@@ -539,7 +538,6 @@ static void build_subroutines(BuildCtx *ctx)
   |  add DISPATCH, GG_G2DISP
   |  mov PC, [BASE-8]			// Fetch PC of previous frame.
   |  mov_false RA
-  |  mov [DISPATCH+DISPATCH_GL(cur_L)], L:RB
   |  mov RB, [BASE]
   |  mov [BASE-16], RA			// Prepend false to error message.
   |  mov [BASE-8], RB

src/vm_x86.dasc

@@ -654,7 +654,6 @@ static void build_subroutines(BuildCtx *ctx)
   |->vm_unwind_c_eh:			// Landing pad for external unwinder.
   |  mov L:DISPATCH, SAVE_L
   |  mov GL:RB, L:DISPATCH->glref
-  |  mov dword GL:RB->cur_L, L:DISPATCH
   |  mov dword GL:RB->vmstate, ~LJ_VMST_CFUNC
   |  mov DISPATCH, L:DISPATCH->glref	// Setup pointer to dispatch table.
   |  add DISPATCH, GG_G2DISP
@@ -690,7 +689,6 @@ static void build_subroutines(BuildCtx *ctx)
   |  add DISPATCH, GG_G2DISP
   |  mov PC, [BASE-4]			// Fetch PC of previous frame.
   |  mov dword [BASE-4], LJ_TFALSE	// Prepend false to error message.
-  |  mov [DISPATCH+DISPATCH_GL(cur_L)], L:RB
   |  // INTERP until jump to BC_RET* or return to C.
   |  set_vmstate INTERP
   |  jmp ->vm_returnc			// Increments RD/MULTRES and returns.

test/tarantool-tests/CMakeLists.txt

@@ -57,6 +57,7 @@ macro(BuildTestCLib lib sources)
 endmacro()
 
 add_subdirectory(gh-4427-ffi-sandwich)
+add_subdirectory(gh-6189-curL)
 add_subdirectory(lj-flush-on-trace)
 add_subdirectory(misclib-getmetrics-capi)
 

test/tarantool-tests/gh-6189-curL.test.lua

@@ -0,0 +1,25 @@
+local libcurl = require('libcurL')
+local tap = require('tap')
+
+local test = tap.test('gh-6189-curL')
+test:plan(1)
+
+local function cbool(cond)
+  if cond then
+    return 1
+  else
+    return 0
+  end
+end
+
+-- Compile function to trace with snapshot.
+jit.opt.start('hotloop=1')
+cbool(true)
+cbool(true)
+
+pcall(libcurl.error_from_other_thread)
+-- Call with restoration from a snapshot with wrong cur_L.
+cbool(false)
+
+test:ok(true)
+os.exit(test:check() and 0 or 1)

test/tarantool-tests/gh-6189-curL/CMakeLists.txt

@@ -0,0 +1 @@
+BuildTestCLib(libcurL libcurL.c)

test/tarantool-tests/gh-6189-curL/libcurL.c

@@ -0,0 +1,36 @@
+#include <lua.h>
+#include <lauxlib.h>
+
+static lua_State *old_L = NULL;
+
+int throw_error_at_old_thread(lua_State *cur_L)
+{
+	lua_error(old_L);
+	/* Unreachable. */
+	return 0;
+}
+
+static int error_from_other_thread(lua_State *L)
+{
+	lua_State *next_cur_L = lua_newthread(L);
+	old_L = L;
+	/* Remove thread. */
+	lua_pop(L, 1);
+	/* Do not show frame slot as return result after error. */
+	lua_pushnil(L);
+	lua_pushcfunction(next_cur_L, throw_error_at_old_thread);
+	lua_call(next_cur_L, 0, 0);
+	/* Unreachable. */
+	return 0;
+}
+
+static const struct luaL_Reg libcurL[] = {
+	{"error_from_other_thread", error_from_other_thread},
+	{NULL, NULL}
+};
+
+LUA_API int luaopen_libcurL(lua_State *L)
+{
+	luaL_register(L, "libcurL", libcurL);
+	return 1;
+}