Re: [Tarantool-patches] [PATCH luajit] FFI: Fix constructor index resolution in JIT compiler.

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

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Hi, Sergey,

thanks for the patch! LGTM

Sergey

On 3/16/26 13:48, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Reported by Vladimir Davydov and Sergey Kaplun.
>
> (cherry picked from commit 02e29995581cb568a784a9b996cd5e9139cd0737)
>
> This patch is a follow-up for the commit
> 6cee1333ef2b340e1b23e3051b9f433fc803d2ff ("FFI: Resolve metamethods for
> constructors, too."). It allows the use of metamethods of the objects to
> be called on their constructors. Unfortunately, the JIT part isn't
> updated and may cause inconsistencies since it is indexing the field of
> the structure referenced by the given CType.
>
> This patch makes the semantics of JIT the same as for the VM.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#12134
> ---
>
> Branch:https://github.com/tarantool/luajit/tree/skaplun/lj-1441-record-constructor-metamethod
> Related issues:
> *https://github.com/LuaJIT/LuaJIT/issues/1441
> *https://github.com/tarantool/tarantool/issues/12134
>
>   src/lj_crecord.c                              |  2 +
>   ...441-record-constructor-metamethod.test.lua | 79 +++++++++++++++++++
>   2 files changed, 81 insertions(+)
>   create mode 100644 test/tarantool-tests/lj-1441-record-constructor-metamethod.test.lua
>
> diff --git a/src/lj_crecord.c b/src/lj_crecord.c
> index b016eaec..d486ee85 100644
> --- a/src/lj_crecord.c
> +++ b/src/lj_crecord.c
> @@ -881,6 +881,8 @@ again:
>   	  }
>   	  J->base[0] = lj_ir_kint(J, (int32_t)fct->size);
>   	  return;  /* Interpreter will throw for newindex. */
> +	} else if (cd && cd->ctypeid == CTID_CTYPEID) {
> +	  /* Only resolve constants and metamethods for constructors. */
>   	} else if (ctype_isbitfield(fct->info)) {
>   	  if (ofs)
>   	    ptr = emitir(IRT(IR_ADD, IRT_PTR), ptr, lj_ir_kintp(J, ofs));
> diff --git a/test/tarantool-tests/lj-1441-record-constructor-metamethod.test.lua b/test/tarantool-tests/lj-1441-record-constructor-metamethod.test.lua
> new file mode 100644
> index 00000000..370e49e0
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1441-record-constructor-metamethod.test.lua
> @@ -0,0 +1,79 @@
> +local tap = require('tap')
> +
> +-- The test file to demonstrate LuaJIT's incorrect recording of
> +-- the __index metamethod invocation on the cdata's constructor.
> +-- See also:https://github.com/LuaJIT/LuaJIT/issues/1441.
> +
> +local test = tap.test('lj-1441-record-constructor-metamethod'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +
> +test:plan(3)
> +
> +local ffi = require('ffi')
> +
> +ffi.cdef[[
> +  struct test_recursive {int a;};
> +  struct test_finite {int a;};
> +]]
> +
> +local recursive_t = ffi.typeof('struct test_recursive')
> +local finite_t = ffi.typeof('struct test_finite')
> +
> +local MAGIC = 42
> +
> +local function new_recursive()
> +  return ffi.new(recursive_t, 0)
> +end
> +
> +local function new_finite()
> +  return ffi.new(finite_t, 0)
> +end
> +
> +local function index_func_recursive(v)
> +  -- Should raise an error (stack overflow).
> +  return ffi.typeof(v).a
> +end
> +
> +-- Special object to invoke metamethod on the cdata<ctypeid>.
> +local one_more_step = new_finite()
> +
> +local function index_func_finite(v)
> +  if v == one_more_step then
> +    -- XXX: Avoid tail-calls.
> +    local x = ffi.typeof(v).a
> +    return x
> +  else
> +    return MAGIC
> +  end
> +end
> +
> +ffi.metatype(recursive_t, {
> +  __index = index_func_recursive,
> +})
> +
> +ffi.metatype(finite_t, {
> +  __index = index_func_finite,
> +})
> +
> +jit.opt.start('hotloop=1')
> +
> +-- Test the recursive call. Expect the stack overflow error.
> +local o_rec = new_recursive()
> +local result, errmsg
> +for _ = 1, 4 do
> +  result, errmsg = pcall(index_func_recursive, o_rec)
> +end
> +
> +test:ok(not result, 'correct status for recursive call')
> +test:like(errmsg, 'stack overflow', 'correct error message for recursive call')
> +
> +-- Test the finite call. Expect the specific value.
> +local got
> +for _ = 1, 4 do
> +  got = index_func_finite(one_more_step)
> +end
> +
> +test:is(got, MAGIC, 'correct result value on trace for finite call')
> +
> +test:done(true)

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