Re: [Tarantool-patches] [PATCH luajit] Prevent snapshot purge while recording a function header.

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

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Hi, Sergey,

thanks for the patch! LGTM with a minor comment.

Sergey

On 2/25/26 15:11, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Thanks to Sergey Kaplun.
>
> (cherry picked from commit d459c6ce503e880dc30aefb6b61aa7f2124c7a6e)
>
> This patch is a follow-up to the commit
> 7505e78bd6c24cac6e93f5163675021734801b65 "Handle on-trace OOM errors
> from helper functions.". Since this commit, the `pcall()` (and
> `xpcall()`) emits an additional snapshot, but the arguments to the
> pcall-ed function are momentarily purged from the snapshot since they
> are not used. Hence, in several cases `debug.getlocal()` can't find the
> corresponding slot for the previous frame.
>
> This patch prevents purging right after `pcall()`, `xpcall()` recording.
>
> 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-1425-pcall-snap-purge
> Related issues:
> *https://github.com/LuaJIT/LuaJIT/issues/1425
> *https://github.com/tarantool/tarantool/issues/12134
>
>   src/lj_record.c                               |  2 +-
>   .../lj-1425-pcall-snap-purge.test.lua         | 93 +++++++++++++++++++
>   2 files changed, 94 insertions(+), 1 deletion(-)
>   create mode 100644 test/tarantool-tests/lj-1425-pcall-snap-purge.test.lua
>
> diff --git a/src/lj_record.c b/src/lj_record.c
> index ba409a61..e2e86f79 100644
> --- a/src/lj_record.c
> +++ b/src/lj_record.c
> @@ -2120,7 +2120,7 @@ void lj_record_ins(jit_State *J)
>     /* Need snapshot before recording next bytecode (e.g. after a store). */
>     if (J->needsnap) {
>       J->needsnap = 0;
> -    if (J->pt) lj_snap_purge(J);
> +    if (J->pt && bc_op(*J->pc) < BC_FUNCF) lj_snap_purge(J);
>       lj_snap_add(J);
>       J->mergesnap = 1;
>     }
> diff --git a/test/tarantool-tests/lj-1425-pcall-snap-purge.test.lua b/test/tarantool-tests/lj-1425-pcall-snap-purge.test.lua
> new file mode 100644
> index 00000000..353842f1
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1425-pcall-snap-purge.test.lua
> @@ -0,0 +1,93 @@
> +local tap = require('tap')
> +
> +-- Test file to demonstrate incorrect snapshot purge for the
> +-- `pcall()` and `xpcall()`.
> +-- See also:https://github.com/LuaJIT/LuaJIT/issues/1425.
> +
> +local test = tap.test('lj-1425-pcall-snap-purge'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +
> +-- `pcall()` and `xpcall()`.
> +test:plan(2)
> +
> +-- XXX: simplify `jit.dump()` output.
> +local type = type
> +local pcall = pcall
> +local xpcall = xpcall
> +local math_modf = math.modf
> +local debug_getlocal = debug.getlocal
> +
> +local checkers = {}
> +
> +-- Called twice for the pseudo-type that aliases base Lua type via
> +-- checkers map.
> +local function checks(expected_type)
> +  -- Value expected to be `checks_tab()` or `checks_obj()`
> +  -- argument. It is always a table.
> +  local _, value = debug_getlocal(2, 1)
> +  -- Simple stitching function. Additional arguments are needed to
> +  -- occupy the corresponding slot.

It is not clear for me why this corresponding slot is needed.

The bug can be reproduced without double nil's passed to math_modf().

> +  math_modf(0, nil, nil)
> +  -- Start trace now, one iteration only.
> +  -- luacheck: ignore 512
> +  while true do
> +    -- Base type?
> +    if type(value) == expected_type then
> +      return true
> +    end
> +    -- Pseudo types fallbacks to the map.
> +    local checker = checkers[expected_type]
> +    -- For the xpcall.
> +    if checker(value) == true then
> +      return true
> +    end
> +    break
> +  end
> +  error('Unreachable path taken')
> +end
> +
> +-- Need to be pcalled.
> +local function checks_tab(_)
> +  checks('table')
> +end
> +
> +local function checks_tab_p(map)
> +  return pcall(checks_tab, map)
> +end
> +
> +local function nop()
> +end
> +
> +local function checks_tab_xp(map)
> +  return xpcall(checks_tab, nop, map)
> +end
> +
> +local function checks_obj(_)
> +  checks('obj')
> +end
> +
> +local function check_ff(name, checks_func)
> +test:test(name, function(subtest)
> +subtest:plan(1)
> +
> +    checkers['obj'] = checks_func
> +
> +    jit.flush()
> +    jit.opt.start('hotloop=1')
> +
> +    checks_obj({})
> +    -- Forcify stack reallocation on trace in `checks()`. The
> +    -- source stack lacks the needed slot.
> +    coroutine.wrap(function()
> +      checks_obj({})
> +    end)()
> +
> +subtest:ok(true, 'No error for ' .. name)
> +  end)
> +end
> +
> +check_ff('pcall', checks_tab_p)
> +check_ff('xpcall', checks_tab_xp)
> +
> +test:done(true)

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