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

[Igor Munkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Sergey,

I've checked the patchset into all long-term branches in
tarantool/luajit and bumped a new version in master, release/2.11 and
release/2.10.

On 26.09.23, Sergey Bronnikov via Tarantool-patches wrote:
> 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
> 

<snipped>

> -- 
> 2.34.1
> 

-- 
Best regards,
IM