Re: lua 5.4 calls __gc twice, sometimes
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- Subject: Re: lua 5.4 calls __gc twice, sometimes
- From: Payo Nel <payonel@...>
- Date: 2020年4月24日 17:11:27 +0000
quick note: I apologize for my previous messages which were badly html formatted. A friendly member of the community sent me a screenshot of how my email rendered for them. It was a mess, I blame hotmail and I'll be more careful
I wanted to explain that my userdata handlers are using the lua_newuserdata correctly. As a verification of this bug, I added an int to my userdata and add a mark when it is first disposed and I can detect when a previously detected userdata is used again.
```c
static int sid = 0;
struct Plain { int id; }
static int userdata_new(lua_State* L)
{
Plain* p = (Plain*)lua_newuserdata(L, sizeof(Plain));
p->id = 0;
return 1;
}
static int userdata_dispose(lua_State* L)
{
Plain* p = (Plain*)lua_touserdata(L, 1);
assert(p->id == 0);
p->id = ++sid;
return 0;
}
```
on the lua size, I cache the userdata objects
here is the most simple case:
```lua
local cache = setmetatable({}, { __mode = "k" })
local function new_proxy()
local p = userdata_new()
local k = setmetatable({}, { __gc = function(self) local p = cache[self] cache[self] = nil userdata_dispose(p) end })
cache[k] = p
return k
end
do
local k = new_proxy()
end
```
This test will likely never hit the bug. There needs to be stack resizing that swaps the order of __gc from the k value leaving the stack and the cache weak key cleanup.
As I mentioned in previous messages (in horrible html formatting)
1. I found this bug is introduced with commit https://github.com/lua/lua/commit/1d8920dd7f508af5f2fd743678be1327f30c079b
2. My code does not fail on any previous lua version tested back to and including 5.2
I am happy to share source code and repro instructions for my project that hits this bug, but it is definitely not a minimal repro - for which I apologize
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