Re: A GC debugging question

[Dibyendu Majumdar <mobile@...>]

Looks like I was mistaken about Lua 5.3 - as the test in 5.3 is different.

  -- Now, thread and closure are not reacheable any more;
  -- two collections are needed to break cycle
  collectgarbage()
  assert(not collected)
  collectgarbage()
  assert(collected)

so it works as expected!


On Fri, 25 Dec 2020 at 00:52, Dibyendu Majumdar <mobile@majumdar.org.uk> wrote:
>
> Hi,
>
> I am trying to debug the following test in Ravi - this is failing
> after porting the Lua 5.4 GC to Ravi. Note that Ravi still uses the
> old style ref-counted upvalues from Lua 5.3. This test used to pass
> before the port, and also passes in Lua 5.4 (it is part of the test
> suite) so the problem is somewhere in the ported version.
>
> collectgarbage("incremental")
> -- Create a closure (function inside 'f') with an upvalue ('param') that
> -- points (through a table) to the closure itself and to the thread
> -- ('co' and the initial value of 'param') where closure is running.
> -- Then, assert that table (and therefore everything else) will be
> -- collected.
> do
>   local collected = false   -- to detect collection
>   collectgarbage(); collectgarbage("stop")
>   do
>     local function f (param)
>       ;(function ()
>         assert(type(f) == 'function' and type(param) == 'thread')
>         param = {param, f}
>         setmetatable(param, {__gc = function () collected = true end})
>         coroutine.yield(100)
>       end)()
>     end
>     local co = coroutine.create(f)
>     assert(coroutine.resume(co, co))
>   end
>   -- Now, thread and closure are not reacheable any more.
>   collectgarbage()
>   -- FIXME
>   assert(collected)
>   collectgarbage("restart")
> end
>
>
> The final assertion fails.
> If I insert another collection before the assertion then it is successful.
> Fyi I am running with LTESTS enabled, and I have tried inserting
> T.checkmemory() as well - no issues reported.
>
> My question is this: what's the best way to debug this?
> I find that there are so many steps that it is kind of hard to figure
> out how to pinpoint the issue. It seems that the problem might be that
> the coroutine thread is not collected when it should be?
>
> I should perhaps add a debugging facility to dump out all objects that
> are live prior to the collection.
>
> Sorry this is a vague question, I know some of you are very good at
> this sort of debugging, so any pointers would be appreciated.
>
> Regards
> Dibyendu