Lua garbage collection weirdness

gistfile1.txt

~~~ = link from table with weak values
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State before removing 'foobar':

                                  +----------------------------------------+
                                  |                                        |
                                  v                                        |
scheduler ---> wrap ---> t ~~~> thread ---> coroutine --- (up-value) --> foobar ---> udata
 ^                                                                         ^
 |                                                                         |
main ----------------------------------------------------------------------+

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[EDIT] Dependency graph in foobar (makes the bug appear)

   +-------- stack -------------------+
   |                                  |
   v                                  |
foobar ---> udata ---> fenv ---> udata_thread 
              ^         |                       
              |         |
              +---------+

The bug only appears when 'foobar' is in udata_thread's stack.

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Setting 'foobar' to nil

main.foobar = nil
collectgarbage() 

                                  +----------------------------------------+
                                  |                                        |
                                  v                                        |
scheduler ---> wrap ---> t ~~~> thread ---> coroutine --- (up-value) --> foobar ---> 0x0 udata
 ^
 |
main

The coroutine exists and when the scheduler calls
coroutine.resume(wrap.t.thread.co) we SEGFAULT.

********************************************************************************
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main.foobar.thread = nil
main.foobar = nil
collectgarbage()

scheduler ---> wrap ---> t
 ^
 |
main

Coroutine removed with 'foobar': all OK
********************************************************************************
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Complete dependency graph

main.foobar = nil
collectgarbage() 

                                  +----------------------------------------+
                                  |                                        |
                                  v                                        |
scheduler ---> wrap ---> t ~~~> thread ---> coroutine --- (up-value) --> foobar ---> 0x0 udata
 ^
 |
main

The coroutine exists and when the scheduler calls
coroutine.resume(wrap.t.thread.co) we SEGFAULT.

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