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The co_extra_freefuncs are stored in PyThreadState. When calling _PyEval_RequestCodeExtraIndex you are given a thread specific index. The code object can then lose it's last reference on a different thread, and the wrong free function can be called if users of the extra space have made calls to get their index in different orders. 
This can also lead to crashes if the extra thread hasn't yet requested extra indexes either.

See review comments on PR 2015.

User code shouldn't allocate PyInterpreterState and PyThreadState structures, it only uses structures created by the interpreter. Changing the size of PyInterpreterState should be safe. The only possible breaking compatibility is if user code directly access co_extra_user_count, co_extra_freefuncs, async_gen_firstiter or async_gen_finalizer rather than using the API: _PyEval_RequestCodeExtraIndex(), _PyCode_GetExtra(), _PyCode_SetExtra(), _PyEval_GetAsyncGenFirstiter(), _PyEval_SetAsyncGenFirstiter(), _PyEval_GetAsyncGenFinalizer() and _PyEval_SetAsyncGenFinalizer().
Nick's idea about _preserve_36_ABI_1 and _preserve_36_ABI_2 should address concerns about direct access to async_gen_firstiter and async_gen_finalizer. Direct access to co_extra_user_count and co_extra_freefuncs obviously can't be preserved.
PyInterpreterState and PyThreadState are not in the stable ABI. They are opaque types when use limited API. May be they should be made opaque for all user code (in 3.7+).

New changeset 2997fec01ee7300c6d5940e6c55e4ccf9f56f1b5 by Ned Deily (Dino Viehland) in branch '3.6':
[3.6] bpo-30604: Move co_extra_freefuncs to interpreter state to avoid crashes in threads (#2015)
https://github.com/python/cpython/commit/2997fec01ee7300c6d5940e6c55e4ccf9f56f1b5

Avoid using double underscores in C code. C compiler uses names with double underscores for its own needs, and this can lead to conflicts.

New changeset 932946ca14168e556293d2508c8eebb23a56a2b2 by Victor Stinner in branch '3.6':
bpo-30604: Fix __PyCodeExtraState_Get() prototype (#2152)
https://github.com/python/cpython/commit/932946ca14168e556293d2508c8eebb23a56a2b2

I'm setting the stage to 'backport needed', but it really is a 'porting needed' stage :)
The two PRs merged PRs here were made against 3.6.

New changeset f3cffd2b7879d209f982de899b782fb89cfc410a by Yury Selivanov (Dino Viehland) in branch 'master':
bpo-30604: clean up co_extra support (#2144)
https://github.com/python/cpython/commit/f3cffd2b7879d209f982de899b782fb89cfc410a

Should this be closed since the all PRs got merged?

PR 2152 is not yet ported to master.

It doesn't need to be, it's only for 3.6

Closing the issue. Thank you Dino for working on this!

If a test requires ctypes, please skip your test if ctypes is missing. The new test fails on the "x86 Ubuntu Shared 3.x" buildbot which lacks the _ctypes module (for an unknown reason, but does it really matter here? ;-)).
http://buildbot.python.org/all/builders/x86%20Ubuntu%20Shared%203.x/builds/924/steps/test/logs/stdio
test test_code crashed -- Traceback (most recent call last):
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/libregrtest/runtest.py", line 156, in runtest_inner
 the_module = importlib.import_module(abstest)
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/importlib/__init__.py", line 127, in import_module
 return _bootstrap._gcd_import(name[level:], package, level)
 File "<frozen importlib._bootstrap>", line 978, in _gcd_import
 File "<frozen importlib._bootstrap>", line 961, in _find_and_load
 File "<frozen importlib._bootstrap>", line 950, in _find_and_load_unlocked
 File "<frozen importlib._bootstrap>", line 655, in _load_unlocked
 File "<frozen importlib._bootstrap_external>", line 679, in exec_module
 File "<frozen importlib._bootstrap>", line 205, in _call_with_frames_removed
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_code.py", line 218, in <module>
 import ctypes
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/ctypes/__init__.py", line 7, in <module>
 from _ctypes import Union, Structure, Array
ModuleNotFoundError: No module named '_ctypes'

New changeset a4b091e135ccf345cfafdd8477aef897c5214f82 by Victor Stinner in branch 'master':
bpo-30604: Skip CoExtra tests if ctypes is missing (#2356)
https://github.com/python/cpython/commit/a4b091e135ccf345cfafdd8477aef897c5214f82

New changeset cea2174ab7cce01c420b2770562be4c91f1f4e35 by Victor Stinner in branch '3.6':
bpo-30604: Skip CoExtra tests if ctypes is missing (#2356) (#2358)
https://github.com/python/cpython/commit/cea2174ab7cce01c420b2770562be4c91f1f4e35

The test_code is fixed again, so I close the issue.

_PyCode_SetExtra() uses two memory block for code extras. By changing how memory is accessed and allocated, it would be possible to use a single memory block. Was it on purpose to use two memory blocks?
See for example PyTupleObject which uses a single memory block vs PyListObject which uses two memory blocks.
typedef struct {
 PyObject_VAR_HEAD
 PyObject *ob_item[1];
 /* ob_item contains space for 'ob_size' elements.
 * Items must normally not be NULL, except during construction when
 * the tuple is not yet visible outside the function that builds it.
 */
} PyTupleObject;

> _PyCode_SetExtra() uses two memory block for code extras. By changing how memory is accessed and allocated, it would be possible to use a single memory block. Was it on purpose to use two memory blocks?
I discussed with Yury who is not opposed to such change in Python 3.7, so I created bpo-30789.

New changeset 23e7944eba1968bb8432fdc4cc96d4fdd2c1a230 by Victor Stinner in branch 'master':
bpo-30704, bpo-30604: Fix memleak in code_dealloc() (#2455)
https://github.com/python/cpython/commit/23e7944eba1968bb8432fdc4cc96d4fdd2c1a230

New changeset 26daad4ee14693381d84a5235709d22aed1c22ed by Victor Stinner in branch '3.6':
bpo-30704, bpo-30604: Fix memleak in code_dealloc() (#2455) (#2456)
https://github.com/python/cpython/commit/26daad4ee14693381d84a5235709d22aed1c22ed

New changeset c794b643c9172d69afa46f85982befd82511d9df by Ned Deily (Victor Stinner) in branch '3.6':
bpo-30704, bpo-30604: Fix memleak in code_dealloc() (#2455) (#2456)
https://github.com/python/cpython/commit/c794b643c9172d69afa46f85982befd82511d9df