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Tests for greenlets interacting with the CPython trash can API.
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The CPython trash can API is not designed to be re-entered from a
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single thread. But this can happen using greenlets, if something
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during the object deallocation process switches greenlets, and this second
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greenlet then causes the trash can to get entered again. Here, we do this
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very explicitly, but in other cases (like gevent) it could be arbitrarily more
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complicated: for example, a weakref callback might try to acquire a lock that's
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already held by another greenlet; that would allow a greenlet switch to occur.
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See https://github.com/gevent/gevent/issues/1909
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This test is fragile and relies on details of the CPython
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implementation (like most of the rest of this package):
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- We enter the trashcan and deferred deallocation after
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``_PyTrash_UNWIND_LEVEL`` calls. This constant, defined in
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CPython's object.c, is generally 50. That's basically how many objects are required to
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get us into the deferred deallocation situation.
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- The test fails by hitting an ``assert()`` in object.c; if the
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build didn't enable assert, then we don't catch this.
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- If the test fails in that way, the interpreter crashes.
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from a deferred trash can run, it switches greenlets, allocates more objects
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which then also go in the trash can. If we don't save state appropriately,
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nesting gets out of order and we can crash the interpreter.
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:param sequence_number: The ordinal of this object during
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one particular creation run. This is used to detect (guess, really)
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when we have entered the trash can's deferred deallocation.
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