179 lines
7.3 KiB
Python
179 lines
7.3 KiB
Python
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# -*- coding: utf-8 -*-
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"""
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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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"""
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from __future__ import print_function, absolute_import, division
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import unittest
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class TestTrashCanReEnter(unittest.TestCase):
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def test_it(self):
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# Try several times to trigger it, because it isn't 100%
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# reliable.
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for _ in range(10):
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self.check_it()
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def check_it(self): # pylint:disable=too-many-statements
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import greenlet
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from greenlet._greenlet import get_tstate_trash_delete_nesting # pylint:disable=no-name-in-module
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main = greenlet.getcurrent()
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assert get_tstate_trash_delete_nesting() == 0
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# We expect to be in deferred deallocation after this many
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# deallocations have occurred. TODO: I wish we had a better way to do
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# this --- that was before get_tstate_trash_delete_nesting; perhaps
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# we can use that API to do better?
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TRASH_UNWIND_LEVEL = 50
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# How many objects to put in a container; it's the container that
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# queues objects for deferred deallocation.
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OBJECTS_PER_CONTAINER = 500
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class Dealloc: # define the class here because we alter class variables each time we run.
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"""
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An object with a ``__del__`` method. When it starts getting deallocated
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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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"""
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#: Has our deallocation actually run and switched greenlets?
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#: When it does, this will be set to the current greenlet. This should
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#: be happening in the main greenlet, so we check that down below.
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SPAWNED = False
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#: Has the background greenlet run?
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BG_RAN = False
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BG_GLET = None
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#: How many of these things have ever been allocated.
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CREATED = 0
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#: How many of these things have ever been deallocated.
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DESTROYED = 0
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#: How many were destroyed not in the main greenlet. There should always
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#: be some.
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#: If the test is broken or things change in the trashcan implementation,
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#: this may not be correct.
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DESTROYED_BG = 0
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def __init__(self, sequence_number):
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"""
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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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"""
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self.i = sequence_number
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Dealloc.CREATED += 1
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def __del__(self):
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if self.i == TRASH_UNWIND_LEVEL and not self.SPAWNED:
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Dealloc.SPAWNED = greenlet.getcurrent()
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other = Dealloc.BG_GLET = greenlet.greenlet(background_greenlet)
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x = other.switch()
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assert x == 42
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# It's important that we don't switch back to the greenlet,
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# we leave it hanging there in an incomplete state. But we don't let it
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# get collected, either. If we complete it now, while we're still
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# in the scope of the initial trash can, things work out and we
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# don't see the problem. We need this greenlet to complete
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# at some point in the future, after we've exited this trash can invocation.
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del other
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elif self.i == 40 and greenlet.getcurrent() is not main:
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Dealloc.BG_RAN = True
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try:
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main.switch(42)
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except greenlet.GreenletExit as ex:
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# We expect this; all references to us go away
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# while we're still running, and we need to finish deleting
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# ourself.
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Dealloc.BG_RAN = type(ex)
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del ex
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# Record the fact that we're dead last of all. This ensures that
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# we actually get returned too.
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Dealloc.DESTROYED += 1
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if greenlet.getcurrent() is not main:
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Dealloc.DESTROYED_BG += 1
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def background_greenlet():
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# We direct through a second function, instead of
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# directly calling ``make_some()``, so that we have complete
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# control over when these objects are destroyed: we need them
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# to be destroyed in the context of the background greenlet
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t = make_some()
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del t # Triggere deletion.
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def make_some():
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t = ()
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i = OBJECTS_PER_CONTAINER
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while i:
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# Nest the tuples; it's the recursion that gets us
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# into trash.
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t = (Dealloc(i), t)
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i -= 1
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return t
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some = make_some()
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self.assertEqual(Dealloc.CREATED, OBJECTS_PER_CONTAINER)
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self.assertEqual(Dealloc.DESTROYED, 0)
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# If we're going to crash, it should be on the following line.
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# We only crash if ``assert()`` is enabled, of course.
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del some
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# For non-debug builds of CPython, we won't crash. The best we can do is check
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# the nesting level explicitly.
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self.assertEqual(0, get_tstate_trash_delete_nesting())
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# Discard this, raising GreenletExit into where it is waiting.
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Dealloc.BG_GLET = None
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# The same nesting level maintains.
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self.assertEqual(0, get_tstate_trash_delete_nesting())
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# We definitely cleaned some up in the background
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self.assertGreater(Dealloc.DESTROYED_BG, 0)
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# Make sure all the cleanups happened.
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self.assertIs(Dealloc.SPAWNED, main)
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self.assertTrue(Dealloc.BG_RAN)
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self.assertEqual(Dealloc.BG_RAN, greenlet.GreenletExit)
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self.assertEqual(Dealloc.CREATED, Dealloc.DESTROYED )
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self.assertEqual(Dealloc.CREATED, OBJECTS_PER_CONTAINER * 2)
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import gc
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gc.collect()
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if __name__ == '__main__':
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unittest.main()
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