PYTHON-2433 Fix Python 3 ServerDescription/Exception memory leak #520
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When the SDAM monitor check fails, a ServerDescription is created from the exception. This exception is kept alive via the ServerDescription.error field. Unfortunately, the exception's traceback contains a reference to the previous ServerDescription. Altogether this means that each consecutively failing check leaks memory by building an ever growing chain of ServerDescription -> Exception -> Traceback -> Frame -> ServerDescription -> ... objects. This change breaks the chain and prevents the memory leak by clearing the Exception's __traceback__, __context__, and __cause__ fields.
ShaneHarvey
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Rebased to fix test failures caused by PYTHON-2436. |
| # If a bug like PYTHON-2433 is reintroduced then too many | ||
| # ServerDescriptions will be kept alive and this test will fail: | ||
| # AssertionError: 4 != 22 within 5 delta (18 difference) | ||
| self.assertAlmostEqual(initial_count, final_count, delta=5) |
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Nice test. What is behind the choice of using delta=5? Is that number 5 liable to change?
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5 is arbitrary but it allows for some tolerance for race conditions, like having several clients open in the background (creating and destroying ServerDescriptions).
pymongo/server_description.py
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| @@ -69,6 +70,11 @@ def __init__( | |||
| self._me = ismaster.me | |||
| self._last_update_time = _time() | |||
| self._error = error | |||
| # PYTHON-2433 Clear error traceback info. | |||
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Any particular reason you are purging the traceback, etc. in this location as opposed to after emitting the heartbeat failed event in Monitor._check_server? Why preclude any exception on a ServerDescription from having a traceback?
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Yeah it is a bit odd to mutate the exception in the ServerDescription constructor. I moved this logic to the Monitor.
prashantmital
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Nov 21, 2020
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Alternative fixes would be:
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ShaneHarvey
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When the SDAM monitor check fails, a ServerDescription is created from the exception. This exception is kept alive via the ServerDescription.error field. Unfortunately, the exception's traceback contains a reference to the previous ServerDescription. Altogether this means that each consecutively failing check leaks memory by building an ever growing chain of ServerDescription -> Exception -> Traceback -> Frame -> ServerDescription -> ... objects. This change breaks the chain and prevents the memory leak by clearing the Exception's __traceback__, __context__, and __cause__ fields. (cherry picked from commit 6c92e6c)
ShaneHarvey
added a commit
that referenced
this pull request
Nov 21, 2020
When the SDAM monitor check fails, a ServerDescription is created from the exception. This exception is kept alive via the ServerDescription.error field. Unfortunately, the exception's traceback contains a reference to the previous ServerDescription. Altogether this means that each consecutively failing check leaks memory by building an ever growing chain of ServerDescription -> Exception -> Traceback -> Frame -> ServerDescription -> ... objects. This change breaks the chain and prevents the memory leak by clearing the Exception's __traceback__, __context__, and __cause__ fields. (cherry picked from commit 6c92e6c)
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When the SDAM monitor check fails, a ServerDescription is created from
the exception. This exception is kept alive via the
ServerDescription.error field. Unfortunately, the exception's traceback
contains a reference to the previous ServerDescription. Altogether this
means that each consecutively failing check leaks memory by building an
ever growing chain of ServerDescription -> Exception -> Traceback ->
Frame -> ServerDescription -> ... objects.
This change breaks the chain and prevents the memory leak by clearing
the Exception's traceback, context, and cause fields.