Python 3.8: Impossible to navigate in Windows Explorer under Windows Server 2012. #12154
Comments
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Do you have Enable selective registration for UI Automation events and property changes enabled? If so, how does this behave when that's off? |
No, I don't. Enabling selective event registration made the crash way less frequent but it still occurs from time to time. |
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Can this be reproduced with the build from #12155 ? |
Yes, it still happens with this build. |
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Stack trace from crash dump: In short it looks like a UI Automation propertyChange event handler is freeing an IAccIdentity COM interface after it has already been freed. IAccIdentity suggests this is MSAA being proxied by UI Automation. However, we have tried rather hard to stop UIA core from proxying MSAA, especially the mapping of winEvents to UIA propertyChange events, as NVDA already gets them from MSAA itself. |
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I have made a try build of NVDA downgraded to Python 3.7, but otherwise identicfal to NVDA master at time of this comment: |
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@michaelDCurran I can confirm that UIA controls are once again usable under Windows Server 2012 with the try build that you've posted above. |
Fixes #12152 Fixes #12154 Since upgrading to Python 3.8, several serious crashes in NVDA have been reported. Specifically: • NVDA crashing when using the SAPI4 speech synthesizer: #12152 • NVDA crashing when using Windows Explorer on Windows Server 2012: #12154 Both of these issues were traced to stack corruption after a Python callback of NVDA's was called from external libraries. In SAPI4's case, after calling NVDA's implementation of ITTSBufNotifySink::TextDataStarted, and in the Windows Server 2012 case: IUIAutomationPropertyChangedEventHandler::handlePropertyChangedEvent. It seems as though libFFI / Python ctypes is not cleaning the stack properly after executing a Python callback with the stdcall calling convention (ctypes WINFUNCTYPE), where the callback contained at least one argument larger than 4 bytes (E.g. a long long, or a VARIANT struct), and the arguments preceding it were such that this argument was not aligned to an 8 byte boundary. E.g. the callback might be: • callback(void*, long long) or • callback(void*, void*, int, VARIANT) See Python bug: https://bugs.python.org/issue38748 On that bug I have attached a minimal testcase. This bug affects both Python 3.8 and Python 3.9. The bug is most likely in the libFFI project used by Python's ctypes module. Python 3.8 switched to a much more recent and official version of libFFI I believe. Although we do really want to move to Python 3.8+ as soon as we can, right now this bug makes it impossible to do so. We could specifically work around the currently known manifestations by moving some of that code into C++, but that brings its own risks, and we still don't know where else this issue may appear in our code. The appropriate thing to do right now is stay on Python 3.7 until we can work with Python / libFFI to get this resolved. Description of how this pull request fixes the issue: Downgrades to Python 3.7 by referencing Python 3.7 (rather than 3.8) in NVDA's build system. The existing PRs that needed to be reverted were: • Updating brlAPI to a Python 3.8 build: nvaccess/nvda-misc-deps#20 • Switching to using Python's own pgettext: #12109 • calling os.add_dll_directory when loading liblouis: #12020 None of these PRs provided any user visible changes. The rest of the Python 3.8 work, including the switch to a virtual environment etc is all compatible with Python 3.7 and can remain.
Steps to reproduce:
%temp%press enter and attempt to navigate in the newly opened folder.Actual behavior:
Windows error sound plays and NVDA crashes and restarts. If the folder view is still focused after restart NVDA crashes again.
Expected behavior:
Windows Explorer should be navigable with NVDA
System configuration
NVDA installed/portable/running from source:
Tested with both portable and installed
NVDA version:
alpha-21882,528d5708
Windows version:
Windows Server 2012.
Name and version of other software in use when reproducing the issue:
Windows Explorer
Other information about your system:
None relevant.
Other questions
Does the issue still occur after restarting your computer?
Yes
Have you tried any other versions of NVDA? If so, please report their behaviors.
2020.4 works okay, latest Python 3.7 based Alpha works okay.
If add-ons are disabled, is your problem still occurring?
Yes
Did you try to run the COM registry fixing tool in NVDA menu / tools?
Yes with no effect.
The attached zip file contains log at debug level and NVDA crash dump file.
crash.zip
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