Unexpected segfault appearing in PyQt6 6.9.1

[Jakub Fránek]

Hello,

I made a PyQt application which has been working without any issues for
quite some time. I updated PyQt6 from 6.9.0 to 6.9.1 and only now I get a
segmentation fault related to mapping indices with a QSortFilterProxyModel
while the source model is being updated.

What my code did previously was something like this, in Pythonesque
pseudocode:

custom_source_model.beginResetModel()
custom_source_model.load_data(new_data)
source_index = custom_source_model.get_index(some_item)
proxy_index = proxy.mapFromSource(source_index)    # crash occurs here
# irrelevant lines of code which utilize the proxy_index somehow...
custom_source_model.endResetModel()

My understanding is that what I did is incorrect and I should not perform
any operations on the proxy model while the main model is in reset. When I
postpone these index related operations until after the
custom_source_model.endResetModel() call, the segmentation fault does not
occur.

What I would like to know is, what changed in 6.9.1 that makes this
segmentation fault occur? The release notes do not document any change
which would explain this.
The reason why I am asking is twofold: 1) to fully explain the appearance
of this segfault in 6.9.1 and 2) to find out whether there are any other
hidden changes that could uncover pre-existing errors in my code.

Thank you very much.

Kind regards
Jakub
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