[cppyy] Set LowLevelView itemsize to sizeof(T) for fixed multidim arrays

[aaronj0]

For fixed contiguous C arrays (e.g. float a[2][2]), the ndim > 1 path in CreateLowLevelViewT was unconditionally setting view.itemsize to sizeof(void*), which is correct for pointer-to-pointer arrays but does not work for fixed arrays. Fixes #21378

[vepadulano]

This is great! Could you also add a test?

[aaronj0]

This is great! Could you also add a test?

neat, this actually revealed a bug in a test pyunittests-bindings-pyroot-pythonizations-pyroot-array-numpy-views that I wrote for fixed-width integer low level views: #18492 :)

Back then, I didn't realise why I had to read the buffer as 1D and reshape the resulting Py_buffer but now it all makes sense...
In the test, we see the same case, view.len = 16, and view.itemsize = 8 so frombuffer saw 2 elements of size 8, but count=22 forced it to reinterpret the raw bytes as 22 elements anyway. That's a bad pattern..

With this patch, frombuffer correctly sees 4 floats, but count=22 now exceeds the buffer size (as it should). Now that the underlying issue is fixed, we can just use np.array(cpp_arr, dtype=dtype[0]) and it works as expected!

[github-actions]

Test Results

    22 files      22 suites   3d 2h 31m 27s ⏱️
 3 807 tests  3 804 ✅ 1 💤 2 ❌
75 691 runs  75 680 ✅ 9 💤 2 ❌

For more details on these failures, see this check.

Results for commit 22b34ec.

♻️ This comment has been updated with latest results.

[vepadulano]

Thank you!

[aaronj0]

ruff formatting has been addressed
edit: it now complains about the rest of the file, and not the added test itself

[vepadulano]

ruff formatting has been addressed
edit: it now complains about the rest of the file, and not the added test itself

Yep, that's expected. You can choose to either merge this PR as is, or take the chance to ruff format and ruff check the entire file, but then those changes would go in a separate commit. Up to you.

[aaronj0]

ruff formatting has been addressed
edit: it now complains about the rest of the file, and not the added test itself

Yep, that's expected. You can choose to either merge this PR as is, or take the chance to ruff format and ruff check the entire file, but then those changes would go in a separate commit. Up to you.

As we want to minimize the diff with upstream, we should keep the cppyy/test files the same.

Merging, as test failures on mac-beta (h2root) and ubuntu26 (python-distrdf-backends-test_all) can be seen on master

[aaronj0]

/backport to 6.38

[root-project-bot]

Something went wrong with the backport to 6.38: @aaronj0 please see the logs