GeezerVison

picnixz left a comment

Reading or writing a sqlite3.Blob with a negative-step slice such as
blob[9:0:-2] caused a crash (SystemError on older versions, ValueError
on current main) because the C code computed stop - start as the read
length, which is negative for negative steps.

Fix subscript_slice() and ass_subscript_slice() in Modules/_sqlite/blob.c
to handle both positive and negative steps correctly:
- For step > 1: keep reading [start, stop) and indexing forward as before.
- For step < -1: read the contiguous range [start+(len-1)*step, start]
  and index backward with stride -step.

When a negative-step slice has start <= stop (e.g. blob[3:8:-1]), the
slice is empty (slicelength == 0).  Previously ass_subscript_slice()
returned 0 immediately without acquiring the value buffer, so assigning
a non-empty bytes object to such a slice silently succeeded instead of
raising IndexError.

Move PyObject_GetBuffer() and the size check before the len == 0 early
return so that the element-count mismatch is always detected.

Exercise the case where step is negative but start <= stop, which
produces an empty slice.  Verify that:
  - blob[3:8:-1] returns b"" (no crash)
  - blob[3:8:-1] = b"" is a no-op
  - blob[3:8:-1] = b"abc" raises IndexError (wrong size)

Also add blob[5:5:-1] == b"" to cover the start == stop edge case.

serhiy-storchaka left a comment

Classify negative-step slice support as a new feature per reviewer
feedback (serhiy-storchaka):

- Add versionchanged:: 3.16 directive to sqlite3.Blob documentation
  noting that negative-step slices are now supported.
- Add What's New entry under Doc/whatsnew/3.16.rst.
- Update NEWS entry wording to describe the change as a feature.
- Add tests for empty-slice assignment edge cases (blob[5:5] = None
  and blob[5:5] = b'123') to verify TypeError and IndexError are
  raised correctly.
- Rename read_offset to write_offset in ass_subscript_slice() since
  that variable is used as the offset for inner_write(), not a read.

Per devguide markup guidelines, the version argument for versionchanged
should be 'next' during development; the release manager replaces it
with the actual version number at release time.

…ystemError or ValueError'

…slice

…ythongh-150913

The empty-slice assignment validation change (moving the len==0 early
return to after buffer/type/size checks) has been split out to a
separate bug-fix PR targeting issue python#150913.

This branch retains only the negative-step slice feature.

…-sqlite-blob-negative-step-slice

picnixz left a comment

…ice operations

Add test cases for sys.maxsize and -sys.maxsize-1 step values in both
read (test_blob_get_slice_with_negative_step) and write
(test_blob_set_slice_with_negative_step) to cover the unsigned wrap-around
arithmetic path that avoids signed integer overflow.

…in sqlite3.Blob

Extend the existing extreme-step test cases (sys.maxsize / -sys.maxsize-1)
in test_blob_set_slice_with_negative_step to verify the blob contents
were actually written correctly, not just that no crash occurred.

…p-slice

…p-slice

…p-slice

…p-slice

Resolve the conflict in ass_subscript_slice(): keep the region computation
which handles negative and extreme steps, and read it into a plain buffer
as in pythongh-155702, instead of patching the bytes object read from the blob.