sqlite3: Blob slice assignment skips validation for empty slices

[ever0de]

Bug report

Bug description:

sqlite3.Blob slice assignment skips type and size validation when the
target slice is empty (start == stop, or any slice whose computed
length is zero).

As a result, assignments that would normally raise TypeError or
IndexError complete silently when the target slice is empty.

This behavior is inconsistent with non-empty sqlite3.Blob slice
assignment and appears to be caused by an early return in
ass_subscript_slice() before validation is performed.

Reproduction

import sqlite3

con = sqlite3.connect(":memory:")
con.execute("CREATE TABLE t(b BLOB)")
con.execute("INSERT INTO t VALUES(zeroblob(10))")
con.commit()

blob = con.blobopen("t", "b", 1)

# Non-empty slice — expected behavior
blob[0:3] = b"12345" # IndexError
blob[0:3] = None # TypeError

# Empty slice — BUG
blob[5:5] = b"123" # silently succeeds
blob[5:5] = None # silently succeeds

Expected behavior

Empty slice assignments should be validated in the same way as non-empty
slice assignments.

blob[5:5] = b"123"

should raise:

IndexError: Blob slice assignment is wrong size

and

blob[5:5] = None

should raise:

TypeError: a bytes-like object is required, not 'NoneType'

Actual behavior

Both assignments complete successfully. The blob remains unchanged and
no exception is raised.

Root cause

In Modules/_sqlite/blob.c, ass_subscript_slice() returns early when
the computed slice length is zero.

Because this early return occurs before the bytes-like object check and
slice-size validation, invalid assignments are silently accepted for
empty slices.

Related

Discovered during review of #150450 (negative-step slice handling for
sqlite3.Blob).

Review discussion:

#150450 (comment)

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-150913: Fix sqlite3.Blob validation for empty slice assignment #150915
• [3.15] gh-150913: Fix sqlite3.Blob validation for empty slice assignment (GH-150915) #150923
• [3.14] gh-150913: Fix sqlite3.Blob validation for empty slice assignment (GH-150915) #150924
• [3.13] gh-150913: Fix sqlite3.Blob validation for empty slice assignment (GH-150915) #150925