gh-150449: Move empty-slice validation fix to dedicated PR for gh-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 #150913.

This branch retains only the negative-step slice feature.

Author: ever0de

Lib/test/test_sqlite3/test_dbapi.py

@@ -1410,18 +1410,6 @@ def test_blob_set_empty_slice(self):
         self.blob[0:0] = b""
         self.assertEqual(self.blob[:], self.data)
 
-    def test_blob_set_empty_slice_wrong_type(self):
-        # Assigning a non-buffer object to an empty slice must raise TypeError
-        # even when the slice length is zero.
-        with self.assertRaises(TypeError):
-            self.blob[5:5] = None
-
-    def test_blob_set_empty_slice_wrong_size(self):
-        # Assigning a non-empty bytes object to an empty slice must raise
-        # IndexError because the sizes do not match.
-        with self.assertRaisesRegex(IndexError, "wrong size"):
-            self.blob[5:5] = b"123"
-
     def test_blob_set_slice_with_skip(self):
         self.blob[0:10:2] = b"12345"
         actual = self.cx.execute("select b from test").fetchone()[0]
@@ -1448,9 +1436,6 @@ def test_blob_set_slice_with_negative_step(self):
         state_before = bytes(self.blob[:])
         self.blob[3:8:-1] = b""
         self.assertEqual(bytes(self.blob[:]), state_before)
-        # Assigning a non-empty sequence to an empty slice must raise.
-        with self.assertRaisesRegex(IndexError, "wrong size"):
-            self.blob[3:8:-1] = b"abc"
 
     def test_blob_mapping_invalid_index_type(self):
         msg = "indices must be integers"

Modules/_sqlite/blob.c

@@ -540,31 +540,24 @@ ass_subscript_slice(pysqlite_Blob *self, PyObject *item, PyObject *value)
         return -1;
     }
 
+    if (len == 0) {
+        return 0;
+    }
+
     Py_buffer vbuf;
     if (PyObject_GetBuffer(value, &vbuf, PyBUF_SIMPLE) < 0) {
         return -1;
     }
 
-    // For extended slices the right-hand side must have the exact same
-    // element count as the slice, even when that count is zero.
+    int rc = -1;
     if (vbuf.len != len) {
         PyErr_SetString(PyExc_IndexError,
                         "Blob slice assignment is wrong size");
-        PyBuffer_Release(&vbuf);
-        return -1;
-    }
-
-    if (len == 0) {
-        PyBuffer_Release(&vbuf);
-        return 0;
     }
-
-    int rc;
-    if (step == 1) {
+    else if (step == 1) {
         rc = inner_write(self, vbuf.buf, len, start);
     }
     else {
-        rc = -1;
         // Compute the contiguous blob region covering all slice elements, then
         // update each element using the standard size_t-cursor pattern that
         // handles both positive and negative steps via unsigned arithmetic.