[3.15] gh-150449: Raise ValueError for sqlite3.Blob slices with a negative step (GH-155773)

Reading or writing such a slice computed a negative length and failed with
SystemError.  Empty slices, which select nothing, are still allowed.

(cherry picked from commit d9e8d226a3d05ea020256086aee24d21f9792206)

Author: serhiy-storchaka

Lib/test/test_sqlite3/test_dbapi.py

@@ -1396,6 +1396,19 @@ def test_blob_set_slice(self):
         actual = self.cx.execute("select b from test").fetchone()[0]
         self.assertEqual(actual, expected)
 
+    def test_blob_slice_with_negative_step(self):
+        # gh-150449: this used to raise SystemError
+        for sl in (slice(9, 0, -2), slice(None, None, -1), slice(9, None, -2)):
+            with self.subTest(slice=sl):
+                with self.assertRaises(ValueError):
+                    self.blob[sl]
+                with self.assertRaises(ValueError):
+                    self.blob[sl] = b"1" * len(self.data[sl])
+        # an empty slice selects nothing and is still allowed
+        self.assertEqual(self.blob[3:8:-1], b"")
+        self.blob[3:8:-1] = b""
+        self.assertEqual(self.blob[:], self.data)
+
     def test_blob_set_slice_with_step_keeps_bytes_intact(self):
         # The buffer used for the read-patch-write cycle must not be the
         # bytes object read from the blob: for a single byte it is an

Misc/NEWS.d/next/Library/2026-08-14-09-28-21.gh-issue-150449.Nf3Qz1.rst

@@ -0,0 +1,3 @@
+Fix :class:`sqlite3.Blob` slices with a negative step.
+Reading or writing such a slice raised :exc:`SystemError`.
+It now raises :exc:`ValueError`.

Modules/_sqlite/blob.c

@@ -450,6 +450,12 @@ subscript_slice(pysqlite_Blob *self, PyObject *item)
         return PyBytes_FromStringAndSize(NULL, 0);
     }
 
+    if (step < 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "Blob doesn't support slices with a negative step");
+        return NULL;
+    }
+
     if (step == 1) {
         return read_multiple(self, len, start);
     }
@@ -557,6 +563,13 @@ ass_subscript_slice(pysqlite_Blob *self, PyObject *item, PyObject *value)
         return 0;
     }
 
+    if (step < 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "Blob doesn't support slices with a negative step");
+        PyBuffer_Release(&vbuf);
+        return -1;
+    }
+
     int rc = -1;
     if (step == 1) {
         rc = inner_write(self, vbuf.buf, len, start);