bpo-43752: Fix sqlite3 regression for zero-sized blobs with converter…

Lib/sqlite3/test/types.py

@@ -255,7 +255,9 @@ def test_number2(self):
     def test_convert_zero_sized_blob(self):
         self.con.execute("insert into test(cbin) values (?)", (b"",))
         cur = self.con.execute("select cbin from test")
-        self.assertEqual(cur.fetchone()[0], b"blobish")
+        # Zero-sized blobs with converters returns None.  This differs from
+        # blobs without a converter, where b"" is returned.
+        self.assertIsNone(cur.fetchone()[0])
 
 
 class ColNamesTests(unittest.TestCase):

Misc/NEWS.d/next/Library/2021-04-06-21-18-29.bpo-43752.K7qmAF.rst

@@ -0,0 +1,3 @@
+Fix :mod:`sqlite3` regression for zero-sized blobs with converters, where
+``b""`` was returned instead of ``None``. The regression was introduced by
+GH-24723. Patch by Erlend E. Aasland.

Modules/_sqlite/cursor.c

@@ -240,7 +240,6 @@ _pysqlite_fetch_one_row(pysqlite_Cursor* self)
 {
     int i, numcols;
     PyObject* row;
-    PyObject* item = NULL;
     int coltype;
     PyObject* converter;
     PyObject* converted;
@@ -282,18 +281,22 @@ _pysqlite_fetch_one_row(pysqlite_Cursor* self)
          */
         if (converter != Py_None) {
             const void *blob = sqlite3_column_blob(self->statement->st, i);
-            if (blob == NULL && sqlite3_errcode(db) == SQLITE_NOMEM) {
-                PyErr_NoMemory();
-                goto error;
+            if (blob == NULL) {
+                if (sqlite3_errcode(db) == SQLITE_NOMEM) {
+                    PyErr_NoMemory();
+                    goto error;
+                }
+                converted = Py_NewRef(Py_None);
             }
-
-            nbytes = sqlite3_column_bytes(self->statement->st, i);
-            item = PyBytes_FromStringAndSize(blob, nbytes);
-            if (item == NULL) {
-                goto error;
+            else {
+                nbytes = sqlite3_column_bytes(self->statement->st, i);
+                PyObject *item = PyBytes_FromStringAndSize(blob, nbytes);
+                if (item == NULL) {
+                    goto error;
+                }
+                converted = PyObject_CallOneArg(converter, item);
+                Py_DECREF(item);
             }
-            converted = PyObject_CallOneArg(converter, item);
-            Py_DECREF(item);
         } else {
             Py_BEGIN_ALLOW_THREADS
             coltype = sqlite3_column_type(self->statement->st, i);