manufacture empty result for DELETE..RETURNING w/ no description

Established a workaround for what seems to be an intrinsic issue across
MySQL/MariaDB drivers where a RETURNING result for DELETE DML which returns
no rows using SQLAlchemy's "empty IN" criteria fails to provide a
cursor.description, which then yields result that returns no rows,
leading to regressions for the ORM that in the 2.0 series uses RETURNING
for bulk DELETE statements for the "synchronize session" feature. To
resolve, when the specific case of "no description when RETURNING was
given" is detected, an "empty result" with a correct cursor description is
generated and used in place of the non-working cursor.

Fixes: #10505
Change-Id: Ib56f1ec5746e2b3212e563169353bc023db84099

diff --git a/doc/build/changelog/unreleased_20/10505.rst b/doc/build/changelog/unreleased_20/10505.rst
new file mode 100644 (file)
index 0000000..b9ea6b4
--- /dev/null
+++ b/doc/build/changelog/unreleased_20/10505.rst
@@ -0,0 +1,13 @@
+.. change::
+    :tags: bug, mariadb, regression
+    :tickets: 10505
+
+    Established a workaround for what seems to be an intrinsic issue across
+    MySQL/MariaDB drivers where a RETURNING result for DELETE DML which returns
+    no rows using SQLAlchemy's "empty IN" criteria fails to provide a
+    cursor.description, which then yields result that returns no rows,
+    leading to regressions for the ORM that in the 2.0 series uses RETURNING
+    for bulk DELETE statements for the "synchronize session" feature. To
+    resolve, when the specific case of "no description when RETURNING was
+    given" is detected, an "empty result" with a correct cursor description is
+    generated and used in place of the non-working cursor.

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 465f3597cbc368a415c2d3187f5de15564c4490e..92f90774fbe2c343ad973d0b45e8bd2df49d6ff7 100644 (file)
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -999,14 +999,14 @@ output::
     )
 
 """  # noqa
+from __future__ import annotations
 
 from array import array as _array
 from collections import defaultdict
 from itertools import compress
 import re
+from typing import cast
 
-from sqlalchemy import literal_column
-from sqlalchemy.sql import visitors
 from . import reflection as _reflection
 from .enumerated import ENUM
 from .enumerated import SET
@@ -1047,10 +1047,12 @@ from .types import TINYTEXT
 from .types import VARCHAR
 from .types import YEAR
 from ... import exc
+from ... import literal_column
 from ... import log
 from ... import schema as sa_schema
 from ... import sql
 from ... import util
+from ...engine import cursor as _cursor
 from ...engine import default
 from ...engine import reflection
 from ...engine.reflection import ReflectionDefaults
@@ -1062,7 +1064,9 @@ from ...sql import operators
 from ...sql import roles
 from ...sql import sqltypes
 from ...sql import util as sql_util
+from ...sql import visitors
 from ...sql.compiler import InsertmanyvaluesSentinelOpts
+from ...sql.compiler import SQLCompiler
 from ...sql.schema import SchemaConst
 from ...types import BINARY
 from ...types import BLOB
@@ -1166,6 +1170,32 @@ ischema_names = {
 
 
 class MySQLExecutionContext(default.DefaultExecutionContext):
+    def post_exec(self):
+        if (
+            self.isdelete
+            and cast(SQLCompiler, self.compiled).effective_returning
+            and not self.cursor.description
+        ):
+            # All MySQL/mariadb drivers appear to not include
+            # cursor.description for DELETE..RETURNING with no rows if the
+            # WHERE criteria is a straight "false" condition such as our EMPTY
+            # IN condition. manufacture an empty result in this case (issue
+            # #10505)
+            #
+            # taken from cx_Oracle implementation
+            self.cursor_fetch_strategy = (
+                _cursor.FullyBufferedCursorFetchStrategy(
+                    self.cursor,
+                    [
+                        (entry.keyname, None)
+                        for entry in cast(
+                            SQLCompiler, self.compiled
+                        )._result_columns
+                    ],
+                    [],
+                )
+            )
+
     def create_server_side_cursor(self):
         if self.dialect.supports_server_side_cursors:
             return self._dbapi_connection.cursor(self.dialect._sscursor)

diff --git a/lib/sqlalchemy/dialects/mysql/mariadbconnector.py b/lib/sqlalchemy/dialects/mysql/mariadbconnector.py
index df9e84ad51a599e70955a7f3015c08dc0958c126..9730c9b4da354b6c3968866cc04d134fe744217a 100644 (file)
--- a/lib/sqlalchemy/dialects/mysql/mariadbconnector.py
+++ b/lib/sqlalchemy/dialects/mysql/mariadbconnector.py
@@ -80,6 +80,8 @@ class MySQLExecutionContext_mariadbconnector(MySQLExecutionContext):
         return self._dbapi_connection.cursor(buffered=True)
 
     def post_exec(self):
+        super().post_exec()
+
         self._rowcount = self.cursor.rowcount
 
         if self.isinsert and self.compiled.postfetch_lastrowid:

diff --git a/lib/sqlalchemy/testing/suite/test_update_delete.py b/lib/sqlalchemy/testing/suite/test_update_delete.py
index 62776ea6b3602344cf4a837ec585c35848330a5d..2d13bda34aee5ae7f1b8dd7f81e770029e3c7b6f 100644 (file)
--- a/lib/sqlalchemy/testing/suite/test_update_delete.py
+++ b/lib/sqlalchemy/testing/suite/test_update_delete.py
@@ -6,6 +6,7 @@ from ..schema import Column
 from ..schema import Table
 from ... import Integer
 from ... import String
+from ... import testing
 
 
 class SimpleUpdateDeleteTest(fixtures.TablesTest):
@@ -58,5 +59,71 @@ class SimpleUpdateDeleteTest(fixtures.TablesTest):
             [(1, "d1"), (3, "d3")],
         )
 
+    @testing.variation("criteria", ["rows", "norows", "emptyin"])
+    @testing.requires.update_returning
+    def test_update_returning(self, connection, criteria):
+        t = self.tables.plain_pk
+
+        stmt = t.update().returning(t.c.id, t.c.data)
+
+        if criteria.norows:
+            stmt = stmt.where(t.c.id == 10)
+        elif criteria.rows:
+            stmt = stmt.where(t.c.id == 2)
+        elif criteria.emptyin:
+            stmt = stmt.where(t.c.id.in_([]))
+        else:
+            criteria.fail()
+
+        r = connection.execute(stmt, dict(data="d2_new"))
+        assert not r.is_insert
+        assert r.returns_rows
+        eq_(r.keys(), ["id", "data"])
+
+        if criteria.rows:
+            eq_(r.all(), [(2, "d2_new")])
+        else:
+            eq_(r.all(), [])
+
+        eq_(
+            connection.execute(t.select().order_by(t.c.id)).fetchall(),
+            [(1, "d1"), (2, "d2_new"), (3, "d3")]
+            if criteria.rows
+            else [(1, "d1"), (2, "d2"), (3, "d3")],
+        )
+
+    @testing.variation("criteria", ["rows", "norows", "emptyin"])
+    @testing.requires.delete_returning
+    def test_delete_returning(self, connection, criteria):
+        t = self.tables.plain_pk
+
+        stmt = t.delete().returning(t.c.id, t.c.data)
+
+        if criteria.norows:
+            stmt = stmt.where(t.c.id == 10)
+        elif criteria.rows:
+            stmt = stmt.where(t.c.id == 2)
+        elif criteria.emptyin:
+            stmt = stmt.where(t.c.id.in_([]))
+        else:
+            criteria.fail()
+
+        r = connection.execute(stmt)
+        assert not r.is_insert
+        assert r.returns_rows
+        eq_(r.keys(), ["id", "data"])
+
+        if criteria.rows:
+            eq_(r.all(), [(2, "d2")])
+        else:
+            eq_(r.all(), [])
+
+        eq_(
+            connection.execute(t.select().order_by(t.c.id)).fetchall(),
+            [(1, "d1"), (3, "d3")]
+            if criteria.rows
+            else [(1, "d1"), (2, "d2"), (3, "d3")],
+        )
+
 
 __all__ = ("SimpleUpdateDeleteTest",)