[Python-checkins] [3.9] bpo-44594: fix (Async)ExitStack handling of __context__ (gh-27089) (GH-28731)

[miss-islington]

https://github.com/python/cpython/commit/7c2a040a10654d67ff543a55858ba2d7a9f7eea8
commit: 7c2a040a10654d67ff543a55858ba2d7a9f7eea8
branch: 3.9
author: John Belmonte <john at neggie.net>
committer: miss-islington <31488909+miss-islington at users.noreply.github.com>
date: 2021-10-04T23:37:24-07:00
summary:

[3.9] bpo-44594: fix (Async)ExitStack handling of __context__ (gh-27089) (GH-28731)



Make enter_context(foo()) / enter_async_context(foo()) equivalent to
`[async] with foo()` regarding __context__ when an exception is raised.

Previously exceptions would be caught and re-raised with the wrong
context when explicitly overriding __context__ with None..
(cherry picked from commit e6d1aa1ac65b6908fdea2c70ec3aa8c4f1dffcb5)

Co-authored-by: John Belmonte <john at neggie.net>

Automerge-Triggered-By: GH:njsmith

files:
A Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst
M Lib/contextlib.py
M Lib/test/test_contextlib.py
M Lib/test/test_contextlib_async.py

diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index f60f0c274a715..4e8f5f7593917 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -496,10 +496,10 @@ def _fix_exception_context(new_exc, old_exc):
             # Context may not be correct, so find the end of the chain
             while 1:
                 exc_context = new_exc.__context__
-                if exc_context is old_exc:
+                if exc_context is None or exc_context is old_exc:
                     # Context is already set correctly (see issue 20317)
                     return
-                if exc_context is None or exc_context is frame_exc:
+                if exc_context is frame_exc:
                     break
                 new_exc = exc_context
             # Change the end of the chain to point to the exception
@@ -630,10 +630,10 @@ def _fix_exception_context(new_exc, old_exc):
             # Context may not be correct, so find the end of the chain
             while 1:
                 exc_context = new_exc.__context__
-                if exc_context is old_exc:
+                if exc_context is None or exc_context is old_exc:
                     # Context is already set correctly (see issue 20317)
                     return
-                if exc_context is None or exc_context is frame_exc:
+                if exc_context is frame_exc:
                     break
                 new_exc = exc_context
             # Change the end of the chain to point to the exception
diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index ad071bb2a22d3..354ea8b3c32b7 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -777,6 +777,40 @@ def suppress_exc(*exc_details):
         self.assertIsInstance(inner_exc, ValueError)
         self.assertIsInstance(inner_exc.__context__, ZeroDivisionError)
 
+    def test_exit_exception_explicit_none_context(self):
+        # Ensure ExitStack chaining matches actual nested `with` statements
+        # regarding explicit __context__ = None.
+
+        class MyException(Exception):
+            pass
+
+        @contextmanager
+        def my_cm():
+            try:
+                yield
+            except BaseException:
+                exc = MyException()
+                try:
+                    raise exc
+                finally:
+                    exc.__context__ = None
+
+        @contextmanager
+        def my_cm_with_exit_stack():
+            with self.exit_stack() as stack:
+                stack.enter_context(my_cm())
+                yield stack
+
+        for cm in (my_cm, my_cm_with_exit_stack):
+            with self.subTest():
+                try:
+                    with cm():
+                        raise IndexError()
+                except MyException as exc:
+                    self.assertIsNone(exc.__context__)
+                else:
+                    self.fail("Expected IndexError, but no exception was raised")
+
     def test_exit_exception_non_suppressing(self):
         # http://bugs.python.org/issue19092
         def raise_exc(exc):
diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py
index 9d6854c702892..43aaf04744431 100644
--- a/Lib/test/test_contextlib_async.py
+++ b/Lib/test/test_contextlib_async.py
@@ -463,6 +463,41 @@ async def suppress_exc(*exc_details):
         self.assertIsInstance(inner_exc, ValueError)
         self.assertIsInstance(inner_exc.__context__, ZeroDivisionError)
 
+    @_async_test
+    async def test_async_exit_exception_explicit_none_context(self):
+        # Ensure AsyncExitStack chaining matches actual nested `with` statements
+        # regarding explicit __context__ = None.
+
+        class MyException(Exception):
+            pass
+
+        @asynccontextmanager
+        async def my_cm():
+            try:
+                yield
+            except BaseException:
+                exc = MyException()
+                try:
+                    raise exc
+                finally:
+                    exc.__context__ = None
+
+        @asynccontextmanager
+        async def my_cm_with_exit_stack():
+            async with self.exit_stack() as stack:
+                await stack.enter_async_context(my_cm())
+                yield stack
+
+        for cm in (my_cm, my_cm_with_exit_stack):
+            with self.subTest():
+                try:
+                    async with cm():
+                        raise IndexError()
+                except MyException as exc:
+                    self.assertIsNone(exc.__context__)
+                else:
+                    self.fail("Expected IndexError, but no exception was raised")
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst b/Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst
new file mode 100644
index 0000000000000..a2bfd8ff5b51b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst
@@ -0,0 +1,3 @@
+Fix an edge case of :class:`ExitStack` and :class:`AsyncExitStack` exception
+chaining.  They will now match ``with`` block behavior when ``__context__`` is
+explicitly set to ``None`` when the exception is in flight.