Fix task cancellation propagation to subtasks when using sync middlew…

…are (#435)

asgiref/sync.py

@@ -203,6 +203,10 @@ def __call__(self, *args: _P.args, **kwargs: _P.kwargs) -> _R:
         # `main_wrap`.
         context = [contextvars.copy_context()]
 
+        # Get task context so that parent task knows which task to propagate
+        # an asyncio.CancelledError to.
+        task_context = getattr(SyncToAsync.threadlocal, "task_context", None)
+
         loop = None
         # Use call_soon_threadsafe to schedule a synchronous callback on the
         # main event loop's thread if it's there, otherwise make a new loop
@@ -211,6 +215,7 @@ def __call__(self, *args: _P.args, **kwargs: _P.kwargs) -> _R:
             awaitable = self.main_wrap(
                 call_result,
                 sys.exc_info(),
+                task_context,
                 context,
                 *args,
                 **kwargs,
@@ -295,6 +300,7 @@ async def main_wrap(
         self,
         call_result: "Future[_R]",
         exc_info: "OptExcInfo",
+        task_context: "Optional[List[asyncio.Task[Any]]]",
         context: List[contextvars.Context],
         *args: _P.args,
         **kwargs: _P.kwargs,
@@ -309,6 +315,10 @@ async def main_wrap(
         if context is not None:
             _restore_context(context[0])
 
+        current_task = asyncio.current_task()
+        if current_task is not None and task_context is not None:
+            task_context.append(current_task)
+
         try:
             # If we have an exception, run the function inside the except block
             # after raising it so exc_info is correctly populated.
@@ -324,6 +334,8 @@ async def main_wrap(
         else:
             call_result.set_result(result)
         finally:
+            if current_task is not None and task_context is not None:
+                task_context.remove(current_task)
             context[0] = contextvars.copy_context()
 
 
@@ -437,20 +449,38 @@ async def __call__(self, *args: _P.args, **kwargs: _P.kwargs) -> _R:
         context = contextvars.copy_context()
         child = functools.partial(self.func, *args, **kwargs)
         func = context.run
-
+        task_context: List[asyncio.Task[Any]] = []
+
+        # Run the code in the right thread
+        exec_coro = loop.run_in_executor(
+            executor,
+            functools.partial(
+                self.thread_handler,
+                loop,
+                sys.exc_info(),
+                task_context,
+                func,
+                child,
+            ),
+        )
+        ret: _R
         try:
-            # Run the code in the right thread
-            ret: _R = await loop.run_in_executor(
-                executor,
-                functools.partial(
-                    self.thread_handler,
-                    loop,
-                    sys.exc_info(),
-                    func,
-                    child,
-                ),
-            )
-
+            ret = await asyncio.shield(exec_coro)
+        except asyncio.CancelledError:
+            cancel_parent = True
+            try:
+                task = task_context[0]
+                task.cancel()
+                try:
+                    await task
+                    cancel_parent = False
+                except asyncio.CancelledError:
+                    pass
+            except IndexError:
+                pass
+            if cancel_parent:
+                exec_coro.cancel()
+            ret = await exec_coro
         finally:
             _restore_context(context)
             self.deadlock_context.set(False)
@@ -466,7 +496,7 @@ def __get__(
         func = functools.partial(self.__call__, parent)
         return functools.update_wrapper(func, self.func)
 
-    def thread_handler(self, loop, exc_info, func, *args, **kwargs):
+    def thread_handler(self, loop, exc_info, task_context, func, *args, **kwargs):
         """
         Wraps the sync application with exception handling.
         """
@@ -476,6 +506,7 @@ def thread_handler(self, loop, exc_info, func, *args, **kwargs):
         # Set the threadlocal for AsyncToSync
         self.threadlocal.main_event_loop = loop
         self.threadlocal.main_event_loop_pid = os.getpid()
+        self.threadlocal.task_context = task_context
 
         # Run the function
         # If we have an exception, run the function inside the except block

tests/test_sync.py

@@ -852,13 +852,10 @@ def sync_task():
 
 
 @pytest.mark.asyncio
-@pytest.mark.skip(reason="deadlocks")
 async def test_inner_shield_sync_middleware():
     """
     Tests that asyncio.shield is capable of preventing http.disconnect from
     cancelling a django request task when using sync middleware.
-
-    Currently this tests is skipped as it causes a deadlock.
     """
 
     # Hypothetical Django scenario - middleware function is sync
@@ -968,3 +965,159 @@ async def async_task():
         assert task_complete
 
     assert task_executed
+
+
+@pytest.mark.asyncio
+async def test_inner_shield_sync_and_async_middleware():
+    """
+    Tests that asyncio.shield is capable of preventing http.disconnect from
+    cancelling a django request task when using sync and middleware chained
+    together.
+    """
+
+    # Hypothetical Django scenario - middleware function is sync
+    def sync_middleware_1():
+        async_to_sync(async_middleware_2)()
+
+    # Hypothetical Django scenario - middleware function is async
+    async def async_middleware_2():
+        await sync_to_async(sync_middleware_3)()
+
+    # Hypothetical Django scenario - middleware function is sync
+    def sync_middleware_3():
+        async_to_sync(async_middleware_4)()
+
+    # Hypothetical Django scenario - middleware function is async
+    async def async_middleware_4():
+        await sync_to_async(sync_middleware_5)()
+
+    # Hypothetical Django scenario - middleware function is sync
+    def sync_middleware_5():
+        async_to_sync(async_view)()
+
+    task_complete = False
+    task_cancel_caught = False
+
+    # Future that completes when subtask cancellation attempt is caught
+    task_blocker = asyncio.Future()
+
+    async def async_view():
+        """Async view with a task that is shielded from cancellation."""
+        nonlocal task_complete, task_cancel_caught, task_blocker
+        task = asyncio.create_task(async_task())
+        try:
+            await asyncio.shield(task)
+        except asyncio.CancelledError:
+            task_cancel_caught = True
+            task_blocker.set_result(True)
+            await task
+            task_complete = True
+
+    task_executed = False
+
+    # Future that completes after subtask is created
+    task_started_future = asyncio.Future()
+
+    async def async_task():
+        """Async subtask that should not be canceled when parent is canceled."""
+        nonlocal task_started_future, task_executed, task_blocker
+        task_started_future.set_result(True)
+        await task_blocker
+        task_executed = True
+
+    task_cancel_propagated = False
+
+    async with ThreadSensitiveContext():
+        task = asyncio.create_task(sync_to_async(sync_middleware_1)())
+        await task_started_future
+        task.cancel()
+        try:
+            await task
+        except asyncio.CancelledError:
+            task_cancel_propagated = True
+        assert not task_cancel_propagated
+        assert task_cancel_caught
+        assert task_complete
+
+    assert task_executed
+
+
+@pytest.mark.asyncio
+async def test_inner_shield_sync_and_async_middleware_sync_task():
+    """
+    Tests that asyncio.shield is capable of preventing http.disconnect from
+    cancelling a django request task when using sync and middleware chained
+    together with an async view calling a sync function calling an async task.
+
+    This test ensures that a parent initiated task cancellation will not
+    propagate to a shielded subtask.
+    """
+
+    # Hypothetical Django scenario - middleware function is sync
+    def sync_middleware_1():
+        async_to_sync(async_middleware_2)()
+
+    # Hypothetical Django scenario - middleware function is async
+    async def async_middleware_2():
+        await sync_to_async(sync_middleware_3)()
+
+    # Hypothetical Django scenario - middleware function is sync
+    def sync_middleware_3():
+        async_to_sync(async_middleware_4)()
+
+    # Hypothetical Django scenario - middleware function is async
+    async def async_middleware_4():
+        await sync_to_async(sync_middleware_5)()
+
+    # Hypothetical Django scenario - middleware function is sync
+    def sync_middleware_5():
+        async_to_sync(async_view)()
+
+    task_complete = False
+    task_cancel_caught = False
+
+    # Future that completes when subtask cancellation attempt is caught
+    task_blocker = asyncio.Future()
+
+    async def async_view():
+        """Async view with a task that is shielded from cancellation."""
+        nonlocal task_complete, task_cancel_caught, task_blocker
+        task = asyncio.create_task(sync_to_async(sync_parent)())
+        try:
+            await asyncio.shield(task)
+        except asyncio.CancelledError:
+            task_cancel_caught = True
+            task_blocker.set_result(True)
+            await task
+            task_complete = True
+
+    task_executed = False
+
+    # Future that completes after subtask is created
+    task_started_future = asyncio.Future()
+
+    def sync_parent():
+        async_to_sync(async_task)()
+
+    async def async_task():
+        """Async subtask that should not be canceled when parent is canceled."""
+        nonlocal task_started_future, task_executed, task_blocker
+        task_started_future.set_result(True)
+        await task_blocker
+        task_executed = True
+
+    task_cancel_propagated = False
+
+    async with ThreadSensitiveContext():
+        task = asyncio.create_task(sync_to_async(sync_middleware_1)())
+        await task_started_future
+        task.cancel()
+        try:
+            await task
+        except asyncio.CancelledError:
+            task_cancel_propagated = True
+        assert not task_cancel_propagated
+        assert task_cancel_caught
+        assert task_complete
+
+    assert task_executed