fix: give error handlers an open container so @inject works in them

CONTEXT.md

@@ -26,7 +26,8 @@ updates in flight finish first, then the other shutdown handlers run, then it is
 The single `Scope.REQUEST` child the middleware builds for one incoming `Update`, seeded with that
 `Update` and its concrete event. Every handler, dialog getter and dialog callback that the update
 reaches shares this one container, and it is closed when the middleware returns, on the error path
-included.
+included. An error handler runs after that close, so the same middleware on `dispatcher.errors`
+builds it a second child, seeded with the failing `Update` and its event.
 _Avoid_: request container. The unit of work is an update, not an HTTP request, the same reason
 `modern-di` rejects "request" for `Connection`.
 

modern_di_aiogram/main.py

@@ -8,7 +8,7 @@
 import aiogram
 from aiogram import BaseMiddleware, Dispatcher
 from aiogram.dispatcher.event.handler import HandlerObject
-from aiogram.types import TelegramObject, Update
+from aiogram.types import ErrorEvent, TelegramObject, Update
 from aiogram.types.update import UpdateTypeLookupError
 from modern_di import Container, Scope, integrations, providers
 
@@ -50,9 +50,10 @@ async def __call__(
         event: TelegramObject,
         data: dict[str, typing.Any],
     ) -> typing.Any:  # noqa: ANN401
-        context: dict[type[typing.Any], typing.Any] = {Update: event}
+        update = event.update if isinstance(event, ErrorEvent) else event
+        context: dict[type[typing.Any], typing.Any] = {Update: update}
         with contextlib.suppress(UpdateTypeLookupError):
-            context[TelegramObject] = typing.cast("Update", event).event
+            context[TelegramObject] = typing.cast("Update", update).event
         self.in_flight += 1
         try:
             async with self.container.build_child_container(scope=Scope.REQUEST, context=context) as child_container:
@@ -116,6 +117,7 @@ def open_root() -> None:
 
         dispatcher.startup.register(open_root)
     dispatcher.update.outer_middleware(middleware)
+    dispatcher.errors.outer_middleware(middleware)
     if auto_inject:
         dispatcher.startup.register(functools.partial(_inject_router, dispatcher))
     return container

tests/test_inject.py

@@ -6,7 +6,7 @@
 import pytest
 from aiogram import Bot, Dispatcher
 from aiogram.dispatcher.event.bases import UNHANDLED
-from aiogram.types import Message, Update
+from aiogram.types import ErrorEvent, Message, Update
 from modern_di import Container, Group, Scope, providers
 
 from modern_di_aiogram import FromDI, inject, setup_di
@@ -104,6 +104,32 @@ async def handler(
     assert teardowns == ["closed"]  # per-update child closed (finalizer ran) on the error path
 
 
+async def test_inject_resolves_in_an_error_handler(dispatcher: Dispatcher, bot: Bot) -> None:
+    seen: dict[str, typing.Any] = {}
+
+    @dispatcher.message()
+    async def handler(message: Message) -> None:  # noqa: ARG001
+        msg = "boom"
+        raise ValueError(msg)
+
+    @dispatcher.errors()
+    @inject
+    async def on_error(
+        event: ErrorEvent,
+        request_instance: typing.Annotated[DependentCreator, FromDI(Dependencies.request_factory)],
+        event_type: typing.Annotated[str, FromDI(Dependencies.event_type)],
+    ) -> None:
+        seen["exception"] = str(event.exception)
+        seen["request_ok"] = isinstance(request_instance, DependentCreator)
+        seen["event_type"] = event_type
+
+    await dispatcher.emit_startup()
+    await dispatcher.feed_update(bot, make_message_update())
+    await dispatcher.emit_shutdown()
+
+    assert seen == {"exception": "boom", "request_ok": True, "event_type": "Message"}
+
+
 async def test_inject_without_setup_di_names_the_fix(bot: Bot) -> None:
     dispatcher = Dispatcher()