Discard default lifespans before merging

fastapi/routing.py

@@ -213,12 +213,16 @@ def _merge_lifespan_context(
     async def merged_lifespan(
         app: AppType,
     ) -> AsyncIterator[Mapping[str, Any] | None]:
-        async with original_context(app) as maybe_original_state:
-            async with nested_context(app) as maybe_nested_state:
-                if maybe_nested_state is None and maybe_original_state is None:
-                    yield None  # old ASGI compatibility
-                else:
-                    yield {**(maybe_nested_state or {}), **(maybe_original_state or {})}
+        async with AsyncExitStack() as stack:
+            maybe_original_state = await stack.enter_async_context(
+                original_context(app)
+            )
+            maybe_nested_state = await stack.enter_async_context(nested_context(app))
+
+            if maybe_nested_state is None and maybe_original_state is None:
+                yield None  # old ASGI compatibility
+            else:
+                yield {**(maybe_nested_state or {}), **(maybe_original_state or {})}
 
     return merged_lifespan  # type: ignore[return-value]
 
@@ -1823,10 +1827,17 @@ class APIRouter(routing.Router):
             self.add_event_handler("startup", handler)
         for handler in router.on_shutdown:
             self.add_event_handler("shutdown", handler)
-        self.lifespan_context = _merge_lifespan_context(
-            self.lifespan_context,
-            router.lifespan_context,
-        )
+
+        if type(router.lifespan_context) is _DefaultLifespan:
+            return
+
+        if type(self.lifespan_context) is _DefaultLifespan:
+            self.lifespan_context = router.lifespan_context
+        else:
+            self.lifespan_context = _merge_lifespan_context(
+                self.lifespan_context,
+                router.lifespan_context,
+            )
 
     def get(
         self,

tests/test_router_events.py

@@ -171,6 +171,107 @@ def test_router_nested_lifespan_state(state: State) -> None:
     assert state.sub_router_shutdown is True
 
 
+def test_router_nested_lifespan_state_discard_default_lifespan_app(
+    state: State,
+) -> None:
+
+    @asynccontextmanager
+    async def router_lifespan(app: FastAPI) -> AsyncGenerator[dict[str, bool], None]:
+        state.router_startup = True
+        yield {"router": True}
+        state.router_shutdown = True
+
+    @asynccontextmanager
+    async def subrouter_lifespan(app: FastAPI) -> AsyncGenerator[dict[str, bool], None]:
+        state.sub_router_startup = True
+        yield {"sub_router": True}
+        state.sub_router_shutdown = True
+
+    sub_router = APIRouter(lifespan=subrouter_lifespan)
+
+    router = APIRouter(lifespan=router_lifespan)
+    router.include_router(sub_router)
+
+    app = FastAPI()
+    app.include_router(router)
+
+    @app.get("/")
+    def main(request: Request) -> dict[str, str]:
+        assert request.state.router
+        assert request.state.sub_router
+        return {"message": "Hello World"}
+
+    assert state.router_startup is False
+    assert state.sub_router_startup is False
+    assert state.router_shutdown is False
+    assert state.sub_router_shutdown is False
+
+    with TestClient(app) as client:
+        assert state.router_startup is True
+        assert state.sub_router_startup is True
+        assert state.router_shutdown is False
+        assert state.sub_router_shutdown is False
+        response = client.get("/")
+        assert response.status_code == 200, response.text
+        assert response.json() == {"message": "Hello World"}
+
+    assert state.router_startup is True
+    assert state.sub_router_startup is True
+    assert state.router_shutdown is True
+    assert state.sub_router_shutdown is True
+
+
+def test_router_nested_lifespan_state_discard_default_lifespan_child(
+    state: State,
+) -> None:
+
+    @asynccontextmanager
+    async def lifespan(app: FastAPI) -> AsyncGenerator[dict[str, bool], None]:
+        state.app_startup = True
+        yield {"app": True}
+        state.app_shutdown = True
+
+    router = APIRouter()
+
+    app = FastAPI(lifespan=lifespan)
+    app.include_router(router)
+
+    @app.get("/")
+    def main(request: Request) -> dict[str, str]:
+        assert request.state.app
+        return {"message": "Hello World"}
+
+    assert state.app_startup is False
+    assert state.app_shutdown is False
+
+    with TestClient(app) as client:
+        assert state.app_startup is True
+        response = client.get("/")
+        assert response.status_code == 200, response.text
+        assert response.json() == {"message": "Hello World"}
+
+    assert state.app_shutdown is True
+
+
+def test_router_nested_lifespan_state_no_lifespans(
+    state: State,
+) -> None:
+    """Test that if no lifespans are provided, the app still works and the state is empty."""
+    router = APIRouter()
+
+    app = FastAPI()
+    app.include_router(router)
+
+    @app.get("/")
+    def main(request: Request) -> dict[str, str]:
+        return {"message": "Hello World"}
+
+    with TestClient(app) as client:
+        response = client.get("/")
+        assert response.status_code == 200, response.text
+        assert response.json() == {"message": "Hello World"}
+
+
 def test_router_nested_lifespan_state_overriding_by_parent() -> None:
     @asynccontextmanager
     async def lifespan(