Changed exception handlers to always run, but render response only if…

… it's yet to be rendered

starlette/exceptions.py

@@ -81,16 +81,15 @@ async def sender(message: Message) -> None:
             if handler is None:
                 raise exc from None
 
-            if response_started:
-                msg = "Caught handled exception, but response already started."
-                raise RuntimeError(msg) from exc
-
             request = Request(scope, receive=receive)
             if asyncio.iscoroutinefunction(handler):
                 response = await handler(request, exc)
             else:
                 response = await run_in_threadpool(handler, request, exc)
-            await response(scope, receive, sender)
+
+            if not response_started:
+                # Send the response only if not response started already.
+                await response(scope, receive, sender)
 
     def http_exception(self, request: Request, exc: HTTPException) -> Response:
         if exc.status_code in {204, 304}:

tests/test_exceptions.py

@@ -58,11 +58,26 @@ def test_websockets_should_raise():
 
 
 def test_handled_exc_after_response():
-    # A 406 HttpException is raised *after* the response has already been sent.
-    # The exception middleware should raise a RuntimeError.
-    with pytest.raises(RuntimeError):
+    wrapper_used = False
+
+    def handler_wrapper(func):
+        def wrapper(*args, **kwargs):
+            nonlocal wrapper_used
+            wrapper_used = True
+            return func(*args, **kwargs)
+
+        return wrapper
+
+    try:  # Revert state no matter what happens
+        key, func = app._exception_handlers.popitem()
+        app._exception_handlers[key] = handler_wrapper(func)
+
         client.get("/handled_exc_after_response")
 
+        assert wrapper_used
+    finally:
+        app._exception_handlers[key] = func
+
     # If `raise_server_exceptions=False` then the test client will still allow
     # us to see the response as it will have been seen by the client.
     allow_200_client = TestClient(app, raise_server_exceptions=False)