fix: Handle client disconnection for HTTP and WebSocket according to ASGI spec

For HTTP connections:
- If the app is sending data using the send callable, according to the ASGI spec, it should throw an exception
  in case of client disconnection. Previously, even if we processed the client_error message
  and set the http->closed state, it wouldn't throw an error because it wasn't handled.
  This change ensures that the exception is raised as per the ASGI spec.

For WebSocket connections:
- If the app is awaiting on receive, it would get a 'websocket.disconnect' event.
  However, if the app continues to send data using the send callable after receiving this event,
  it wouldn't raise an error because ws->state = NXT_WS_DISCONNECTED was never set in that case.
  According to the ASGI spec, if send is called after receiving a 'websocket.disconnect' event or on a closed client,
  it should raise an exception. This change ensures that the exception is raised as per the ASGI spec.

Author: gourav-kandoria

src/nxt_unit.c

@@ -74,7 +74,7 @@ static int nxt_unit_request_check_response_port(nxt_unit_request_info_t *req,
 static int nxt_unit_send_req_headers_ack(nxt_unit_request_info_t *req);
 static int nxt_unit_process_websocket(nxt_unit_ctx_t *ctx,
     nxt_unit_recv_msg_t *recv_msg);
-static int nxt_unit_process_client_error(nxt_unit_ctx_t *ctx, 
+static int nxt_unit_process_client_error(nxt_unit_ctx_t *ctx,
     nxt_unit_recv_msg_t *recv_msg);
 static int nxt_unit_process_shm_ack(nxt_unit_ctx_t *ctx);
 static nxt_unit_request_info_impl_t *nxt_unit_request_info_get(
@@ -1736,7 +1736,7 @@ nxt_unit_process_client_error(nxt_unit_ctx_t *ctx, nxt_unit_recv_msg_t *recv_msg
     if (req == NULL) {
         return NXT_UNIT_OK;
     }
-    
+
     lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
 
     if (lib->callbacks.close_handler) {

src/python/nxt_python_asgi_http.c

@@ -367,6 +367,11 @@ nxt_py_asgi_http_response_body(nxt_py_asgi_http_t *http, PyObject *dict)
                             "Unexpected ASGI message 'http.response.body' "
                             "sent, after response already completed");
     }
+    
+    if (nxt_slow_path(http->closed)) {
+        return PyErr_Format(PyExc_RuntimeError,
+                            "Connection Closed ");
+    }
 
     if (nxt_slow_path(http->send_future != NULL)) {
         return PyErr_Format(PyExc_RuntimeError, "Concurrent send");

src/python/nxt_python_asgi_websocket.c

@@ -984,9 +984,9 @@ nxt_py_asgi_websocket_close_handler(nxt_unit_request_info_t *req)
         return;
     }
 
-    if (ws->receive_future == NULL) {
-        ws->state = NXT_WS_DISCONNECTED;
+    ws->state = NXT_WS_DISCONNECTED;
 
+    if (ws->receive_future == NULL) {
         return;
     }