public CompletableFuture<?> stop() {
    if (httpServer == null) {
      return CompletableFuture.completedFuture(null);
    }

    final CompletableFuture<?> resultFuture = new CompletableFuture<>();
    httpServer.close(
        res -> {
          if (res.failed()) {
            resultFuture.completeExceptionally(res.cause());
          } else {
            httpServer = null;
            resultFuture.complete(null);
          }
        });
    return resultFuture;
  }
Stops the GraphQL HTTP service.

<p>This method stops the HTTP server that was created and started by the start() method. If the
server is not running, this method will do nothing.

@return a CompletableFuture that will be completed when the server is successfully stopped.
ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/graphql/GraphQLHttpService.java