.net core中Grpc使用报错：The response ended prematurely.

　　当我们调用Grpc是出现下面的一堆异常时，一般是由于LTS导致的：　　

　　Call failed with gRPC error status. Status code: 'Unavailable', Message: 'Error starting gRPC call. HttpRequestException: An error occurred while sending the request. IOException: The response ended prematurely.'.
　　fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
　　Grpc.Core.RpcException: Status(StatusCode="Unavailable", Detail="Error starting gRPC call. HttpRequestException: An error occurred while sending the request. IOException: The response ended prematurely.", DebugException="System.Net.Http.HttpRequestException: An error occurred while sending the request.
　　---> System.IO.IOException: The response ended prematurely.
　　at System.Net.Http.HttpConnection.FillAsync()
　　at System.Net.Http.HttpConnection.ReadNextResponseHeaderLineAsync(Boolean foldedHeadersAllowed)
　　at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
　　--- End of inner exception stack trace ---
　　at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
　　at System.Net.Http.HttpConnectionPool.SendWithNtConnectionAuthAsync(HttpConnection connection, HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
　　at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
　　at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
　　at System.Net.Http.DiagnosticsHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
　　at Microsoft.Extensions.Http.Logging.LoggingHttpMessageHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
　　at Microsoft.Extensions.Http.Logging.LoggingScopeHttpMessageHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
　　at System.Net.Http.HttpClient.FinishSendAsyncUnbuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
　　at Grpc.Net.Client.Internal.GrpcCall`2.RunCall(HttpRequestMessage request, Nullable`1 timeout)")
　　at Grpc.Net.Client.Internal.HttpClientCallInvoker.BlockingUnaryCall[TRequest,TResponse](Method`2 method, String host, CallOptions options, TRequest request)
　　.........

　　首先我们知道，Grpc是基于HTTP/2作为通信协议的，而HTTP/2默认是基于LTS/SSL加密技术的，或者说默认需要https协议支持（https=http+lts/ssl），而HTTP/2又支持明文传输，即对http也是支持，但是一般需要我们自己去设置。

　　当我们使用Grpc时，又不去改变这个默认行为，那可能就会导致上面的报错。

　　在.net core开发中，Grpc要支持http，我们需要显示的指定不需要LTS支持，官方给出的做法是添加如下配置（比如客户端（调用方）在ConfigureServices添加）：　　

    public void ConfigureServices(IServiceCollection services)
    {
        //显式的指定HTTP/2不需要TLS支持
        AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport", true); 
        AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2Support", true);

        services.AddGrpcClient<Greeter.GreeterClient>(nameof(Greeter.GreeterClient), options =>
        {
            options.Address = new Uri("http://localhost:5000");
        });

        ...
    }