dotnet-httpclient - 使用 "AddPolicyHandler"时 Polly 策略不起作用

Question

我有一个应用程序请求经过身份验证的服务，需要通过 access_token .
我的想法是使用 Polly 重试 access_token已过期。
我在 .NET Core 3.1 应用程序中使用 Refit (v5.1.67) 和 Polly (v7.2.1)。
服务注册如下:

services.AddTransient<ExampleDelegatingHandler>();

IAsyncPolicy<HttpResponseMessage> retryPolicy = Policy<HttpResponseMessage>
    .Handle<ApiException>()
    .RetryAsync(1, (response, retryCount) =>
    {
        System.Diagnostics.Debug.WriteLine($"Polly Retry => Count: {retryCount}");
    });

services.AddRefitClient<TwitterApi>()
    .ConfigureHttpClient(c =>
    {
        c.BaseAddress = new Uri("https://api.twitter.com/");
    })
    .AddHttpMessageHandler<ExampleDelegatingHandler>()
    .AddPolicyHandler((sp, req) =>
    {
        //this policy does not works, because the exception is not catched on 
        //"Microsoft.Extensions.Http.PolicyHttpMessageHandler" (DelegatingHandler)
        return retryPolicy;
    });

public interface TwitterApi
{
    [Get("/2/users")]
    Task<string> GetUsers();
}

public class ExampleDelegatingHandler : DelegatingHandler
{
    protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        try
        {
            return await base.SendAsync(request, cancellationToken);
        }
        catch (Exception)
        {
            //Why do not catch the exception?
            throw;
        }
    }
}

重试策略不起作用!
分析这个问题，我意识到异常没有被 HttpClient 的 DelegatingHandler 捕获。 .自 AddPolicyHandler语句正在生成 DelegatingHandler ( PolicyHttpMessageHandler ) 来执行策略并且在那里没有捕获异常，策略永远不会执行。我意识到问题只发生在异步场景中，可以发送请求。在同步场景中它有效(例如:超时)。
为什么内部没有捕获异常DelegatingHandler ??
我附上了一个模拟 Twitter 调用的示例项目。
https://www.dropbox.com/s/q1797rq1pbjvcls/ConsoleApp2.zip?dl=0
外部引用:
https://github.com/reactiveui/refit#using-httpclientfactory
https://www.hanselman.com/blog/UsingASPNETCore21sHttpClientFactoryWithRefitsRESTLibrary.aspx
https://docs.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.1

Answer 1

TL;DR:AddPolicyHandler 的顺序和 AddHttpMessageHandler确实很重要。

我用 HttpClient 重现了这个问题(所以没有 retrofit )。
用于测试的类型化 HttpClient

public interface ITestClient
{
    Task<string> Get();
}

public class TestClient: ITestClient
{
    private readonly HttpClient client;
    public TestClient(HttpClient client)
    {
        this.client = client;
    }
    public async Task<string> Get()
    {
        var resp = await client.GetAsync("http://not-existing.site");
        return "Finished";
    }
}

测试用 Controller

[ApiController]
[Route("[controller]")]
public class TestController : ControllerBase
{
    private readonly ITestClient client;

    public TestController(ITestClient client)
    {
        this.client = client;
    }

    [HttpGet]
    public async Task<string> Get()
    {
        return await client.Get();
    }
}

用于测试的 DelegateHandler

public class TestHandler: DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        try
        {
            return await base.SendAsync(request, cancellationToken);
        }
        catch (System.Exception ex)
        {
            _ = ex;
            throw;
        }
    }
}

订购 #1 - 处理程序、政策
启动

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();
    services.AddTransient<TestHandler>();
    services.AddHttpClient<ITestClient, TestClient>()
        .AddHttpMessageHandler<TestHandler>() //Handler first
        .AddPolicyHandler(RetryPolicy()); //Policy second
}

private IAsyncPolicy<HttpResponseMessage> RetryPolicy()
    => Policy<HttpResponseMessage>
    .Handle<HttpRequestException>()
    .RetryAsync(1, (resp, count) =>
    {
        Console.WriteLine(resp.Exception);
    });

执行顺序

TestController的 Get

TestClient的 Get

TestHandler的 SendAsync的 try

RetryPolicy的 onRetry

TestHandler的 SendAsync的 catch

TestController的 Get失败 HttpRequestException (内部:SocketException)

因此，这里不会触发重试策略。
订购 #2 - 政策、处理程序
启动

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();
    services.AddTransient<TestHandler>();
    services.AddHttpClient<ITestClient, TestClient>()
        .AddPolicyHandler(RetryPolicy()) //Policy first
        .AddHttpMessageHandler<TestHandler>(); //Handler second
}

private IAsyncPolicy<HttpResponseMessage> RetryPolicy()
    => Policy<HttpResponseMessage>
    .Handle<HttpRequestException>()
    .RetryAsync(1, (resp, count) =>
    {
        Console.WriteLine(resp.Exception);
    });

执行顺序

TestController的 Get

TestClient的 Get

TestHandler的 SendAsync的 try

TestHandler的 SendAsync的 catch

RetryPolicy的 onRetry

TestHandler的 SendAsync的 try

TestHandler的 SendAsync的 catch

TestController的 Get失败 HttpRequestException (内部:SocketException)

因此，这里重试策略已被触发。