IApiFilter丢失AsyncLocal值

[pengweiqhca]

private sealed class TestFilter : IApiFilter
{
    private static readonly AsyncLocal<bool> V = new();

    public Task OnRequestAsync(ApiRequestContext context)
    {
        V.Value = true;

        return Task.CompletedTask;
    }

    public Task OnResponseAsync(ApiResponseContext context)
    {
        Assert.True(V.Value); // 此处将失败，应该成功

        return Task.CompletedTask;
    }
}

解决办法需要重构ApiRequestExecuter

下面是可能的解决办法（未验证，仅限参考）：

private static Func<ApiRequestContext, Task<ApiResponseContext>> BuildFilterPipe(ApiRequestContext context)
{
    Func<ApiRequestContext, Task<ApiResponseContext>> executor = static async request =>
    {
        using var requestAbortedLinker = new CancellationTokenLinker(request.HttpContext.CancellationTokens);

        return await ApiRequestSender.SendAsync(request, requestAbortedLinker.Token).ConfigureAwait(false);
    };

    executor = context.ActionDescriptor.FilterAttributes.Reverse()
        .Aggregate(executor, (current, filter) => Build(filter, current));

    executor = context.HttpContext.HttpApiOptions.GlobalFilters.Reverse()
        .Aggregate(executor, (current, filter) => Build(filter, current));

    static Func<ApiRequestContext, Task<ApiResponseContext>> Build(IApiFilter filter,
        Func<ApiRequestContext, Task<ApiResponseContext>> next)
    {
        return async context =>
        {
            await filter.OnRequestAsync(context).ConfigureAwait(false);

            var response = await next(context);

            await filter.OnResponseAsync(response).ConfigureAwait(false);

            return response;
        };
    }

    return executor;
}

[xljiulang]

return async context =>
{
    await filter.OnRequestAsync(context).ConfigureAwait(false);
    var response = await next(context);
    await filter.OnResponseAsync(response).ConfigureAwait(false);
    return response;
};

这里的处理不是正确的：

1. 在OnRequestAsync里创建的AsyncLocal不会影响到 async context{}方法的执行上下文，然而OnResponseAsync捕捉的是 async context{}方法的执行上下文。
2. 并不是每个filter要发起一笔请求，而是一笔请求需要多个filter来协同处理

正确的实现，应该是每个OnXXXAsync()的逻辑（不只是Filter的)，包装为一个中间件，然后按照现在的foreach先后顺序，把他们串起来编排，m1调用m2，m2调用m3这样的逻辑传递。考虑到这个编排过程依赖于ApiRequestContext自身的ActionDescriptor和HttpContext.HttpApiOptions两个重要属性，这就只能每次请求时编排和创建与Action关联的中间件委托，不管使用不使用Action来做中间件委托的缓存，都会带来一些性能损耗。

[pengweiqhca]

那可以添加一个如下的IApiPipeFilter接口，使用者只需要实现IApiPipeFilter即可

static Func<ApiRequestContext, Task<ApiResponseContext>> Build(IApiFilter filter,
    Func<ApiRequestContext, Task<ApiResponseContext>> next) =>
    context => (filter as IApiPipeFilter ?? new ApiFilterWrapper(filter)).ExecuteAsync(context, next);

interface IApiPipeFilter : IApiFilter
{
  Task<ApiResponseContext> ExecuteAsync(ApiRequestContext context, Func<ApiRequestContext, Task<ApiResponseContext>> next);

  Task IApiFilter.OnRequestAsync(ApiRequestContext context) => Task.CompletedTask;

  Task IApiFilter.OnResponseAsync(ApiResponseContext context) => Task.CompletedTask;
}

class ApiFilterWrapper(IApiFilter filter) : IApiPipeFilter
{
  public async Task<ApiResponseContext> ExecuteAsync(ApiRequestContext context, Func<ApiRequestContext, Task<ApiResponseContext>> next)
  {
      await filter.OnRequestAsync(context);
      var response = await next(context);
      await filter.OnResponseAsync(response);
      return response;
  }
}

private sealed class TestFilter : IApiPipeFilter
{
    private static readonly AsyncLocal<bool> V = new();

    public async Task<ApiResponseContext> ExecuteAsync(ApiRequestContext context, Func<ApiRequestContext, Task<ApiResponseContext>> next)
    {
        V.Value = true;

        var response = await next(context);
        
        Assert.True(V.Value);

        return response;
    }
}

[xljiulang]

好想法，我们可以在WebApiClientCore.Abstractions项目新建一个public delegate Task<ApiResponseContext> RequestDelegate(ApiRequestContext request);的请求委托，并为ApiActionDescriptor增加一个public RequestDelegate RequestDelegate {get;}的属性， 创建和修改RequestDelegate属性的过程，在DefaultApiActionDescriptor中实现，步骤为：

1.注入IOptionsMonitor获取GlobalFilters，拿GlobalFilters和Attribute类型的Filter以及ApiRequestExecutor(去掉了filter执行）一起build出RequestDelegate ；
2. 监听IOptionsMonitor的变化，变化之后执行步骤1的逻辑；

这样做之后，至少在Filter，应该能够使用AsyncLocal了

[xljiulang]

@pengweiqhca

public async Task<ApiResponseContext> ExecuteAsync(ApiRequestContext context, Func<ApiRequestContext, Task<ApiResponseContext>> next)
  {
      await filter.OnRequestAsync(context);
      var response = await next(context);
      await filter.OnResponseAsync(response);
      return response;
  }

上面这种在OnRequestAsync里异步之后给AsyncLocal设置值，在OnResponseAsync里是获取不到的。

    public async Task<ApiResponseContext> ExecuteAsync(ApiRequestContext context, Func<ApiRequestContext, Task<ApiResponseContext>> next)
    {
        V.Value = true;

        var response = await next(context);
        
        Assert.True(V.Value);

        return response;
    }

上面这种代码，完全不需要AsyncLocal（你可以把V当作是普通的Nullable<bool>类型)，任何访问的变量，可以被捕获变成async await的上下文。

[pengweiqhca]

第二种AsyncLocal是要在HTTP管道中使用到AsyncLocal值，此处只是用于举例