如何安全地拦截自定义 Owin 中间件中的响应流

Question

我正在尝试编写一个简单的OWIN 中间件，以拦截响应流。我想要做的是用自定义的基于 Stream 的类替换原始流，在那里我将能够拦截对响应流的写入。

但是，我遇到了一些问题，因为我不知道响应何时被链中的内部中间件组件完全写入。永远不会调用 Stream 的 Dispose 覆盖。所以我不知道什么时候该执行我的处理，这应该发生在响应流的末尾。

这是一个示例代码：

public sealed class CustomMiddleware: OwinMiddleware
{
    public CustomMiddleware(OwinMiddleware next)
        : base(next)
    {
    }

    public override async Task Invoke(IOwinContext context)
    {
        var request = context.Request;
        var response = context.Response;

        // capture response stream

        var vr = new MemoryStream();
        var responseStream = new ResponseStream(vr, response.Body);

        response.OnSendingHeaders(state =>
        {
            var resp = (state as IOwinContext).Response;
            var contentLength = resp.Headers.ContentLength;

            // contentLength == null for Chunked responses

        }, context);

        // invoke the next middleware in the pipeline

        await Next.Invoke(context);
    }
}

public sealed class ResponseStream : Stream
{
    private readonly Stream stream_; // MemoryStream
    private readonly Stream output_; // Owin response
    private long writtenBytes_ = 0L;

    public ResponseStream(Stream stream, Stream output)
    {
        stream_ = stream;
        output_ = output;
    }

    ... // System.IO.Stream implementation

    public override void Write(byte[] buffer, int offset, int count)
    {
        // capture writes to the response stream in our local stream
        stream_.Write(buffer, offset, count);

        // write to the real output stream
        output_.Write(buffer, offset, count);

        // update the number of bytes written

        writtenBytes_ += count;

        // how do we know the response is complete ?
        // we could check that the number of bytes written
        // is equal to the content length, but content length
        // is not available for Chunked responses.
    }

    protected override void Dispose(bool disposing)
    {
        // we could perform our processing
        // when the stream is disposed of.
        // however, this method is never called by
        // the OWIN/Katana infrastructure.
    }
}

正如我在上面代码中的 cmets 中提到的那样，我可以想到两种策略来检测响应是否完整。

a) 我可以记录写入响应流的字节数并将其与预期的响应长度相关联。但是，对于使用分块传输编码的响应，长度是未知的。

b) 当在响应流上调用Dispose 时，我可以确定响应流已完成。但是，OWIN/Katana 基础架构从不对替换的流调用 Dispose。

我一直在调查Opaque Streaming，以了解操纵底层 HTTP 协议是否是一种可行的方法，但我似乎不知道 Katana 是否支持不透明流。

有没有办法实现我想要的？

Answer 1

我认为您不需要子类流，但是您可以通过以下方式阅读响应。只需确保此中间件是 OWIN 管道中的第一个中间件，这样它将是最后一个检查响应的中间件。

using AppFunc = Func<IDictionary<string, object>, Task>;

public class CustomMiddleware
{
    private readonly AppFunc next;

    public CustomMiddleware(AppFunc next)
    {
        this.next = next;
    }

    public async Task Invoke(IDictionary<string, object> env)
    {
        IOwinContext context = new OwinContext(env);

        // Buffer the response
        var stream = context.Response.Body;
        var buffer = new MemoryStream();
        context.Response.Body = buffer;

        await this.next(env);

        buffer.Seek(0, SeekOrigin.Begin);
        var reader = new StreamReader(buffer);
        string responseBody = await reader.ReadToEndAsync();

        // Now, you can access response body.
        Debug.WriteLine(responseBody);

        // You need to do this so that the response we buffered
        // is flushed out to the client application.
        buffer.Seek(0, SeekOrigin.Begin);
        await buffer.CopyToAsync(stream);
    }
}

顺便说一句，据我所知，从 OwinMiddleware 派生不是一个好的做法，因为 OwinMiddleware 是特定于 Katana 的。但是，这与您的问题无关。