Throw on wrong Content-Length

[greenEkatherine]

Fixes #1412

Throwing an exception when Content-Length is set but doesn't match with actual length in request. This allows to avoid unclear hangings as described in the issue above.

[MihaZupan]

Looks good overall, just left some minor comments

[Tratcher]

There's a scenario where we don't copy the Content-Length:

reverse-proxy/src/ReverseProxy/Forwarder/HttpTransformer.cs

Lines 61 to 73 in 6eba401

	// https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.3
	// If a message is received with both a Transfer-Encoding and a
	// Content-Length header field, the Transfer-Encoding overrides the
	// Content-Length. Such a message might indicate an attempt to
	// perform request smuggling (Section 9.5) or response splitting
	// (Section 9.4) and ought to be handled as an error. A sender MUST
	// remove the received Content-Length field prior to forwarding such
	// a message downstream.
	if (httpContext.Request.Headers.ContainsKey(HeaderNames.TransferEncoding)
	&& httpContext.Request.Headers.ContainsKey(HeaderNames.ContentLength))
	{
	proxyRequest.Content?.Headers.Remove(HeaderNames.ContentLength);
	}

Can StreamCopyHttpContent capture the ContentLength from its own headers on start?

[greenEkatherine]

I considered it but StreamCopyHttpContent get it before transformation (step 2 vs step 3):

reverse-proxy/src/ReverseProxy/Forwarder/HttpForwarder.cs

Lines 287 to 293 in cde14a6

	// :: Step 2: Setup copy of request body (background) Client --► Proxy --► Destination
	// Note that we must do this before step (3) because step (3) may also add headers to the HttpContent that we set up here.
	var requestContent = SetupRequestBodyCopy(context.Request, isStreamingRequest, activityToken);
	destinationRequest.Content = requestContent;
	// :: Step 3: Copy request headers Client --► Proxy --► Destination
	await transformer.TransformRequestAsync(context, destinationRequest, destinationPrefix);

Did I miss something?

[Tratcher]

You're correct on the ordering, but you can capture the header at the start of SerializeToStreamAsync, you don't need it in the constructor.

[greenEkatherine]

Got it, I moved capturing out of the constructor.

[Tratcher]

Looks good overall. My comments are mostly style and flow.