Expand length verification

[Noarkhh]

Make verify_content_length recognize and inform if body length is too long

[gBillal]

Hi @Noarkhh
Since you're working on fixing the verifiy_content_length, there's a bug that occurs sometimes because of this function in (https://github.com/membraneframework/membrane_rtsp/blob/master/lib/membrane_rtsp/transport/tcp_socket.ex#L56).

When sending a play request using the same session for rtp data. Some IP cameras send an RTSP response along an RTP packet on one network packet which causes the function above to loop forever.

[Noarkhh]

Hi @gBillal

I think when this function was written it was assumed that the only traffic on the TCP connection would be RTSP communication. In my experience with RTP over TCP with RTSP some servers start streaming and send some RTCP and RTP packets even before the response to PLAY request. I think the best solution to this is to use RTSP.play_no_response and capture the response using RTP.RTSP.Decapsulator.

Purpose of this PR was essentially to expand the verification to include cases, where there is no Content-Length header, but there was more data anyway. I needed it specifically for membraneframework/membrane_rtp_plugin#166, so that capturing RTSP responses in cases of interleaved RTP would work correctly.

By the way I was getting ready to start implementing a RTSP Source and creating membrane_rtsp_plugin, since the RTSP functionality is kind of repetetive, but then I randomly stumpled upon your implementation 😅 I forked and started adapting it, so you can expect a PR soon, since we'd be happy if we could make it work for our use cases too :)

[gBillal]

Using play_no_response make sense in this case since the rtsp_decapsulator will parse the rtsp response at some point.

For membrane_rtsp_plugin I also found myself repeating the same code over to use RTSP, this is why I decided to create an element to reuse it. I had a hunch that you guys will create a similar element, this is why I haven't published it on hex.

[mat-hek]

btw, can you change this if to when (a guard), to reduce nesting?