implement a flush capability for data channels

[cuchaz]

Hello again,

In my application, it is useful to know when a data channel message has "finished" sending (but not necessarily received by the remote peer), so I've implemented a flush-like capability. It works by inserting a special flush meta-message into the processing pipeline (including the SCTP stream) which can be polled later using a specialized function. If I've implemented this correctly, the flush message should not be pollable until all the socket datagrams corresponding to all previous data channel message are also pollable.

This may not be the simplest way to implement a feature like this, but it's the first way I've found so far that works. It modifies a lot of different layers of the processing pipeline, so I realize it may not be such a welcome change.

Would you be interested in adding a flush feature to data channels? The attached PR is a draft implementation to illustrate the idea, but I don't know if it's really the best way to do this. Maybe you know of a better way? Or maybe this is a good enough starting point and we work on it from here.

Thanks,
Jeff

[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 108 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.11%. Comparing base (9feb4a3) to head (1541c17).

Files with missing lines	Patch %	Lines
rtc/src/peer_connection/handler/sctp.rs	40.62%	38 Missing ⚠️
rtc/src/peer_connection/handler/mod.rs	5.26%	18 Missing ⚠️
rtc/src/peer_connection/handler/datachannel.rs	7.14%	13 Missing ⚠️
rtc-sctp/src/association/stream.rs	0.00%	9 Missing ⚠️
rtc/src/peer_connection/message/internal.rs	0.00%	9 Missing ⚠️
rtc/src/data_channel/mod.rs	0.00%	8 Missing ⚠️
rtc/src/peer_connection/handler/demuxer.rs	0.00%	8 Missing ⚠️
rtc-sctp/src/queue/pending_queue.rs	88.37%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   71.17%   71.11%   -0.07%     
==========================================
  Files         442      442              
  Lines       67330    67452     +122     
==========================================
+ Hits        47922    47966      +44     
- Misses      19408    19486      +78     

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