Remove the offer sdp from peer.Join. fix #409

[cloudwebrtc]

Separate the offer sdp parameter in the peer.Join method.
This allows for a more flexible signal design. The current modification does not affect the existing ion-sfu signaling but separates the first offer SDP from the join method. When the join fails, we don’t have to operate pc or create an offer on the client-side.

before:

                       answer, err := peer.Join(payload.Join.Sid, offer)
                        if err != nil {
                                switch err {
                                case sfu.ErrTransportExists:
                               ...
                        }

after:

                       err = peer.Join(payload.Join.Sid)
                        if err != nil {
                                switch err {
                                case sfu.ErrTransportExists:
                                  return sig.SendReply(JoinReply{Success: false, Error: sfu.ErrTransportExists })
                                case sfu.RoomIsFull:
                                  return sig.SendReply(JoinReply{Success: false, Error: sfu.RoomIsFull })
                                 ...
                        }
                        return sig.SendReply(JoinReply{Success: true, Error: nil })

                       ......

                       // recv description.
                       answer, err := peer.Answer(offer)
                       if err != nil {
                               return status.Errorf(codes.Internal, fmt.Sprintf("answer error: %v", err))
                       }

Reference issue

Fixes #409

[codecov]

Codecov Report

Merging #410 (26782ca) into master (7925192) will decrease coverage by 0.21%.
The diff coverage is 63.49%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
- Coverage   41.35%   41.14%   -0.22%     
==========================================
  Files          21       21              
  Lines        2048     2022      -26     
==========================================
- Hits          847      832      -15     
+ Misses       1099     1087      -12     
- Partials      102      103       +1     

Impacted Files	Coverage Δ
pkg/buffer/buffer.go	22.26% <0.00%> (ø)
pkg/sfu/session.go	49.47% <30.76%> (+7.47%)	⬆️
pkg/sfu/peer.go	58.25% <53.84%> (-3.62%)	⬇️
pkg/sfu/downtrack.go	32.86% <83.87%> (+5.91%)	⬆️
pkg/sfu/subscriber.go	63.76% <100.00%> (-6.35%)	⬇️
pkg/sfu/router.go	48.58% <0.00%> (+1.12%)	⬆️

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[billylindeman]

It was originally this way, and @tarrencev added it to the join to streamline the process awhile back. thoughts @tarrencev

[tarrencev]

i'm ok with these changes. before i wanted to simplify things as much as possible. but perhaps we should be more flexible

[adwpc]

LGTM