Cannot add a candidate without specifying either sdpMid or sdpMLineIndex - Webrtc Wasm

[dcam0050]

Your environment.

• Version:
  Ubuntu 21.10 (Offer)
  Ubuntu 20.04 (Answer)
  go 1.16 GOOS=wasm GOARCH=js
  pion v3.1.23

• Browser (Offer)
  Google Chrome - Ubuntu 21.10 - Version 97.0.4692.99 (Official Build) (64-bit)
  Firefox - Ubuntu 21.10 - 97.0 (64-bit)

• Other Information - stacktraces, related issues, suggestions how to fix, links for us to have context

What did you do?

I modified the pion to pion example in order to run the answer on a pc while running the offer inside of wasm. Candidate exchange follows trickle-ice for handling ice candidates as they arrive

What did you expect?

Expected a connection to be made and data transmission to start.

What happened?

Received this error in Firefox:

Uncaught (in promise) TypeError: Cannot add a candidate without specifying either sdpMid or sdpMLineIndex
    valueCall http://localhost:9090/wasm_exec.js:399
    _resume http://localhost:9090/wasm_exec.js:588
    _makeFuncWrapper http://localhost:9090/wasm_exec.js:599
    valueCall http://localhost:9090/wasm_exec.js:399
    _resume http://localhost:9090/wasm_exec.js:588
    _makeFuncWrapper http://localhost:9090/wasm_exec.js:599

And this error in google chrome:

wasm_exec.js:399 Uncaught (in promise) TypeError: Failed to execute 'addIceCandidate' on 'RTCPeerConnection': Candidate missing values for both sdpMid and sdpMLineIndex
    at syscall/js.valueCall (wasm_exec.js:399:31)
    at syscall_js.valueCall (main.wasm:0x180ab9)
    at syscall_js.Value.Call (main.wasm:0x17de8f)
    at github.com_pion_webrtc_v3.__PeerConnection_.AddICECandidate (main.wasm:0x9413b6)
    at wasm_pc_f_loop (main.wasm:0xe8903)
    at wasm_export_resume (main.wasm:0xe88e4)
    at global.Go._resume (wasm_exec.js:588:23)

When running both offer and answer on the same computer a data connection is created and data is sent but if the answer lies on a different computer over the internet, a connection is not established.

[dcam0050]

After some additional testing, changing to pion/webrtc v2.2.26 results in the same error on the browser side however a connection is established and data channels are created and operate correctly.

Changing to v3.0.1 however gives the same result as v3.1.23 - ICE connection status does not become connected and data cannot be sent between answer and offer.

[RustPanda]

Hello. The problem is this piece of code:
cecandidate.go#L159

// ToJSON returns an ICECandidateInit
// as indicated by the spec https://w3c.github.io/webrtc-pc/#dom-rtcicecandidate-tojson
func (c ICECandidate) ToJSON() ICECandidateInit {
	zeroVal := uint16(0)
	emptyStr := ""
	candidateStr := ""

	candidate, err := c.toICE()
	if err == nil {
		candidateStr = candidate.Marshal()
	}

	return ICECandidateInit{
		Candidate:     fmt.Sprintf("candidate:%s", candidateStr),
		SDPMid:        &emptyStr,
		SDPMLineIndex: &zeroVal,
	}
}

RTCIceCandidate/sdpMid#value

Note: Attempting to add a candidate (using addIceCandidate()) that has a value of null for both sdpMid and sdpMLineIndex will throw a TypeError exception.

I ran into this problem in WebRTC-rs, what values should these fields be set to?