Async AcceptConnection

[nixxquality]

Summary

This PR replaces AcceptConnection with an async version. The original functionality is left in place, but marked as Obsolete.

Motivation & Context

An async function lets you make a time-consuming call like a check with a database or on-disk ban list without causing a server freeze on connections.

Fixes: #1563

Implementation Details

Discussion happened in Discord: https://discord.com/channels/833983068468936704/895597253316714497/1468245963356508322
The initial implementation was just a string where null means no rejection.
Further discussion led to a new ConnectionRequestResult struct with static defaults so it can be expanded in the future if need be.

Note: I have no idea where the ConnectionRequestResult struct should go. I can either follow instructions or you can eel free to pull in the PR and edit it to your hearts content.

Screenshots / Videos (if applicable)

(Note: this video was recorded before the rework but not much has changed)

2026-02-03_15-20-52.mp4

Sample projects:

• rejectconnection initial.zip
• rejectconnection struct.zip

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂

[badandbest]

I'd recommend squashing the commits into one when you're done

[badandbest]

You could combine both SceneNetworkingSystem.AcceptConnection methods by making the obsolete INetworkListener.AcceptConnection return a ConnectionRequestResult.Reject. This would simplify other parts of the class.

Suggested change

	var result = await c.AcceptConnection( channel );
	if ( !result ) return result;
	#pragma warning disable CS0618 // Games still use the old method.
	var denialReason = "";
	if ( !c.AcceptConnection( channel, ref denialReason ) )
	return ConnectionRequestResult.Reject( denialReason );
	#pragma warning restore CS0618
	var result = await c.AcceptConnection( channel );
	if ( !result ) return result;

[nixxquality]

The double functions exist because AcceptConnection is a public member of the public abstract partial class GameNetworkSystem. Changing that would be a breaking change.
https://github.com/Facepunch/sbox-public/blob/69a2985c8645c3b94a70175596e8f86f78e91706/engine/Sandbox.Engine/Systems/Networking/System/GameNetworkSystem/GameNetworkSystem.cs

[badandbest]

Is it even possible for games to get access to GameNetworkSystem?

[nixxquality]

Not sure what you'd use it for, but again, it is public so yes it can be accessed.

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[nixxquality]

As an aside, I noticed that if an exception is thrown in the AcceptConnection function (both current and new) then the connecting user gets stuck in the loading screen until he gives up and presses cancel.

I could fix that in this PR as well, if you would like me to.

[badandbest]

if an exception is thrown in the AcceptConnection function then the connecting user gets stuck in the loading screen until he gives up and presses cancel

Yeah you should probably fix that right away