[Fix Incoming] Seriously damaging change to Online Subsystems in 4.20

[Stephen_Ellis]

This was logged as Unreal Engine Issues and Bug Tracker (UE-63326) and believed to be fixed with the 4.20.3 hotfix, however new information is showing that may not be the case.

I’ve pulled some conversation from private channels that may help.

For those using source code via GitHub:

If you change the serialization of the type in both places to serialize it as a string it should work.

In FUniqueNetIdRepl::MakeReplicationData, change the two places Type is serialized from

1. if (TypeHash == TypeHash_Other)
2. {
3. Writer << Type;
4. }

to:

1. if (TypeHash == TypeHash_Other)
2. {
3. FString TypeString = Type.ToString();
4. Writer << TypeString;
5. }

In FUniqueNetIdRepl::NetSerialize, change the two places Type is serialized from

1. if (TypeHash == TypeHash_Other)
2. {
3. Ar << Type;
4. if (Ar.IsError() || Type == NAME_None)
5. {
6. bValidTypeHash = false;
7. }
8. }

to:

1. if (TypeHash == TypeHash_Other)
2. {
3. FString TypeString;
4. Ar << TypeString;
5. Type = *TypeString;
6. if (Ar.IsError() || Type == NAME_None)
7. {
8. bValidTypeHash = false;
9. }
10. }

For binary users, this might be a viable workaround

If you happen to use FUniqueNetIdString in your custom online subsystem then it might be possible. You could provide one of the compiled in FNames of an online subsystem to the Type paramter of the FUniqueNetIdString ctor in your online subsystem. I tested this with “LIVE” in our case. If this built in subsystem is not loaded the serialization code will fall back to FUniqueNetIdString. (“LIVE” is a good match in this case as it also uses FUniqueNetIdString so it doesn’t matter if it’s loaded or not.)