Stop a client authoring the clock its own shots are judged against

[themuffinator]

Engine half of the multiplayer lag-compensation work. Pairs with openQ4-game #14, which is merged.

The sanitisation

usercmd.gameTime arrives from the client and reaches the game unchanged, so anything that measures a command's age against the server clock becomes a dial the client can turn. Overwriting it with the frame index the server already filed the command under is a no-op for an honest client — that is exactly what idAsyncClient sends — and it removes the dial permanently.

Staleness detection is unaffected: a slot that was never received still holds a stamp from a whole backup window ago, so DuplicateUsercmd still fires. Worth noting the obvious exploit is the mirror of the intuitive one — stamping the value low is self-defeating because DuplicateUsercmd overwrites it, so the live vector was stamping it high.

The contract

Lag compensation was pinned by nothing in either repository, which is how it came to ship with a rewind that could not measure what it was for. tools/tests/mp_lag_compensation_contract.py pins three agreements:

• the estimate must not be derived from the command's age (a structural constant — the server executes a client's command at that client's own frame index, after RunFrame has advanced time past it);
• the hit target must be restored before anything acts on the hit — damage, decal, impact effect, the effect's PVS routing, ragdoll;
• a discontinuity must invalidate the history.

It also pins net_mpLagCompensation at 0, so the default cannot be flipped by accident — only deliberately, by editing the assertion alongside it. Registered in both workflows and the local validation profile.

Pin bump

OPENQ4_GAMELIBS_SHA moves to 6804902, which is the tree these commits were measured against and carries all three merged multiplayer changes (openQ4-game #11, #12, #14). The contract test asserts code that exists only there.

🤖 Generated with Claude Code

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